Aggression in Humans and Other Primates: Biology, Psychology, Sociology 9783110291360, 9783110291339

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Aggression in Humans and Other Primates Edited by Hans-Henning Kortüm & Jürgen Heinze

Aggression in Humans and Other Primates Biology, Psychology, Sociology Edited by Hans-Henning Kortüm & Jürgen Heinze

DE GRUYTER

ISBN 978-3-11-029133-9 e-ISBN 978-3-11-029136-0 Library of Congress Cataloging-in-Publication Data A CIP catalog record for this book has been applied for at the Library of Congress. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at http://dnb.dnb.de. © 2013 Walter de Gruyter GmbH, Berlin/Boston Printing and binding: Hubert & Co. GmbH & Co. KG, Göttingen Cover image: Hemera/Thinkstock ⬁ Printed on acid-free paper Printed in Germany www.degruyter.com

 

Contents Editors‘ Preface

.................................................................................................... vii

List of Contributors

.............................................................................................

Aggression in Humans and Other Animals – A Biological Prelude     JÜRGEN HEINZE

ix

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Aggression/Violence in Humans and Other Primates – A Historian’s Prelude .........................................................................................     HANS-HENNING KORTÜM

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What Theoretical Biology has to say on Aggression in Humans and Animals ........................................................................................ 23     PETER HAMMERSTEIN Aggression in Humans and Other Primates – Biology, Psychology, Sociology .............................................................................................................. 41     DIETMAR ZINNER AND BRANDON C. WHEELER Explaining Aggression: The Ultimate-Proximate Problem ........................... 87     CLAIRE EL MOUDEN   Human Sex Differences in Aggression from the Perspective of Sexual Selection ................................................................................................... 101     JOHN ARCHER The Logic of Aggression – Reciprocal Altruism, Psychological Games and the Persistence of Conflict .......................................................................... 121     LUCIANO ANDREOZZI What causes Large-Scale Variation in Homicide Rates?     MANUEL EISNER

............................... 137

One flew over the Cuckoo’s Nest: Violence, Uncertainty, and Safety     MANFRED J. HOLLER AND BARBARA KLOSE-ULLMANN Index

......... 163

..................................................................................................................... 181

Editors’ Preface There is always much talking about cooperation among different disciplines, but it does seldom happen. Especially the scientific dialogue between Natural Sciences and Humanities is extremely difficult. The ways of thinking and arguing are so different. Nevertheless, the editors of this book – a biologist and an historian – are convinced that such a dialogue between different science cultures could be possible and – what seems of even greater importance – could be a fruitful one. As a step forward, they organised a symposium at the University of Regensburg in September 2011 in order to bring together scientists belonging to different disciplines. The congress papers here collected are all dealing with (in a biological understanding) aggression and (in a wider understanding) violence. Even if it seems too optimistic to think that a common theory could be formulated for such different sciences as Evolutionary Biology, Neurobiology, Psychology, History, Criminology, Cultural Studies etc., the editors believe in the existence of quite a lot of similarities in explaining and understanding aggression and violence. This could further more cooperation in the future. We have to thank the Regensburger Universitätsstiftung for financial support, Dr Stephanie Dawson and Julia Lauterbach (Publishing House DeGruyter) and Katharina Fritsch (University of Regensburg) for all their support. Last but not least, we have to thank our authors. Due to their readiness to give us their conference papers in a short time after the conference, it was possible to publish the book so quickly.

Regensburg, October 2012 Jürgen Heinze

Hans-Henning Kortüm

List of Contributors LUCIANO ANDREOZZI

Professor, Department of Economics at University of Trento. JOHN ARCHER

Professor, School of Psychology at University of Central Lancashire. MANUEL EISNER

Professor, Institute of Comparative & Developmental Criminology at University of Cambridge. CLAIRE EL MOUDEN

Post-Doctoral Prize Research Fellow, Department of Zoology at Oxford University. PETER HAMMERSTEIN

Professor, Institute for Theoretical Biology at Humboldt University Berlin. JÜRGEN HEINZE

Professor, Institute of Zoology at University of Regensburg. MANRED J. HOLLER

Professor (em.), Institute of Economics, at University of Hamburg. BARBARA KLOSE-ULLMANN

Writer and Freelancer at an Academic Publisher, Munich. HANS HENNING KORTÜM

Professor, Institute of History at University of Regensburg. BRANDON C. WHEELER

Postdoctoral Research Fellow, Cognitive Ethology Lab at German Primate Center Göttingen. DIETMAR ZINNER

Senior Scientist, Research Group Cognitive Ethology at German Primate Center Göttingen.

Aggression in Humans and Other Animals – A Biological Prelude JÜRGEN HEINZE Murder – riot – amok – torture. Whenever we browse our daily newspaper, switch on our TV or google for news in the internet we stumble upon endless reports about violence and hostility in our neighborhood, our city, or somewhere else on our planet. Homo sapiens, the subjective pinnacle of evolution, appears to be a particularly aggressive species. Conflict and violence seem to form the common threads running through our society, from economy to the fine arts (see also chapter by Holler). Human history is characterized by a series of killings, wars, and genocides and large parts of human culture are centered on brutality, sadism, and warfare. Without crime, books rarely become bestsellers, movies without bloodshed rarely turn into Oscar-winning blockbusters, and the libretti of operas or musicals simply remain unexciting without jealousy, rivalry and murder. Lupus est homo homini – miserable us. Contests among conspecifics in animals other than H. sapiens have long been thought to be ritualized and undamaging. While aggression abounds between species – predators kill prey, parasites sterilize and mutilate their hosts – intraspecific dominance interactions, territorial disputes, and conflict over access to mating partners seem to result in injuries or death only rarely. Though in particular male animals display frightening armaments, such as strong mandibles, pronged antlers or elongated canines, already Charles Darwin (1871) observed that such weapons “seem singularly ill-fitted for fighting:” the horns of the Oryx antelope are directed backwards rather than pointing towards the opponent in a head-on encounter. The antlers of stags, which seem to intertwine during fights rather than stab the attacker, provide another example for the odd construction of animal weaponry. And even among carnivores, such as wolves, fights usually end with the loser displaying its most vulnerable parts in a submissive posture and the dominant not using its fangs, though these are perfectly designed by evolution to kill. Konrad Lorenz and other biologists in the mid 20th century concluded that animals “not only do not damage each other, but often constitute a community of interests” (Lorenz 1973). The presumed reluctance of animals to kill conspecifics was interpreted as “moral-analogue behavior,” selected for the good of the group or even the whole species. But humans were considered to be different (Lorenz 1973).

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With the decline of group selectionist thinking in the 60es and 70es and the corresponding advance of the gene-centered view of evolution (Williams 1966, Hamilton 1972), more attention was given to field studies that had revealed examples of fatal fighting and killing among animals. Headlines of sensationist journals in an animal world might read “Husband devoured after mating!” or “New alpha in the pride lacerates stepchildren!” or “Brothers killed in battle over sex with their sisters!” – only that these news would not make it to the front page, as they simply describe the daily business of praying mantis, lions, and fig wasps. Given the right conditions, animals of many species regularly kill conspecifics. Models of game theory, such as the simple hawk-dove game (see chapter by Zinner and Wheeler), suggest that lethal fighting may be stable in evolution whenever the killing is associated with a low risk of injury or death to the killer or when a contested resource is extremely valuable (Enquist & Leimar 1990, chapter by El Mouden). For example, after taking over a harem, a male lion may kill the helpless cubs of his predecessor. In this way he ensures that the lionesses are no longer “wasting” energy and time with nursing the offspring of another male and quickly will be available for mating. Similar infanticide has been observed in numerous other species, including monkeys and apes (Hrdy 1979, Hausfater & Hrdy 2008, chapter by Zinner and Wheeler). Males of fig wasps and Cardiocondyla ants engage in lethal combat for access to females (Hamilton 1979, Heinze & Hölldobler 1993, Cook et al. 1997). Mating occurs in a ”seraglio situation” (Hamilton, 1979): males and females eclose in a confined space – the interior of a fig fruit in the case of fig wasps or small chambers in the soil or plant material in Cardiocondyla. While the females are winged and disperse after mating, the males are wingless and never leave their place of birth. They can only increase their fitness, i.e., their reproductive success, by preventing rival males from mating, even if this is associated with a high risk to their own survival. Withdrawal from a fight equals a zero chance of mating. Hence, fights quickly escalate, as to kill a rival even when seriously injured oneself is always better than surrender. In what on a first glance appears to be the opposite way, a male black widow spider may similarly increase his fitness by allowing to be cannibalized by his mating partner. Because his mating organs are destroyed during sperm transfer, he has only a very limited chance of mating again with another female. By sacrificing himself, the male spider may increase the amount of resources available to the female for the investment into their joint offspring (Johns & Maxwell 1997). The emergence of group-living animals is one of the major transitions in evolution (Maynard Smith & Szathmáry 1995), in which cooperation and division of labor among individuals led to a new level of selection. Correspondingly, social evolution adds a new facet to animal aggression. In solitary animals, such as fig wasps, individuals engage in occasionally fatal one-to-one encounters. In social animals, such as many primates or social insects, the whole society may engage in violent fights with other societies (Wilson 1975).

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Teamwork and mutual help within the group are typically associated with aggression against non-members – it is us vs. them. The highly social ants are notoriously famous for “warfare” and raiding. Encounters among neighboring colonies of honeypot ants may leave hundreds of dead ants at territory borders, and hundreds of immature workers may be pillaged from the defeated colony and integrated into the workforce of the winner (Hölldobler 1976, Kronauer et al. 2003). Similar aggression against non-members is known from group-living mammals, such as primates: chimpanzees form raiding parties, which scout into neighboring territories and eventually kill members of other groups (chapter by Zinner and Wheeler). At the same time, cooperation within the group is associated with internal conflict among group members. Animal societies risk being exploited by selfish individuals, who pursue their own interests at a cost for the whole group. For example, rather than helping to gather food or to defend the nest against intruders, an individual in a paper wasp nest might selfishly focus on the production of its own offspring. If all wasps behaved in such a way, the whole group would fall apart – a typical case of the “tragedy of the commons” (Wenseleers et al. 2005). Social animals could only become successful in evolution once the disruptive conflicts among selfish group members had been resolved or suppressed. Even the apparently so harmonious and disciplined colonies of ants and honey bees, since Biblical times cited as examples for human civilization, are stabilized by a complex system of mutual surveillance and policing (Ratnieks & Wenseleers 2005). Ant workers, which in an experimental situation are allowed to challenge the reproductive monopoly of the queen and produce own offspring, are quickly identified and attacked by other workers, and their eggs are destroyed (Heinze 2004). Many insect societies thus appear as well-organized, conflict-free superorganisms when seen from outside, but are police states for an individual group member: “big sister is watching you.” What does this all tell us about violence and aggression in our species? Human aggressiveness appears to be less exceptional than was thought by philosophers such as Titus Maccius Plautus or Thomas Hobbes. Aggression and violence abound, and the animal world, as observed by “Darwin’s bulldog” Thomas Henry Huxley (1888), “is on the same level as the gladiator’s show.” E.O. Wilson (1978), commenting on the daily life of hamadryas baboons, suspected that if they “had nuclear weapons, they would destroy the world in a week.” In fact, humans are probably less aggressive than what could be expected for a primate with our peculiar life history of living in more or less monogamous pairs embedded in larger social communities. Given that conflict is one of the driving forces of evolution, one might jump to the conclusion that we behave like we do because violence, aggressiveness, and even “lust to kill” constitute parts of our evolved behavioral repertoire. Similar claims in the last chapter of E.O. Wilson’s pioneering book “Sociobiology” (Wilson 1975) were fiercely rejected by influential social scientists, anthropologists, and geneticists, who published an endless number of articles and

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books against this assumed biological determinism of humanity (e.g. Montagu 1980, Lewontin et al. 1984, see also Segestråle 2000). The controversy rekindled in the 90es, when comparable ideas about the biological foundation of human behavior were presented in more modern terminology as “evolutionary psychology” (Barkow et al. 1992, Rose & Rose 2001), and to a lesser extent again in the late 90es, when Wilson (1998) in “Consilience” claimed that a strictly biological approach to human behavior ought to replace the social sciences and humanities. Several critics of such sociobiological reasoning have argued that by creating civilization, the complex human brain and its enormous capacity of learning and imitation have long withdrawn modern H. sapiens from the grip of natural selection. Instead of our genetic legacy, the influences of society and culture were fully responsible for our behavior. Indeed, both history and psychological experiments have shown how social environment may lead to differences in the level of aggression and violence. Most of us have not yet violently beat up others, run amok, or vandalized a phone booth, but we probably would have done so if we had grown up in a slum or if we had been ordered to do so by an unquestioned authority. What perhaps needs most explanation is therefore “not why some people are criminals but why most people are not” (Wilson 1993). In the famous experiment conducted by Stanley Milgram, volunteers were told they were taking part in a study about the effect of punishment on learning. They were to teach a series of word pairs to a “learner” hidden by a screen, who later had to choose the missing word from a list of four possible answers when only the first word of each pair was given. When ordered by a scientist to punish incorrect or missing responses with electric shocks of increasing intensity, most “teachers” did so – not knowing that the shocking device was just a dummy and the “learner” was a member of the experimenter’s team, who only simulated the response to the fake shocks. Some participants paused at 135 V after hearing the learner scream and asked to stop the experiment, but continued when assured that it was critically important to do so and that they were not held responsible. Almost two thirds of the participants administered “final shocks” of 450 V (Milgram 1974). The experiment has been repeated at different places and with different details, but always with the same depressing results. Does this prove that humans are born as “tabulae nudae,” onto which society and education can imprint everything? If so, why have all attempts to establish a completely unaggressive and peaceful society failed, and why is the homicide rate in primeval societies often romanticized as free of aggression similar to that in the Western world (see chapter by Eisner)? And why are men usually more prepared to engage in physical aggression than females (see chapter by Archer)? Might not our willingness to conform and to punish under appropriate environmental and social conditions be an innate consequence of our specific life history as a social primate?

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The debate – nature vs. nurture – has been on for more than a century. It is indeed one of the oldest and most controversial quarrels between sciences and humanities. Sincere attempts to find out how to best study and interpret human qualities have repeatedly been intermingled with accidental or voluntary misinterpretations of fundamental terms, such as “heritability,” wrong accusations, and ideology-driven manipulation of data (e.g. Gould 1996). Many researchers now agree that the nature / nurture dichotomy is artificial. As D.O. Hebb reportedly mused, the debate is akin to the question, “which contributes more to the area of a rectangle, its length or its width?” Clearly, both natural and cultural evolution have shaped and are still shaping human behavior, and neither the exclusive sequencing of the human genome nor the exclusive analysis of societal influences on early infantile experience will produce a full understanding of why we behave in the ways we do. It is therefore unfortunate when biological findings about a more or less pronounced genetic influence on certain human qualities are popularized by the media as the discovery of “a gene for alcoholism,” “a gene for intelligence”, or “a language gene”. Complex traits are usually affected by a large number of genes and additionally may show strong gene-environment interactions. This is nicely illustrated by the story about the “warrior gene” monoaminoxidase A, coding for an enzyme that breaks down neurotransmitters in the brain. A certain variant, MAOA-L, has been found to be associated with antisocial behavior in males and was quickly linked to the formation of gang culture in the US. Subsequent studies have shown, however, that MAOA-L is quite common in the population and associated with an increased probability of exhibiting violent behavior only in certain situations and / or in people who have been maltreated during childhood: a clear example of the interwoven effects of nature and nurture (Yong 2010, see also chapter by El-Mouden). Obviously, additional interdisciplinary and transdisciplinary efforts are needed to complete our understanding of why we behave in the ways we do. The present volume was inspired by a fruitful symposium in September 2011 in Regensburg, financially supported by Regensburger Universitätsstiftung. It summarizes the views of several experts in the field of primate aggression and adds interesting puzzle stones to the emerging picture of the cultural and biological foundations of aggression in humans and other primates.

References Barkow, J. H., Cosmides, L. & Tooby, J. (Eds.) (1992). The Adapted Mind: Evolutionary Psychology and the Generation of Culture. Oxford: Oxford University Press. Cook, J. M., Compton, S. G., Herre, E. A. & West, S. A. (1997). Alternative Mating Tactics and Extreme Male Dimorphism in Fig Wasps. Proceedings of the Royal Society of London, B 264, 747-754.

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Darwin, C. (1871). The Descent of Man, and Selection in Relation to Sex. London: John Murray.c Enquist, M. & Leimar, O. (1990). The Evolution of Fatal Fighting. Animal Behaviour, 39, 1-9. Gould, S. J. (1996). The Mismeasurement of Man. 2nd edition. New York: W.W. Norton & Co. Hamilton, W. D. (1979). Wingless and Fighting Males in Fig Wasps and Other Insects. In: M. S. Blum & N. A. Blum (Eds.), Sexual Selection and Reproductive Competition in Insects (pp. 167-210). London: Academic Press. Hamilton, W. D. (1972). Altruism and Related Phenomena, mainly in Social Insects. Annual Review of Ecology and Systematics, 3, 193-232. Hausfater, G. and Hrdy, S. B. (Eds.) (2008). Infanticide: Comparative and Evolutionary Perspectives. Piscataway, N.J.: Aldine Transactions. Heinze, J. (2004). Reproductive Conflict in Insect Societies. Advances in the Study of Behavior, 34, 1-57. Heinze, J. & Hölldobler, B. (1993). Fighting for a Harem of Queens: Physiology of Reproduction in Cardiocondyla Male Ants. Proceedings of the National Academy of Sciences of the USA, 90, 8412-8414. Hölldobler, B. (1976). Tournaments and Slavery in a Desert Ant. Science, 192, 912-914. Hrdy, S. (1979). Infanticide among Animals: A Review, Classification, and Examination of the Implications for the Reproductive Strategies of Females. Ethology and Sociobiology, 1, 13-40. Huxley, T. H. H. (1888). Struggle for Existence and Its Bearing on Man. In Collected Essays, vol. IX. London: Macmillan. Kronauer, D. J. C., Gadau, J. & Hölldobler, B. (2003). Genetic Evidence for Intra- and Interspecific Slavery in Honey Ants (genus Myrmecocystus). Proceedings of the Royal Society of London B, 270, 805-810. Johns, P. M. & Maxwell, M. R. (1997). Sexual Cannibalism: Who benefits? Trends in Ecology and Evolution, 12, 127-128. Lewontin, R. C., Rose, S. & Kamin, L. J. (1984). Not in Our Genes. Biology, Ideology, and Human Nature. New York: Pantheon. Lorenz, K. (1973). Die acht Todsünden der zivilisierten Menschheit. München: Piper. Maynard Smith J. & Szathmáry, E. (1995). The Major Transitions in Evolution. Oxford: Oxford University Press. Milgram, S. (1974). Obedience to Authority. An Experimental View. New York: Harper & Row. Montagu, A. (Ed.) (1980). Sociobiology examined. Oxford: Oxford University Press. Ratnieks, F. L. W. & Wenseleers, T. (2005). Policing Insect Societies. Science, 307, 54-56. Rose, H. & Rose, S. (Eds.) (2001). Alas Poor Darwin: Arguments against Evolutionary Psychology. London: Vintage. Segestråle, U. (2000). Defenders of the Truth. The Sociobiology Debate. Oxford: Oxford University Press. Wenseleers, T., Tofilski, A. & Ratnieks, F. L. W. (2005). Queen and Worker Policing in the Tree Wasp Dolichovespula sylvestris. Behavioral Ecology and Sociobiology, 58, 80-86. Williams, G. C. (1966). Adaptation and Natural Selection. Princeton, N.J.: Princeton University Press. Wilson, E. O. (1978). On Human Nature. Cambridge, Mass.: Harvard University Press. Wilson, E. O. (1975). Sociobiology. Cambridge, Mass.: Harvard University Press. Wilson, E. O. (1998). Consilience. The Unity of Knowledge. London: Vintage. Wilson, J. Q. (1993). The Moral Sense. New York: The Free Press.

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Yong, E. (2010). Dangerous DNA: The Truth about the 'Warrior Gene'. New Scientist, (2755).

Aggression/Violence in Humans and Other Primates – A Historian’s Prelude HANS-HENNING KORTÜM

I. Definition Problems There is a general consent that aggression is one of the most important factors in the history of mankind. In contrast to biologists and psychologists, scientists from the field of Humanities, like social and cultural scientists, prefer to speak of violence instead of aggression. But defining and explaining “violence” seems a hopeless undertaking. There is no consent about an exact definition but a — so to speak — violent dispute what violence really means (Imbusch 2002). Violence is so multifaceted, and each discipline seems to have a definition of its own. Therefore, at this point I will renounce to present a definition which could not be satisfying at all. Instead, I will use the expressions “aggression” and “violence” indistinctively.

II. Popular Attractiveness of Aggression/Violence Despite of its indefiniteness violence/aggression has never lost its attractiveness, neither in public nor among scientists. War as a form of organized collective violence has been and still is one of the main, even the main issue in literature and in historiography. In regard to the dramatic, frequent lethal impact warfare has on human life, this fact cannot surprise anybody: “War has been with us ever since the dawn of civilization. Nothing has been more constant in history than war” (Aumann 2005). This is why it seems, at first glance, a little bit surprising, when last year (2011) a book was published which tries to proof that compared to the past, a decrease of violence has taken place in modern times. The book ends with a very optimistic outlook into a future with no or only little violence. It was one of the great successes on the book market, not only in the United States but also in Europe: Steven Pinker’s The Better Angels of our Nature. The Decline of Violence in History and its Causes (Pinker 2011a, Pinker 2011b). The message of this voluminous study with over 700 pages makes clear that contrary to the widespread opinion that violence has increased especially in the 20th century, which

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was labelled “the Age of Extremes” (Hobsbawm 1994) by the renowned British historian Eric Hobsbawm, and also contrary to widespread scepticism concerning future global conflicts and even wars in the 21st century (Lundestad & Njølstad 2003), Pinker believes in what he names “the forces of modernity – reason, science, humanism, individual rights” (Pinker 2011a: 694). In his opinion these forces are responsible for “the decrease of violence”. It may be legitimate to explain the book’s success written by a psychologist also with a psychological motive: after decades and decades of intensive research and storytelling of violence in history, the wider public is very pleased to hear the good message that after all the evil in history, mankind will soon have a nearby future without any or with only little violence. As the renowned German sociologist Wolfgang Sofsky has rightly remarked, the firm “belief to live in an era of moral and ethical progress is an expression of historical presumption and blindness belonging to the mythology of modern civilization” (Sofsky 1996: 223) “The decline of violence”, so evident for Pinker, could in his opinion be recorded even and expressively in the disastrous 20th century – which is really a very surprising statement in regard to World War I and II and in regard to the numberless sanguinary conflicts afterwards. It is at the same time a rather misleading interpretation of this extreme century, even if one likes to accept the caveat of the German historian Dieter Langewiesche, who is advising against an overestimation of total numbers and is pleading for its relativisation in regard to the population (Langewiesche 2006). Following a conservative estimation, the 20th century caused the death of about 230 million people in consequence of wars and conflicts (Leitenberg 2006: 14). Nevertheless, the alleged decline of violence is described as the result of a long historical trend already starting after the Dark Ages of prehistory, ancient, medieval and early modern times, which were full of violence, if one accepts Pinker’s historical interpretations. Special aspects of his study, however, shall not be criticized at this point as the reviews written by scientists have only been launched recently (e.g. Gray 2011, Ziemann 2012). It seems much more important to stress some of the underlying historical and biological assumptions of this book. Concerning the development of human history, Pinker’s view is a very optimistic one. As a critic has rightly observed in his internet review, the author seems comparable with Voltaire’s naive Pangloss (Proyect 2011). In his famous Enlightenment satire the Frenchman ridicules persons like Candide and his ever optimistic teacher Pangloss, who were convinced that they were actually living in the best of all possible worlds, i.e. in the wonderful enlightened 18th century. This optimistic evolutionary understanding of human history, which allegedly tends always towards a world of lesser violence, is the key issue of Pinker’s arguing. He is relying on a theory which was already developed in the 30s of the last century by sociologist Norbert Elias and is normally known as the “civilizing process” (‘Zivilisationstheorie’) (Elias 1939, 1969/1982). Today this theory is called into question since the historical assumptions made by Elias concerning

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the medieval times were full of stereotypes, which were considered to be old fashioned already in his own time and which, therefore, did not describe historical realities (Schwerhoff 1998, Ziemann 2012). Other fundamental aspects of Elias’ anthropological assumptions were also heavily criticized, e.g. that the feeling of shame was be a modern development, and was thus totally unknown in pre-modern times (Duerr 1998). Nevertheless, Elias’ theory has gained acceptance among scientists until today (Eisner 2001, 2003). In his article “What causes Large-Scale Variation in Homicide-Rates?” Manuel Eisner is expressively using arguments to explain the decline of homicide rates, which we can already find in Elias’s works. Undisputedly, the rise of the state’s monopoly of violence has reduced homicide rates drastically. No one would contradict the thesis that the risk to be killed by a gangster has dramatically decreased in modern societies. Consequently, violence used by private persons was discriminated as illegal. But monopolizing the licence to kill could have also disastrous consequences for those parts of society which are officially, i.e. by the state, stigmatized, discriminated and excluded from social and political participation. Especially the history of the 20th century is full of lethal state violence against persons and groups. It was officially declared that they were not parts of the society, but intruders and enemies not belonging to the core of the nation and therefore had to be annihilated. Violence organized by state institutions could be even more effective and therefore even more dangerous compared to private violence, as modern bureaucratic states have all the necessary means at hand to kill in an extremely effective way (Förster 2009: 75). What we could really learn from the history of violence is the fact that there is a need to distinguish carefully between the different species of violence: Who uses which sort of violence against whom and why is he doing so? (Imhof 2002: 34-37). We should always remember that “under any circumstances, violence is the result of a social relation, mainly between the perpetrator and the victim, but often including bystanders as a third party. It is possible and necessary to differentiate even further, and to distinguish between short-lived violent encounters, the specification of roles in organized violence such as in the military or organized crime, and the general level of violence in a given society. Yet in any of these configurations, violence is a ‘fait social’ in the original meaning of the term ...” (Ziemann 2012).

With regard to this, we should distinguish very carefully between “homicide” as a moralistic and legal category of today’s jurists and criminalists and “homicide” as a mere indicator of death rates or body counts of combatants and noncombatants in times of war. Still another question has to be answered in this context: Could one even be so far going as Eisner is in his article pleading “that there is a continuum from the more individual ‘traditional’ types of pathological murder to the organized elimination of whole population groups”? Detailed studies of crime in the Third Reich have shown that there was quite a normal, and in no sense an ab-

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normal or even pathological, background to persons who routinely killed the people they had been ordered to kill. They were all ordinary people like me and you, husbands with wives and children, ordinary policemen, like the men of the Hamburg Policebataillion 101, deployed to South-East Poland in summer 1942. In July 1942 they were ordered to kill all the Jewish women, babies, little children and old people at that time concentrated in the little village of Józefów. Before the mass execution started in the morning of July 13th, the battalion commander, Major Trapp, told his men explicitly to signalize if anybody could not cope with this emotional pressure, which he for his part was not able to stand to. But only few of the ca. 500 policemen took the option of refusing to take part in the execution of ca. 1500 Jews in the nearby forest. The behaviour of the policemen seems explainable by the fatal influence of authority, conformity pressure and indoctrination, we are normally inclined to follow (Browning 1991). Later, in the 60s of the last century, the fatal influence of authority conformity pressure and indoctrination could exercise on humans was also proven experimentally under laboratory conditions. The so called Milgram experiment successfully induced “ordinary people” to use force, i.e. electric shocks against other people allegedly following an authority’s request, in this case a scientist’s, pretending the experiment would have a scientific goal (Browning 1991: 58-64). To give just a last example: Ordinary male Americans were deployed as soldiers in the Vietnam War. They committed war crimes against Vietnamese civilians to a great extent. They were killing noncombatants, babies, children, women and old people mercilessly. Nevertheless, they were faithful husbands and caring fathers in their civilian life in the USA (Greiner 2007).

III. Biological Understanding of Aggression Following Pinker’s thesis it seems evident that not only the above mentioned ‘forces of modernity’ are responsible for the decline of violence. Another influence, which the author calls “the inner demons” and “the better angels” of mankind in the last two chapters of his book, seems to be responsible. Here Pinker is mentioning genetical and neurobiological aspects of the use of violence: Which are, so to speak, the good factors in our brain restraining the use of violence, and which are the bad ones furthering the practice of violence? It seems that we have arrived at a crucial point right now: What could the humanities traditionally concerned with aggressive and often lethal violence of humans in the past and in the present really learn from biological research on aggression? First of all, it seems necessary to inform the reader about the actual state of biological research concerning aggression among animals. This seems important, because there are still many ideologically influenced myths about aggression among animals in the wider public. The overviews given by Hammerstein and Zinner show very distinctly that long established romantic con-

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ceptions of the peaceful and harmonious living together among conspecifics are now obsolete. What strikes historians and social scientists perhaps most is the specific kind of expert language used by biologists describing aggressions among animals. For instance, natural scientists speak of raids, battles and even wars, of mutilating and killing, of weapons and armaments (Heinze 2003a, 2003b, 2010). This very metaphorical language used by biologists marks the similarities which are evident between human and non-human aggression/violence. But could we learn anything from these similarities? Are these only similarities on a more or less onomasiological or phenomenological level, or do these similarities go even deeper? Is human violence really comparable to non-human aggression? The answer is quite straightforward: Yes, in some respects it is, which I am going to discuss in the following paragraph.

IV. Similarities between Non-Human Aggression and Human Violence First of all, aggression and violence seem to be comparable under the aspect of sex. As John Archer shows in his article, a strong relationship between males and aggression exists: males prefer more direct, i.e. physical, often very risky forms of aggression against one another, especially when they are young – a behaviour which could be explained by the process of sexual selection. Even today there is a strong tendency among modern military bureaucracies that female soldiers should not be deployed in the immediate frontline, but first of all in the army medical corps. A hard-core military historian has even expressively denied the ability of female soldiers to fight effectively, even in such a modern army like the Israelian (van Creveld 2001). But times are changing. Nowadays women are even practising martial arts, like boxing and wrestling, and modern military bureaucracies like the German Bundeswehr are introducing new patterns in searching for female pilots: On posters attractive young ladies are handling the gearing sticks of fighter jets. How should we understand this development? Is it “only” a successful overcoming of old fashioned, culturally inherited gender roles forbidding active male warrior behaviour for women? Or is it a behaviour “against nature” with its sexual selection process, if women are trained to fight aggressively? - In his article the psychologist Archer is warning us to underestimate biological factors. But what does this mean? Are males really the ideal warriors they had been in former times and are still today in existing small-scale societies in South America? The anthropologist E. A. Smith judges “proven ability in lethal fighting” as a signal which would enhance the mating chances of a warrior becoming more attractive for women and at the same time deterring other possible male rivals (Smith 2003: 415). If this statement is true, it could help us understanding why still today (some) women are favouring physical strong males and assessing men in uni-

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form as very attractive ones. Do they prefer more or less unconsciously such males and is such a behavior genetically deeply backed up? The second point seems to be of even greater importance: the rationality of aggression. Biologists are using game theoretical models to explain a certain concrete behaviour in a conflict. This model originally developed as an instrument of analyzing economic behaviour was introduced into biology mainly by John Maynard Smith since the Seventies of the last century. To explain it with the help of an “over simplified model of animal contests” (Maynard Smith 1985: 77): often animals had the option to choose between alternative ‘strategies’: the ‘aggressive’ falk strategy and the ‘peaceful’ dove strategy (mixed strategies). Which strategy will be chosen depends on calculation: “Assessment of each other’s ‘resource holding potential’ ... defines of each contestant a critical probability of winning, above which fighting or escalation is the favoured strategy, and below which withdrawal, flight or submission is the better, more economic alternative. Escalation of a fight ensues when both contestants assess their probability of winning as positive.”

as Dietmar Zinner and Brandon Wheeler are showing in their article. Conflicting animals gain benefits by winning their fight and have costs (most dramatically: will be killed) if losing it. The benefits and costs will influence the population. It will come to an increase or to a decrease of fitness for the species of animals involved in such conflicts. An equilibrium will be reached “when the average pay-offs for a hawk are equal to the average pay-offs for a dove “(Zinner-Wheeler). This situation is given when the animals are not playing always the same strategy but are changing their strategy. The animals are playing the dove strategy with a certain probability and playing the falk strategy with a certain probability in regard to its assumed pay-offs. Some of the facts stated by biologists concerning aggression among animals could also be applied by social scientists and historians in regard to human conflicts like warfare, where the responsible people could be imagined as “hawks” and “doves”. Organized violence in its most effective way, i.e. wars, happened in history when the commanders-in-chief were convinced that they could win their wars by playing the hawk strategy. But wars were lost when commanders-in-chief dramatically underestimated the resource holding potential of their enemies, and therefore it would have been better not to choose the aggressive “hawk” strategy, but to choose the “dove” strategy and to fly away when confronted with a superior “hawk”: Often wars did not break out because each party assessed the enemy’s striking force as so strong that no real chance of winning the war was left. This is exactly what could be called “strategy”, namely “the theory of interdependent action” (Schelling 1960: 16). “Strategy” in this sense should not be confused with “strategy” in the military sense meaning a specific behaviour in wartimes. “Strategy” in a game theoretical approach means that the opponents, i.e. the players, are choosing their strategies in regard to each other: “... the best course of action for each participant

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depends on what he expects the others to do” (Schelling 1960: 9-10) what can even have tragic consequences as we shall see later on. A colleague of Thomas Schelling, like him a Nobel laureate in economics in 2005, the mathematician Robert J. Aumann also declared expressively that it would be a great fault to understand war as something irrational (Aumann 2005). It seems to be an historical truth that “no wars take place by accident ... Government leaders, and insurgents as well, make war by calculation” (Leitenberg 2006: 65). Also the rational analysis of aggressions and conflicts in a pure economic sense, by calculating possible costs and possible benefits of aggressive behaviour, helps a lot to understand why human aggressions and wars happen. Even in the Dark Ages of medieval Europe wars were only waged on the condition that there would be a good return of investment for the warlords and their warriors (Kortüm 2010): For instance, as long as an active military resistance was not organized between the end of the 8th century and the beginning 11th century, the Vikings invaded the seashores of Central, Western and Southern Europe year after year. Deliberately, they chose a hawk strategy and killed many helpless monks, raped nuns and killed ‘civilians’. But they chose the dove strategy and came to arrangements with their enemies, and even settled down peacefully in their enemies’ country whenever they realized that they were confronted with an adequate adversary, this means when the probability to win was low. To give just two other examples: The Norman Duke, William the Conqueror, was rewarded with the English throne when he shipped over the Channel in 1066, and his followers were given manors and magnificent estates. It would be difficult to calculate in detail the great investments (shipbuilding, payment for the warriors, their expensive weapons and their extremely expensive horses) made by the Norman, duke William, and his allies over the years, but they were comparatively low regarding the return on investment. – And the Third Reich began its aggressive wars only after a period of intensive rearmament which was started after 1933, when the main part of the leading elites and the Führer himself were convinced that there was a good chance to win the wars against their European neighbours. These wars were also the reaction of an economical desperate situation of the Third Reich after its rearmament program. The leading elites were dreaming that the lack of resources could be counterbalanced by winning new ones in the conquered territories (Förster 2007: 84). Stressing the rational causes of aggression and violence could further the analysis and helps a lot to counter the extensively used mystification and moralization, one can find in Pinker’s book with its tendency to distinguish between ‘good angels’ and ‘bad demons’ inside men. The game theory even permits to understand more and also complex situations. For example, we have to differentiate between so called one-shot-games and repeated games. The behaviour of players, i.e. the opposing parties, could be heavily influenced by calculating the probability that the play could be repeated after a time. Therefore, it could be rational to weaken one’s enemy deci-

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sively. This option was made use of by the Allies after World War I imposing heavy disarmament conditions on Germany in the Versailles Treaty of 1919 to prevent the Germans from waging war in the future. Another even more radical and more effective option could be to destroy one’s enemy’s ability to act aggressively completely. This option was suggested by the American secretary of Treasure Henry Morgenthau in 1944 proposing to divide Germany into two States and destroying its heavy industry. Contrary to that and more positively, game theory helps also us to understand, why cooperation instead of aggression/violence could be possible: “Each individual keeps a cooperative behaviour, because he anticipates today that the others will stop cooperating with him tomorrow if he defects today” as Andreozzi says in his article. This could explain, why sometimes enemies are treating each other more or less in an humanitarian way. Even the fighting could come to an end because today’s winner could become tomorrow’s loser. Therefore, it would be better not to fight at all. This is exactly what happened in the First World War at the Western Front, when after a period of heavy fighting the advance of the German armies was stopped by French and British troops latest at the end of 1914. Now the character of war changed totally: trench warfare, interrupted by some offensives in every year, dominated almost until the end of war in 1918. Sometimes the enemies, French and British infantry on one side, German infantry on the other side, were separated only by a short distance of some hundred yards or so from each other. This circumstance facilitated and supported a sort of cooperation between the enemies. Due to fruitless repeated small offensives they soon learned that an attack of their own troops would be quickly answered by the enemy by a counter-attack on the next day, and conquered trenches would be lost again immediately. This led to the so called live-and-let-live system, which was frequent in the direct front lines at the Western Front and made every day’s life a little bit easier for the soldiers (Axelrod 1984). Rationality is in no way restricted to modern warfare. This impression could arise, when looking at the historical examples of World War I, which was just mentioned. But rational behavior dominates even the actors in tribal warfare (Jürg Helbling 2011). Interestingly enough, we can observe the same strategy of conflict in tribal conflicts as in the times of Cold War. As Thomas Schelling (1960) has convincingly argued: The strategy of deterrence will function at its best, if the threat to use force is believable for the threatened. Then he will deter from any aggression. The plausibility of a threat could be achieved, if the person who threatens commits himself to fulfilling the threat, i.e. to use violence. This led to the Cold War Situation: no hot war broke out in Europe. But it was a great problem during the Cuba Crisis 1962, when the Soviet Union leader Nikita Khruschev did not believe in the reputation of the American President John F. Kennedy being a tough guy. Kennedy had accepted the building of the Berlin Walls in 1961. Therefore, he wrongly thought that the soft guy Kennedy would also accept the installation of Soviet missiles on Cuba.

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Having a ‘tough guy’ image is also used by tribes as a strategy to deter potential enemies from attack. The aggressors should realise that the attacked tribe will “retaliate immediately and at any costs”, because the reputation of the attacked tribe as a ‘tough guy’ is at stake (Helbling 2011: 162). Not reacting to an attack by a counter-attack would result in the attacked tribe losing its face, i.e. its reputation. A third similarity in regard to conflict and warfare among animals and humans concerns the aspect of ritualization. Ritualized behaviour among conspecifics was frequently observed by biologists and was explained as a strategy to minimize or even to avoid the costs aggression is normally linked to. The rituals displayed by the opponents help them to assess exactly each others’ resource holding power and in the best case to prevent them from starting a risky fight (Zinner & Wheeler). Historians have noticed ritualized aspects of human warfare, too. Manoeuvres and military reviews have the function to impress potential enemies and to act as a deterrent. The practice of keeping prisoners of war alive and treating them more or less correctly, is also a widespread ritual, which is often described by many military historians (Afflerbach & Strachan 2012). Not killing one’s adversary is routinely practised by such soldiers who know each other’s behaviour and are therefore in a biological sense ‘conspecifics’. Soldiers would fight until their last breathe take if they did not have the chance to surrender with the prospect not to be killed, but to be treated honestly by the winner: “Fearing to be killed if being captured, German and Japanese soldiers usually fought with extreme determination”, as Andreozzi supposes in his contribution. So the ritualization of surrender reduces the cost of fighting for the winner, and the defeated party earns its profit being offered the possibility to survive (Kortüm 2012). The ritualization of surrender furthers cooperation: Each side taking its prisoners could be sure that the rules of combat will be observed. Contrary, ritualization of warfare does not take place when the opposing parties are not ‘conspecifics’, or to express it in the terminology of Military historians, when the enemies are living in different cultures and are not sharing common standards of behaviour. In the result this is inter-cultural warfare characterized by extremely high rates of cruelty and disorder (Kortüm 2006). For instance, the Vietnam War (1964-1973) was characterized by such a deficiency of routines in an enormous extent (Greiner 2007). Cooperation in the sense of limiting cruelty among enemies is also very difficult, when so-called second order believes are erroneous and therefore misleading. Instead of cooperation it will lead to reciprocal punishment. Andreozzi is giving a convincing example in regard to warfare: opponent A thinks that his opponent B will kill his prisoners of war. At the same time opponent B is sure, that A will kill his prisoners of war (First order beliefs). A thinks also that his opponent B believes that he (A) kills his prisoners and B thinks that his opponent A believes that he (B) kills his prisoners (second order beliefs). The result is negative reciprocity: both sides will never stop the killing

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of their prisoners of war, since each of the opponents has erroneous beliefs over the thinking of one another.  

V. Explaining the Reasons for Aggression/Violence After discussing some striking similarities between human and non human behaviour in regard to aggression/violence, the crucial question why such behaviour does happen remains to be answered. Biologists are used to ask for reasons in a much elaborated way why acts of aggression do happen among animals. Since Tinbergen (1963) they normally distinguish between proximate and ultimate explanations of aggression, just as Claire El Mouden explains in her article. Ultimate explanations given by biologists want to understand the deeper reasons for aggression regarding the problem of natural and sexual selection. And since the times of Charles Darwin, the so-called fitness plays an prominent role. Biologists understand selection as a long term process by stressing the important role of genetic factors. Oversimplifying matters, the species with the best reproduction potential, i.e. with the best fitness, will come out on top. It seems evident that such biological explanations cannot be transferred into a serious scientific approach to explain human violence and its role in history. It would reduce human history to a “natural” process with the survival of the ‘fittest’ society, nation or state. It seems not consistent with all we know about human history. This does not mean to exclude that – what the ethnologist Jürg Helbling describes as a survival and adaptation process of local groups in a “given social environment” (Helbling 2011: 161) – such “adaptations” do happen in history, but which in no way should be regarded as “natural”, “biological” “selection processes”. It must be emphasized that now many biologists are regarding the socalled antinomy between nature and culture as obsolete and senseless (see Heinze in his article). Instead of it the actual situation is characterized by an intensified cooperation between biologists and anthropologists and ethnologists who are exploring small-scale societies in South America and other regions. The scientists are looking how different cultural practises in such tribal societies came into being and which role thereby nature plays. It seems that regarding culture only as a variable would in no way reflect the strong relationship and interdependency between culture and nature adequately (Hammerstein 2003). Following Claire El Mouden in her article the question whether “it is human mind that drive the change, not natural selection” hasn’t yet pinned down. But what seems clear in the eyes of an historian: the course of history and the success of single societies were and are not dependent on high reproduction rates. Now, emerging societies like the Chinese are favouring low birth rates as they fear the fatal economic and political consequences of an ever increasing

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population. Not without reasons since the end of the 19th century, however, such a biological interpretation of human history has found its strong supporters among intellectuals, scientists, politicians with fatal consequences in regard to the widespread genocides in the disastrous 20th century. This period began with the genocide among the Hereros committed between 1904 and 1908 by German troops. Its commander General Lothar von Trotha was expressively speaking of a “race war”. The century ended with genocides in Somalia, Rwanda and Former Yugoslavia. But there can be no doubt about the fact that the climax of genocide was reached by the Holocaust committed by Germans during the era of the Third Reich (Förster 2009: 83-85, Johnson 2011). Biologically influenced thinking can even be found in today’s lamenting over the comparatively low birth rates and its suspected fatal consequences for modern societies (“Die Deutschen sterben aus!”). But limited off-spring rates could also be positively judged as a sign for a rational adaptation to the limiting factors of a given environment. A greater number of offspring could not be so well fed by its parents compared with a lesser number of offspring. The chance for surviving of all or of most young birds will increase if the genetic possible maximum of offspring is not reached (Hammerstein 1991). Serious historians would not accept the idea that aggression/violence has happened in history so often out of “ultimate causes”. In other words: They cannot see any deeper sense in violence/aggression. Only supporters of a teleological view of the world are able to see such a sense and thereby to explain and to legitimize violence in history. This does not mean that historians will ever stop to ask for the concrete reasons of aggression/violence in a specific historical situation, which biologists on their part would call the “proximate causes” of aggression. At this point, biologists and historians seem to be very close together. It seems a banality, which nevertheless should never be forgotten: The conflict over limited resources is fundamental, among humans and non human animals. This gives mankind no hope at all that aggression/violence will stop one day in regard to limited resources. Humans and other primates have in common that no conflicts and no war take place “by accident”, but “strategically”, which means: “by calculation”. The difference between humans and other primates is that only the further have moral sentiments and they are to be held responsible for their deeds.

References Afflerbach, H. & Strachan, H. (Eds.) (2012). How Fighting ends: A History of Surrender. Oxford: Oxford University Press. Aumann, R. (2005). War and Peace. Prize Lecture. December 8, 2005. www.nobelprize. org/nobel_prizes/economics/laurates/2005/aumann-lecture (Date accessed: 30 September, 2012). Axelrod, R. (1984). The Evolution of Cooperation. New York: Basic Books.

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Browning, C. (1992). Ordinary Men: Reserve Police Batallion 101 and the Final Solution in Poland. New York: Harper Collins. Duerr, H.-P. (1988, 1990, 1993, 1997, 2002). Der Mythos vom Zivilisationsprozess, 5 vol. Frankfurt a. M.: Suhrkamp. Eisner, M. (2001). Modernization, Self-Control and Lethal Violence. The Long-Term Dynamics of European Homicide Rates in Theoretical Perspective. British Journal of Criminology, 41, 618-638. Eisner, M. (2003). Long-Historical Trends in Violent Crime. Crime and Justice; A Review of Research, 30, 83-142. Elias, N. (1939). Über den Prozess der Zivilisation, 2 vol. Basel: Verlag Haus zum Falken. Elias, N. (1969/1982). The Civilizing Process. Vol. I. The History of Manners. Oxford: Blackwell. The Civilizing Process. Vol. II. State Formation and Civilization. Oxford: Blackwell. Förster, S. (2009). Krieg und Genozid: Überlegungen zum Problem extremer Gewalt in universalhistorischer Perspektive. Mittelweg 36, 18, 71-87. Gray, J. (2012). Delusions of Peace. www.prospectmagazine.co.uk./magazine (Date accessed: 19 September 2012). Greiner, B. (2007). Krieg ohne Fronten. Die USA in Vietnam. Hamburg: Hamburger Edition. Heinze, J. (2003a). Live and let die: Why Fighter Males of the Ant Cardiocondyla kill Each Other but tolerate Their winged Rivals. Behavioral Ecology, 14 (1), 54-62. Heinze, J. (2003b). Arms Races between Social Parasits and Their Hosts: Geographic Patterns of Manipulation and Resistance. Behavioral Ecology, 14 (1), 80-88. Heinze, J. (2010). The Police are not the Army: Context-Dependent Aggressiveness in a Clonal Ant. Biology Letters, 6 (2010), 329-332. Helbling, J. (2011). The Tactical Use of Cruelty in Tribal Warfare. In: T. Trotha (Ed.), On Cruelty (pp. 149-173). Cologne: Rüdiger Köppe Verlag. Hobsbawm, E. (1993). The Age of Extremes. The Short Twentieth Century, 1914-1991. London: Michael Joseph. Hobsbawm, E. (2002). War and Peace in the 20th Century and beyond. In: G. Lundestad & O. Njölstad (Eds.), Proceedings of the Nobel Centennial Symposium (pp. 25-40). Singapore: World Scientific Publishing. Imbusch, P. (2002). Der Gewaltbegriff. In: W. Heitmeyer & H. Hagan (Eds.), Internationales Handbuch der Gewaltforschung (pp. 26-57). Wiesbaden: Westdeutscher Verlag. Johnson, E. A. (2011). Criminal Justice, Coercion and Consent in ‘Totalitarian’ Society: The Case of National Socialist Germany. British Journal of Criminology, 51, 599-615. Kortüm, H.-H. (Ed.) (2006). Transcultural Wars from the Middle Ages to the 21th Century. Berlin: Akademie Verlag. Kortüm, H.-H. (2010). Kriege und Krieger 500 - 1500. Stuttgart: Kohlhammer. Kortüm, H.-H. (2012). Introduction: Surrender in Medieval Times. In: H. Afflerbach & H. Strachan (Eds.), How Fighting ends. A History of Surrender (pp. 41-54). Oxford: Oxford University Press. Langewiesche, D. (2006). Eskalierte die Kriegsgewalt im Laufe der Geschichte? In: J. Baberowski (Ed.), Krieg, Revolution und Gewalt im 20. Jahrhundert (pp. 12-36). Göttingen: Vandenhoeck & Ruprecht. Leitenberg, M. (2006). Deaths in Wars and Conflicts in the 20th Century. Cornell University Peace Studies Program. Occasional Paper 29, 3rd ed. Ithaca, NY: Cornell University Press.

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Lundestad, G. and Njøstad, O. (Eds.) (2002). War and Peace in the 20th Century and beyond. Proceedings of the Nobel Centennial Symposium. Singapore: Word Scientific Publishing. Pinker, S. (2011a). The Better Angels of our Nature. The Decline of Violence in History and Its Causes. London: Viking. Pinker, S. (2011b). Gewalt. Eine neue Geschichte der Menschheit. Frankfurt a.M.: S. Fischer Verlag. Proyect, L. (2011). Steven Pinker = Hobbes + Pangloss. www.Louisproyect.wordpress.com (Date accessed: 4 October, 2011). Schelling, T. (1960). The Strategy of Conflict. Cambridge, Massachusetts: Harvard University Press. Schwerhoff, G. (1998). Zivilisationsprozess und Geschichtswissenschaft. Norbert Elias’ Forschungsparadigma in historischer Sicht. Historische Zeitschrift, 266, 561-605. Smith, E. A. (2003). Human Cooperation: Perspectives from Behavioral Ecology. In: P. Hammerstein (Ed.), Genetic and Cultural Evolution of Cooperation (pp. 401-427). Cambridge, Massachusetts/London: The MIT Press. Sofsky, W. (1996). Traktat über die Gewalt. Frankfurt a.M.: S. Fischer Verlag. Tinbergen, N. (1963). On the Aims and Methods of Ethology. Zeitschrift für Tierpsychologie, 20, 410-433. van Creveld, M. (2001). Men, Women and Warfare. Do Women belong in the Front Line? London: Cassel and Co. Ziemann, B. (2012). Review of Histories of Violence (review no. 1232). www.history.ac.uk/ review 1232 (Date accessed: 18 September, 2012).

What Theoretical Biology has to say on Aggression in Humans and Animals PETER HAMMERSTEIN

I. Introduction: Does Biology matter at all? Only a few hundred years ago, the homicide rates in Europe (Eisner, 2003, 2009, 2011) were outrageously high compared to now, travelers routinely fell victim to robbery, and cities needed to protect themselves through walls against violent invaders. Today, the few remaining city walls serve as tourist attractions, and European life has become so safe that we no longer wish to carry a sword for self-defense. Obviously, this dramatic decline in everydayviolence cannot be explained through genetic changes in our biological makeup and is best interpreted as an achievement of “civilization” with its legal, educational, and other institutions. Why, then, bother about biology? Any reflection on the usefulness of human institutions must include a serious attempt to understand how these institutions generate culturally desired and undesired effects. This requires a realistic picture of “human nature”, one that captures our biological predisposition to engage in cooperative social relationships as well as our biological potential for aggression and violence. So the question is not whether nature has made us “good” or “bad”, but how nature has shaped the mental mechanisms that induce cooperative or antagonistic behavior depending on circumstances. This is how biology matters in the study of aggression. Behavioral biologists have conducted numerous empirical studies to explore how environmental conditions can favor or disfavor animal violence. Many results of this research are particularly well understood in the light of game theory (Maynard Smith 1982, Hammerstein & Riechert 1988, Hammerstein & Selten 1994). Looking through the eyes of this theory, the pattern of how animal behavior systematically varies with circumstances appears like a “strategy”. Equipped with the appropriate strategy, animals can respond adaptively to the different situations in which they may find themselves. In some situations violent action would augment the actor’s expected lifetime reproductive success. From an evolutionary biologist’s point of view, these are the situations likely to trigger violent actions in animals. The underlying assumption is here that animals have acquired their “strategic competence” through natural selection.

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In this idealized picture of the animal world, it is the evolutionary process that chooses “smart” strategies, while the animal only executes its strategy but does not choose it. Obviously, such a picture can be misleading. A chess player, for example, makes her own strategic choice based on reasoning, and evolution could not specifically prepare her for this game because our Stone Age ancestors never played it. Human behavior in general seems to be far less “preprogrammed” than that of animals. Some caution is, therefore, needed when applying the lessons from evolutionary biology to human affairs. Rather than being “directly” shaped by evolution, human behavior is so strongly influenced by social learning that no biologist can seriously deny the important role culture plays in forming our habits. We know from empirical studies, however, that learning as such is biologically preprogrammed. Animal species differ strongly in what, when and how they learn, what cues they rely on, how they generalize, and what they try to imitate (see Hammerstein & Boyd 2012 for a review). They are all prepared to learn efficiently those things that matter most under species-typical circumstances. A distinctive human feature is, for example, our innate preparedness for “enculturation”. Evolution has installed the learning machinery needed for enculturation and set strong biases and limitations to this machinery. It is due to these biases and limitations that our biology cannot completely be overridden by cultural imperatives. If this were the case it would perhaps suffice to teach all children to be well-behaved and so they would. No legal institution for punishment would then be needed to keep violence and other forms of crime in check. Last but not least, the emotions with their undeniable evolutionary history make it particularly interesting to look at human aggression through the eyes of a biologist. In order to understand the phenomenon of road rage, for example, we would have no chance to explain this phenomenon in terms of coldblooded reasoning. It is the emotion anger that prompts car drivers to lash out at others or even shoot them. In the U.S. more than a thousand such events are reported every year (Mizell 1997). “Blinded by anger” the drivers perform costly acts of retaliation that are preposterous to common sense. Fortunately, the influence of anger on our behavior is constrained by another mental mechanism, the emotion fear. So biology matters once again, but let us not throw out the baby with the bathwater. Aggressive acts are not always influenced by strong emotions and can sometimes best be understood as resulting from “cold-blooded reasoning”. A criminal may, for example, decide in a sober state of mind to kill the witnesses of a crime in order to protect himself from prosecution. But even this criminal’s cold-blooded reasoning could be understood as a cognitive process that is instrumentalized by the biological “whispering within” that tells him to do all he can to save his neck.

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II. The Meandering of Biological Thought on Aggression Compared to now, ethologists in the early 1960s had little quantitative information about animal fighting in natural environments. This made it easy for “cock-and-bull stories” to proliferate within the profession. One such story claims that animals would not intentionally kill members of the same species, and that their fighting would resemble a “fair contest” where opponents avoid inflicting injury on each other. With this story in mind, any lethal combat observed in cages had to be interpreted as an artifact of captivity. Lorenz (1963) was the most prominent representative of classical ethology who proclaimed an innate inhibition for animals to kill members of the same species. He had to admit, of course, that for Homo sapiens this inhibition fails to work properly. In Lorenz’s view humans kill each other because their innate adaptations got eroded in recent evolutionary times through a hypothetical process of “selfdomestication”. This line of reasoning is utterly misleading because an inhibition that does not exist cannot erode. As we know today from field studies on chimpanzees, our closest animal relatives are certainly not inhibited to kill members of their own species. Groups of chimpanzees have often been observed to conduct lethal raids on neighboring communities (Wilson et al. 2004). Their cruel actions have an “evolutionary logic” in that they provide measurable benefits in the competition for space and mates (Mitani et al. 2010). Chimpanzees are not the only animals known for violence. It seems to be a rather general feature of the animal world that individuals in possession of weapons will use them in situations where this enhances their success in the evolutionary sense. The term success here refers to an individual’s biological contribution to future generations. This contribution is typically measured as the expected number of offspring that survive to adulthood – a quantity denoted as fitness by evolutionary biologists. There are more subtle ways of defining fitness, but the formulation given here captures the core idea behind this concept, which plays a central role in the theorizing of evolutionary biologists. The most important breakthrough in the understanding of animal aggression was indeed achieved by systematically exploring under natural conditions what the fitness consequences of aggression and violence really are. It is now textbook wisdom, for example, that in lions and several primate species, males systematically kill the existing young of a new female partner. When performing such a cruel act, male killers destroy infants sired by other males and thus do not harm their own fitness. Moreover, the infanticide can increase the fitness of these killers because female partners will no longer reserve resources for infants after these infants have been killed. Please note that this argument does not simply carry over into the human world. It only happens on very rare occasions that stepfathers kill stepchildren. Unlike male lions, human killers would face a high risk of being ostracized or finding themselves in jail. Even more importantly, a human male who kills the child of a partner will often be rejected by this partner – a rejection uncommon in lions. As rarely as infanti-

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cide happens in the human world, however, it is committed far more frequently by stepfathers than by genetic fathers (Daly & Wilson 1988). Both the rarity and structure of human infanticide are well in line with biological theory. As we have seen so far, classical ethology was barking up the wrong tree with its ideas on the evolution of violence. I am tempted to add that when it rains it pours. There are similar problems with classical accounts of the mental mechanisms behind aggressive behavior. Without mentioning it explicitly, Lorenz (1963) had the “psychohydraulic model” of an aggressive drive in mind when he wrote his influential book on the theme of this chapter. Lorenz himself compared that drive model to an old-fashioned water closet, where water dribbles into a tank and builds up a “reaction specific energy”. Environmental stimuli then release the flush, causing a discharge of this energy through aggressive action. The more water in the tank, the less of a stimulus it takes to release the flush. In case of a very high water level, no stimulus is needed for the flush to come into effect. Aggression then occurs spontaneously. Lorenz thus postulated an endogenous urge to act aggressively, and he firmly believed that this urge automatically builds up over time. If this were the case, the author of the present chapter would perhaps have to attack an arbitrary person for no reason because he has not acted aggressively for a long time. It is this “for no reason” that shows the absurdity of Lorenz’s theory. Our contemporary picture of animal behavior is one in which aggression occurs because it would under natural circumstances – at least on average – increase the aggressor’s fitness. We thus expect the mental machinery to serve as a mediator translating this ultimate function of aggression into pragmatic decisions that can be made in real time and with limited knowledge. Not surprisingly, Lorenz’s drive model of aggression has never found substantial empirical support. It must be said, though, that he was one of the first researchers who made major efforts to create a theoretical background for the field of ethology that was still very young at the time.

III. Fighting in Toads and the Power of Methodological Individualism A common misunderstanding of evolutionary biology is the idea that natural selection typically acts for the good of the species. This idea persisted for a long time in the main stream of last century’s zoological literature, and it strongly inspired classical ethologists who tried to explain behavior in terms of its benefit to the species. Contemporary biology, however, takes seriously the fact that selection can operate at multiple levels, ranging from genes to societies and populations. Under many if not most circumstances in nature, the power of selection is particularly strong at the level of the individual. This is, roughly speaking, why biologists now tend to look mainly “through the eyes of the individual” in order to understand behavioral traits and their evolution.  

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The power of this methodological individualism in biology becomes visible, for example, if we take a glance at mating behavior in the common toad (Bufo bufo). During each mating season a fraction of the female population appears at ponds in order to spawn. Males have already congregated around these ponds and are waiting for opportunities to mate. They outnumber the females by far and are in strong local competition with one another. Let us watch the scene at a given pond. What happens when a female toad arrives? The first male she encounters grasps her back and clings to it. Not having much of a choice, she continues her travel to the spawning site, transporting the male as a load. The female’s situation can aggravate when other males join in and a struggle between these males takes place on her back. Sometimes three to six males are found wrestling on the same back. This can have lethal consequences for the female as these wrestling males lead to a small but non-negligible risk of drowning (Davies & Halliday 1979). Whenever drowning of the female occurs, the males are destroying the very mate they are competing for. It would, therefore, seem farfetched to interpret the male toads’ wrestling style as something that serves the species. Why then are the fights literary carried out on the female’s back? In order to answer this question, we need to look at the mating scenario through the eyes of a focal individual male. Imagine this male to be confronted with a situation where a female arrives with a male partner. What if the focal male attempts wrestling on the female’s back, what if not? We have to compare the consequences these decisions would have on the focal male’s fitness. Behavior A: The focal male refrains from attacking the male on the female’s back. His fitness gain through this female is zero. Behavior B: The focal male engages in a struggle on the female’s back.

If she gets drowned in the struggle, his fitness gain through this female is zero again – no less! In addition, he may win, spawn with the female and end up with a positive fitness gain. Looked at it this way and assuming some heritability of A and B, natural selection would indeed favor the behavior B despite the fact that is so offputting to the human observer. There are, of course, a few hidden assumptions behind our simple calculations. For example, we have taken it for granted that males will never meet the same female again. But the chance for this to happen is very small indeed, and more realistic models can still explain the selective advantage of B over A. I wish to emphasize that methodological individualism is the crucial key to understanding the toads’ bizarre behavior. Why is this the case? If male toads really had been “taught by evolution” to care about females, populations, or their species, we would perhaps see them wrestle next to a female but certainly not on her back.

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IV. Confronting Historical Myths with the Sobering Facts on Aggression Historically, not only non-human animals but also humans themselves were, for a long time, believed to act for the good of the species. In literature, the term “noble savage” stands for this idealization of our “uncivilized ancestors” as being “innately good” and “not corrupted” by civilization. Glorification of the noble savage is one of the main themes in the work of Jean-Jacques Rousseau and, more generally, in the Romantic writings of the 18th and 19th century. It is certainly more appealing to believe in the myth of the noble savage than to accept the sobering facts about our human ancestors. Facts, however, strongly suggest the demise of the noble savage. It is now documented, for example, that massacres among humans have occurred for at least the last ten thousand years (the approximate time span for which modern forensic methods can properly be used). One of the more recent cruel events is the Crow Creek Massacre between Indian groups, which occurred well before the days of Columbus crossing the Atlantic. Crow Creek is an archeological site in an area now part of South Dakota. At this site the remains of 486 skeletons were found near a former village. Judging from perimortal damages on the skeletons, and on other facts, the inhabitants of this village must have been slaughtered in a bloodbath conducted by invaders (Willey & Emerson 1993). Perhaps the most important evidence in disfavor of the noble savage stems from studies of tribal communities that still live under circumstances comparable to our human ancestry. Chagnon (1988) summarizes more than two decades of research on the Yanomamo Indians of the Amazonas and comes to the horrifying conclusion that 44 percent of the males at age 25 or older have participated at least once in the killing of other humans. In addition, he reports that approximately 30 percent of adult male deaths are due to violence. Needless to say that the Yanomamo homicide rates are vastly higher than those reported for modern Western societies. Does all this killing among the Yanomamo have logic? Or is it perhaps due to an overshooting aggressive drive à la Lorenz? According to Chagnon, demographic data indicate that men who have killed have (a) more wives and (b) more offspring than men who have not killed. With all the caveats given in the introductory section, these facts are consistent with explanations at the level of biological evolution that are standard in biological studies of violence. The demise of the noble savage is paralleled by something I think one should call the “demise of the noble animal”. A noble animal would engage in ritualized fights that resemble a sports event rather than anything like a stabbing match. But contrary to the ideas expressed by classical ethologists, it is misleading to compare fighting in animals with a sports event where the opponents comply with rules of fair play. Fighting in red deer, for example, may look like fencing but in these antagonistic interactions each individual carefully

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avoids offering a vulnerable flank to the opponent. Individuals who fail to do manage this task can receive a dangerous blow. Clutton-Brock et al. (1979) estimate that up to 6 percent of rutting stags are permanently injured each year. Similar to red deer, Canadian musk ox also fight for access to females. According to Wilkinson & Shank (1977) up to 6 percent of the adult bulls incur lethal injuries from fighting in a single mating season. There are numerous other examples of violence in the animal kingdom including the almost warfare-like behavior in chimpanzees already mentioned above (e.g. Manson & Wrangham 1991). Groups of males patrol territorial boundaries, often seeking out isolated members of adjacent groups for attack. They employ stealth, invade foreign territory, and target out-group males as well as some out-group females. Young females who have not yet given birth are not attacked but are forced to travel with the aggressors. Similar warfarelike behavior can be found in human tribal communities. Chagnon (1988) describes, for instance, raids conducted by Yanomamo Indians that resemble astonishingly well those of the chimpanzees. This is particularly interesting since, from all we know, the biology of the Yanomamo is essentially the same as that of people in Western societies. Almost paradoxically, the warfare among human small-scale societies may have promoted the genetic and cultural evolution of human cooperation (Bowles 2009). The reason is that group members rely on each other in aggressive interactions with other groups. This mutual dependence can make it necessary for group members to worry not only about their own well-being but also about that of other group members, so that cooperation evolves primarily as an in-group phenomenon. It is known from experiments in social psychology, for example, that there are major differences between in-group and outgroup behavior of human subjects. A famous such experiment, the prison simulated in Stanford by Haney et al. (1973), had to be stopped earlier than planned because intergroup aggression had risen to an unbearable degree.

V. Evolutionary Game Theory as an Explanatory Tool When looking at the mating behavior of toads in section III, it helped tremendously to study this behavior from the perspective of the individuals involved and of the costs and benefits of behavioral alternatives. There are conceptual tools to pursue this kind of research program in a very systematic way. These tools have been developed in game theory, a discipline designed to view interactions through the eyes of all individuals involved and drawing particular attention to conflicts of interest. Starting with a seminal paper by Maynard Smith & Price (1973) biologists have created their own branch of game theory. This branch differs from the “original” by laying the decision about strategies into the hands of natural selection instead of assuming that animals are able to perform cognitively demanding strategic analyses by themselves.

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As we will see, putting “rationality” into the hands of natural selection instead of the individual actors presents a powerful new approach. Even humans have a hard time dealing with the complexity of strategic reasoning. This is why classical game theory is an academic discipline and requires the rigor of mathematical modeling. Let us first look at a ridiculously simple and yet intriguing example of a game-theoretic model. A game called chicken used to be fashionable among American teenagers in the 1950s. Two car drivers play this game and approach each other on a collision course. The first one who turns the steering wheel and swerves from the course loses the game. The other driver then receives a reward. If neither of them “chickens out”, a crash occurs. Even though the drivers are allowed to split the reward in this case, this will not compensate for the cost of repairing their vehicles. Of course, both may turn the steering wheel, in which case they can split the reward without having any costs. This example can be translated into a mathematical model in which the players each have two strategies, namely S1 “remain on the collision course” and S2 “turn the steering wheel to avoid a collision”. Since each of the two players has these alternatives, four combinations of strategies are possible and – depending on the players’ choices – four different courses of action can thus occur. Since the payoffs to a player are defined by the course of action, they can be written down in a “two by two” scheme called payoff matrix. Rather than going through this simple exercise, let us discuss how the game of chicken can be analyzed. Game theorists have developed their agenda for analyzing a game like the one just described. The main idea is to compute so called best response strategies. For example, if it were known that the opponent would turn the steering wheel, the best response would be to remain on the collision course. Conversely, were the opponent known not to chicken out, the best response would be to turn the wheel. The next step on the analytical agenda is to look for pairs of strategies, one for each player, such that each of them is a best response to the other one. A pair of strategies with these two properties (Nash 1951) is called a Nash equilibrium. At first sight there are only two Nash equilibria. Both are of the same structure; one of the players chickens out and the other does not. Given the symmetry of the situation, these Nash equilibria are problematic because it cannot be decided at the theoretical level who of the two players should be the aggressor and who the chicken. In classical game theory the search is therefore continued in a larger strategy space, where mixed strategies are taken into account. To play a mixed strategy means to use a random device and play the strategies S1 and S2 with certain probabilities, such as 1/3 and 2/3. Mixed strategies are a way to make one’s own behavior unpredictable. Much to the enjoyment of classical game theorists, it is now possible to identify a Nash equilibrium in which both players play the same mixed strategy. When playing this symmetric Nash equilibrium each of the drivers remains on the collision course with

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some probability and those who observe the scene have chance of observing a crash. We now see what is intricate about the game under consideration. Both players could in principle chicken out and share the reward without risking the cost of car repair. But if each of them believed in the opponent’s peaceful action, they would individually have an incentive to remain on the collision course. In contrast, a Nash equilibrium is characterized by the property that no single player would have an incentive to undermine it through deviant behavior. It is now easy to demonstrate why game theory matters in evolutionary biology. Let us reinterpret the game of chicken as follows: (i) Payoffs are defined in terms of fitness (ii) Strategies are conceived as heritable traits (iii) Natural selection determines the frequency of strategies in a population To cut a long story short, the property (iii) can be made precise by the concept of an evolutionarily stable strategy. The idea is here that for a given game the evolutionary process would eventually come to a halt in a population because the strategy now played by its members does not offer incentives for deviant behavior. As long as everyone plays this strategy, nothing can be gained from switching to a different one. Mutant strategies will then be selected against, as long as they occur at small enough frequencies. It is almost self-evident from the last paragraph that an evolutionarily stable strategy will be a strategy found in a Nash equilibrium of the game that gave rise to the evolutionary problem. This is a general insight about the “marriage” between classical game theory and evolutionary biology. It is neither confined to the game of chicken, nor to the case of two as opposed to several or many players.

VI. Lessons from Evolutionary Game Theory Evolution promotes and limits fighting in animals. In the work of Maynard Smith (1982), the famous game of chicken appears in a slightly modified way as the “Hawk-Dove” game. This reinvention of a famous paradigm is understandable because Maynard Smith and Price (1973) had introduced the concept of an evolutionarily stable strategy with the purpose of analyzing animal contests. Specifically, they were asking why these contests sometimes but not always escalate into injurious fighting where, figuratively speaking, the opponents both remain on the collision course. Chicken (Hawk-Dove) is the simplest of all the possible models that allow us to investigate how natural selection may have shaped fighting strategies. The simplicity of this game lies in the fact that there are only two extreme alternatives, namely to go for a

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physical fight (remain on the collision course) or to refrain from physical fighting (turn the steering wheel). We know that with the assumptions made above neither of these two “pure” strategies can be evolutionarily stable because the best response would always be the other one. The mixed strategy solution of the chicken game, however – where individuals make biased random choices – is evolutionarily stable; the higher the reward for winning, the stronger is the bias towards physical fighting. In addition, a population mix (biologists call this a polymorphism) of the two pure strategies is stable if the same bias can be found in the frequencies at which these pure strategies occur in the population. For example, if the evolutionarily stable mixed strategy lets each individual fight or refrain from fighting with equal probabilities, then a mixture of 50% individuals that always fight and 50% that always refrain from fighting can be evolutionarily stable as well. Is there any stable “Hawk-Dove” polymorphism in nature? The well known coexistence of two radically different male morphs in fig wasps certainly has this flavor. Within the same fig wasp species two fundamentally different male morphologies can be found. One male morph is winged and prepared for migrating away from the fig where they hatched. The other morph has no wings but impressive arms instead and stays within the natal fig. The latter morph is specifically designed to kill members of the same species. Hamilton (1979) describes battlefields of fig wasps, where in some figs more than half the males died from the consequences of inter-male combat. It must be emphasized that the fig wasp example needs more elaborate modeling than the simple Hawk-Dove (chicken) game can offer. But the HawkDove game can be seen as the point of origin for any guided tour through the world of models on animal fighting. More realistic games have to take into account, for example, that opponents may differ in their size, age, sex, or status, as well as in many other aspects that can influence the costs and benefits of aggressive behavior. Asymmetries play a key role in conflict resolution. We have learned from studies in evolutionary game theory that animal contests in which no damage occurs are often not as peaceful as they appear. Deterrence often occurs in the sense that one contestant would have inflicted serious injury on the other if the latter had not surrendered or escaped at some crucial point. But who can deter whom? If one contestant is much stronger than its opponent, it seems obvious that this individual could easily deter the other. A strategy “risk the collision if stronger” and “avoid collision if weaker” can indeed be evolutionary stable unless the value of winning is, for example, high for the weaker and low for the stronger contestant. In a population where all individuals play this strategy, conflicts are conventionally resolved on the basis of power asymmetries, but the willingness of the stronger to “fight for its interests” is crucial for the stability of this convention. Let us now look at an apparently irrelevant asymmetry. Two equally strong and equally hungry animals are in conflict over a food item but one of

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them was the first to arrive at the site where this item is located. The order in which these competitors appear at the food site is irrelevant for the payoffs in the game but the “first versus second” asymmetry is nevertheless of great importance. Maynard Smith & Parker (1976) analyzed situations of this kind and showed that a strategy “fight if you are the first” and “do not fight if you are second” can be evolutionarily stable. With a sense of humor Maynard Smith called this the Bourgeois strategy. Now, what happens if the first to be present at a resource is weaker than the second? Hammerstein (1981) showed that within limits the Bourgeois strategy remains evolutionarily stable and can be used for conflict resolution even in the presence of additional asymmetries influencing conflict outcome. In order to understand the stability of a Bourgeois strategy, imagine two car drivers at an intersection. They are playing a game of chicken in the sense that both can either risk a collision or stop their car. Usually the one coming from the right pushes the accelerator, the other hits the break – at least according to most countries’ traffic rules regarding right of way. If this is the common behavior pattern in a population of drivers, then it is indeed best to follow this convention even in the absence of sanctions for rule violation. We are here confronted with a Bourgeois strategy in our everyday life, where the payoffirrelevant asymmetry is not “first versus second” but “coming from the right versus the left”. In the animal kingdom, the evolutionary stability of the Bourgeois strategy is important if one wants to understand why animals often behave as if they were the owner of something, let it be a territory, food, or a mating partner. The phenomenon of ownership arises here in the complete absence of any property rights and legal institutions. Classical ethologists were, of course, aware of the fact that territory-holders defend their territories. What they overlooked, though, is the role that a payoffirrelevant asymmetry like “first versus second” can play in explaining these facts. Because of this lack of game-theoretic understanding, ethologists felt obliged to explain successful territorial defense either in terms of a “home bias” or by resorting to the very problematic argument that an effort already made to establish a territory would bias the value of winning in the owner’s favor. Critics have called this use of a “backward logic” the Concorde fallacy – after the famous supersonic airplane which, even though its future flights were predicted to be unprofitable, was allowed to keep on flying because of the Billions already invested in its development. Evolutionary biologists have come to use a “forward logic” instead, where the value of winning is defined as the individual’s future benefits from winning. The study of asymmetric contests then leads to a new concept of home bias that lies in innate behavioral propensities, e.g. “be bold and fight if first, be shy and do not fight if second”. Realistic payoffs inform us about conflict resolution and animal societies. The theory of asymmetric contests has a wide range of applications in different parts of the animal kingdom, ranging from spiders and insects to birds and

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mammals. Many biological studies have demonstrated indeed that asymmetries are often decisive for conventional conflict resolution (e.g. Wilson 1961, Kummer et al. 1974, Davies 1978, Yasukawa & Bick 1983, Crespi 1986). Empirically it is quite a challenge, however, to estimate quantitatively the costs and benefits that matter if one really wants to put contest models to the test. Hammerstein & Riechert (1988) analyze data from a long-term field study on spider territorial behavior to achieve exactly this goal. The spiders under investigation live in a harsh desert environment in which potential sites for spider webs are scarce and differ in quality. Competition for these web sites causes a great number of territorial interactions. At an average site (ensuring a moderate food supply), a contest usually ends without escalating into an injurious fight. For small differences in weight it is the owner and otherwise the heavier individual, that typically wins the contest. Escalated fights are rare and occur mainly at locations where food is particularly abundant. The spiders thus behave qualitatively according to the asymmetric contest model analyzed by Hammerstein (1981).

Figure 1: Fighting in the desert spider Agelenopsis aperta. The left individual is an intruder who challenges the owner of a territory. Typically the opponents do not touch each other and keep a minimum distance of approximately two centimeters. The web is used as a “scale” in order to determine relative body weight. If there is an important weight difference, the heavier spider usually wins. Otherwise ownership is respected in most cases. Escalated fights occur mainly over territories that are particularly valuable. Hammerstein & Riechert (1988) show how a game theoretic model can explain these observations. Parameters, such as the various costs of fighting or the different values of winning were estimated from field data. This was one of the first studies in which the existence of a Nash equilibrium (evolutionarily stable strategy) could be demonstrated quantitatively on the basis of field data. (Picture from Hammerstein & Selten 1994, drawn by Dieter Schmidl).

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The authors use their field data to estimate all game payoffs in territorial contests between female owners and intruders as changes in the expected lifetime number of eggs laid. Eggs are here the best empirically accessible proxy for fitness, and payoff estimation is complicated by the fact that contests over territories take place long before eggs are laid. Subsequent interactions occur so that a spider that wins a territorial contest today may lose tomorrow against another intruder. Conversely, today’s loser may succeed tomorrow in defeating the owner of another territory. In the focal study area, web site tenancy crucially affects survival probability, fighting ability, and the rate at which eggs are produced. Hammerstein & Riechert (1988) apply their payoff-estimates to investigate meticulously whether the interpretation of spider conflict resolution via role asymmetries withstands quantitative scrutiny. Acting as if they were real estate brokers, the authors compare different categories of territories and ask, for example, how much each category will be worth to a spider. The daily weight gain is about 5.1 mg for an average site and 7.9 mg for an excellent location. Feeding these values into a model for spider life histories, it turns out that the benefits of being in possession of these web-sites (increments in expected fitness) are 20 mg and 72 mg egg mass for the respective sites. In other words, while the daily weight gains at an excellent site are only about 50 percent above that of an average site, the estate value in terms of egg mass return is more than tripled. Why is there such a strong multiplier effect? In the spider population under investigation, owners of excellent sites grow faster than the average spider. Therefore, an originally “richer” spider will have better chances of defending its wealth than an otherwise equal “poorer spider”. Anthropomorphically speaking, the gap between rich and middle class spiders increases substantially over time – a phenomenon not unheard of in human societies. We learn from this example that realistic costs and benefits in terms of fitness tell us a lot more than just the validity of a model under scrutiny. Signaling and assessment precede escalation. When two cars approach an intersection from different angles and the drivers have to decide who should go first, there is not much of an ambiguity about who is coming from the right and who from the left. But such quick and clear unambiguity is far from common among conflict-relevant asymmetries. Instead, the relevant asymmetries are often revealed through phases of mutual assessment, as examined by Enquist & Leimar (1983) in a game-theoretic study. Many empirical studies support their approach. Clutton Brock & Albon (1979) show, for example, that when red deer stags compete for females in the rut, their contests typically start with a phase where they roar at each other. It often happens that one of the contestants retreats after this phase, and escalated fights will rarely start without previous roaring. How do we know that roaring occurs to impress an opponent? In red deer the vocal tract’s resonance frequencies are in principle a good indicator of body

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size. These frequencies depend on the length of the vocal tract above the larynx. During roaring the stags lower their larynx to the sternum (Fitch & Reby 2001). This translocation elongates the vocal tract to the maximum and decreases the resonance frequencies. Lowering the larynx, therefore, exaggerates the acoustically perceived body size as much as possible. It seems unlikely that such a subtle mechanism has evolved for no good reason. Probably, it evolved as a means of signaling a “larger-than-life-size” to opponents in order to deter them from engaging in an injurious fight. Once this behavioral novelty had been acquired by all red deer, however, the resulting lower resonance frequencies became an honest signal about body size again. Larynx lowering thus can be regarded as the trace of an acoustic arms race that took place long ago. In a famous experiment Davies & Halliday (1978) demonstrate that common toads (Bufo bufo) also use vocalizations to assess the size of rivals in malemale interactions. When a male attacks a pair of toads on their way to the spawning site (as described in section III), the defender usually croaks and tries to kick the aggressor away. The fundamental frequency of a croak is a good indicator of body size but this alone does not tell us whether a rival would pay any attention to this vocalization. In the experiment defenders are, therefore, silenced with the help of a soft rubber band and their croaks replaced by taperecorded croaks from large and small males. And indeed, the intensity of attacks by a rival is highest for the high pitch sounds recorded from small toads and lower for low pitch sounds recorded from bigger toads. It thus seems that rivals really do pay attention to the defender’s croak when they assess their chances of success in this struggle over mating. Emotions matter in the logic of deterrence. Almost any theory is faced with problems that it should be able to tackle but for which the appropriate tools are missing. Classical game theory is essentially blind to emotions and is almost proud of it, since rational decision makers are meant to make very smart decisions without being hampered by feelings and the like. In a caricature, this attitude can be described as “what matters are the payoffs and some solid logic and mathematics, full stop”. In contrast, biology as a natural science has always promised to try and understand all the facts. There is one exceptional area of research, however, where classical game theory has informed biology about the outstanding role of an emotion in politics and in our everyday life. This is the research on the logic of deterrence (see Hammerstein & Boyd 2012 for a review). It all started in the days of the Cold War, where NATO and the Warsaw Pact forces faced each other from opposite sides of the Iron curtain. What would have happened, had the Soviets launched a conventional attack on Germany? Due to an imbalance of power at the level of conventional forces, NATO forces would have been unable to stop the invasion. NATO doctrine therefore held that such an invasion would have to be countered by a massive nuclear attack on the Soviet Union. It was believed at the time that this threat of nuclear retaliation was sufficient to deter the Kremlin from entering NATO

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territory because the masters of the Kremlin were smart enough to foresee the disaster they would end up with. It was also believed that the American president would actually hit the red button to launch the missiles. Schelling (1960) and other game theorists understood rather quickly that there was a fatal flaw in the deterrence argument. In a nutshell, it turned out that logic of this deterrence would only work if the Kremlin had reasons to believe that the American president would behave in an irrational way. In order to understand why the president’s launching of a nuclear attack would have been irrational, let us assume in a thought experiment that the Warsaw Pact forces have begun to enter the western part of Germany. What decision will the American president take? He has the choice to either lose Europe or end the world including his own country. Even with great sympathy for Europe, a rational president weighing the costs and benefits in this situation cannot possibly push the red button. The rational masters of the Kremlin had anticipated this, of course, and this is why they felt safe when entering NATO territory. It is easy to capture this scenario with simple game-theoretic models and there is no doubt that in a world of rational players nuclear deterrence fails to work because it is not backed up by any firm commitment to launch the dreadful nuclear missiles in case of an attack. One could say that in order for this deterrence to work properly, the masters of the Kremlin had to believe that even the American president might make a stupid move. And they may indeed have believed that. How then can such deterrence be made binding and thus convincing. Hirshleifer (1987) argued, that in our everyday life, the emotion anger can act as a commitment device through which threats come true. If a potential aggressor knows that aggression will anger his opponent, he knows that his opponent will retaliate even if, at that point, retaliation does not pay. As a result, the potential aggressor may be deterred from attacking his opponent for fear of retaliation. Everyday experience tells us indeed that, indeed, angry people often do not consider the cost and benefits of their actions in rational ways. Psychological experiments (see Lerner and Tiedens 2006 for a review) have demonstrated, for example, that risks seem lower to people in a state of anger than they would appear in a state of “cold cognition”. Anger is probably not a human specialty. It seems, for example, that even the desert spiders discussed above can lock into “mental” states that act as a commitment device in deterrence. At this point I have to admit, that we are left with the problem to find out how such mental mechanisms have evolved in the first place. The integration of evolutionary and mechanistic approaches to behavior is one of the major themes in modern biology.

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Acknowledgement I am very grateful to my colleague Arnulf Koehncke who helped me with making the biology in this chapter transparent for non-biologists.

References Bowles, S. (2009). Did Warfare among Ancestral Hunter-Gatherers affect the Evolution of Human Social Behaviors? Science, 324, 1293-1298. Chagnon, N. A. (1988). Life Histories, Blood Revenge, and Warfare in a Tribal Population. Science, 239, 985-992. Clutton-Brock, T. H. & Albon, S. D. (1979). The Roaring of Red Deer and the Evolution of Honest Advertisement. Behaviour, 69, 145-170. Clutton-Brock, T. H., Albon, S. D., Gibson, R. M. & Guinness, F. E. (1979). The Logical Stag: Adaptive Aspects of Fighting in Red Deer (Cervus elaphus L.). Animal Behaviour, 27, 211-225. Crespi, B. J. (1986). Size Assessment and Alternative Fighting Tactics in Elaprhothrips tuberculatus (Insecta: Thysanoptera). Animal Behaviour, 34, 1335. Daly, M. & Wilson, M. (1988). Homicide. New York: Aldine de Gruyter (1988). Davies, N. B. (1978). Territorial Defence in the Speckled Wood Butterfly (Pararge aegeria): The Resident always wins. Animal Behaviour, 26, 138-147. Davies, N. B. & Halliday, T. M. (1978). Deep Croaks and Fighting Assessment in Toads Bufo bufo. Nature, 274, 638-685. Davies, N. B. & Halliday, T. M. (1979). Competitive Mate Searching in Male Common Toads, Bufo bufo. Animal Behaviour, 27, 1253-1267. Eisner, M. (2003). Long-Term Historical Trends in Violent Crime. Crime and Justice; A Review of Research, 30, 83-142. Eisner, M. (2009). The Uses of Violence: An Examination of some Cross-Cutting Issues. International Journal of Conflict and Violence, 3, 40-59. Eisner, M. (2011). Killing Kings: Patterns of Regicide in Europe, 600-1800 AD. British Journal of Criminology, 51, 556-577. Enquist, M. & Leimar, O. (1983). Evolution of Fighting Behaviour: Decision Rules and Assessment of Relative Strength. Journal of Theoretical Biology, 102, 387-410. Fitch, W. T. & Reby, D. (2001). The Descended Larynx is not uniquely Human. Proceedings of the Royal Society London B, 268, 1669-1675. Hamilton, W. D. (1979). Wingless and Fighting Males in Fig Wasps and Other Insects. In M. S. Blum & N. A. Blum (Eds.), Sexual Selection and Reproductive Competition in Insects (pp. 167-220). London: Academic Press. Hammerstein, P. (1981). The Role of Asymmetries in Animal Contests. Animal Behaviour, 29, 193-205. Hammerstein, P. & Boyd, R. (2012). Learning, Cognitive Limitations, and the Modelling of Social Behavior. In P. Hammerstein & J. R. Stevens (Eds.), Evolution and the Mechanisms of Decision Making (pp. 319-343). Cambridge, MA: MIT Press. Hammerstein, P. & Riechert, S. E. (1988). Payoffs and Strategies in Territorial Contests: ESS Analyses of Two Ecotypes of the Spider Agelenopsis aperta. Evolutionary Ecology, 2, 115-138.

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Hammerstein, P. & Selten, R. (1994). Game Theory and Evolutionary Biology. In R. J. Aumann & S. Hart (Eds.), Handbook of Game Theory with Economic Applications, Vol.2 (pp. 929-993). Amsterdam: Elsevier. Haney, C., Banks, W. C. & Zimbardo, P. G. (1973). Interpersonal Dynamics in a Simulated Prison. International Journal of Criminology and Penology, 1, 69–97. Hirshleifer, J. (1987). Economics from a Biological Viewpoint. Journal of Law and Economics, 20, 1–52. Kummer, H., Götz, W. & Angst, W. (1964). Triadic Differentiation: An Inhibitory Process protecting Pair Bonds in Baboons. Behaviour, 49, 62-87. Lerner, J. S. & Tiedens, L. Z. (2006). Portrait of the Angry Decision Maker: How Appraisal Tendencies shape Anger’s Influence on Cognition. Journal of Behavioral Decision Making, 19, 115–137. Lorenz, K. (1963). Das sogenannte Böse: Zur Naturgeschichte der Aggression. Wien: Borotha-Schöler. Manson, J. H. & Wrangham, R. W. (1991). Intergroup Aggression in Chimpanzees and Humans. Current Anthropology, 32, 369-377. Maynard Smith, J. & Price, G. R. (1973). The Logic of Animal Conflict. Nature, 246, 15-18. Maynard Smith, J. (1982). Evolution and the Theory of Games. Cambridge: Cambridge University Press. Maynard Smith, J. & Parker, G. A. (1976). The Logic of Asymmetric Conflict. Animal Behaviour, 24, 159-175. Mitani, J. C., Watts, D. P. & Amsler, S. J. (2010). Lethal Intergroup Aggression leads to Territorial Expansion in Wild Chimpanzees. Current Biology, 20, R507-R508. Mizell, L. (1997). Aggressive Driving. In Aggressive Driving Three Studies. Washington, DC: AAA Foundation for Traffic Safety. Nash, J. F. (1951). Non-Cooperative Games. Annals of Mathematics, 54, 286-295. Schelling, T. (196). The Strategy of Conflict. Cambridge, MA: Harvard University Press. Wilkinson, P. F. & Shank, C. C. (1976). Rutting-Fight Mortality among Musk Oxen on Banks Island, Northwest Territories, Canada. Animal Behaviour, 24, 756-758. Willey, P. & Emerson, T. E. (1993). The Osteology and Archeology of the Crow Creek Massacre. Plains Anthropologist, 38, 227-269. Wilson, F. (1961). Adult Reproductive Behavior in Asolcus basalis (Hymenoptera: Selonidae). Australian Journal of Zoology, 9, 739-751. Wilson, M. L., Wallauer, W. R. & Pusey, A. E. (2004). New Cases of Intergroup Violence among Chimpanzees in Gombe National Park, Tanzania. International Journal of Primatology, 25, 523-549. Yasukawa, K. & Bick, E. I. (1983). Dominance Hierarchies in Dark-Eyed Juncos (Junco hyemalis): A Test of a Game-Theoretic Model. Animal Behaviour, 31, 439-448.

Aggression in Humans and Other Primates — Biology, Psychology, Sociology DIETMAR ZINNER AND BRANDON C. WHEELER

I. Introduction A gang of eight males from the Kasakela community travelled purposefully towards the boundary of their territory, crossed the border and pushed silently further, monitoring carefully the neighbourhood. From time to time they stopped and listened. Then they spotted an adult male of the neighbouring Kahama community. They faced him up and attacked him brutally. After twenty minutes they desisted from their victim and moved back into their own territory. The victim died after some days due to his severe injuries. Similar events followed and after five years, the Kahama community was wiped out and its territory was taken over by the Kasakela community. While this sounds like an incident from hundreds of years ago between two clans in the Scottish Highlands or perhaps even a media report of atrocities from one of the many conflicts still taking place today, the actors from this story are not even humans, but are our closest primate relatives – the chimpanzees. Jane Goodall made these observations in the 1970s in Gombe National Park in Tanzania (Goodall et al. 1979, Goodall 1986). For Goodall, as for many other students of animal behaviour, these observations were irritating and shocking. Not only would chimpanzees kill neighbours of their own kind (i.e., ‘conspecifics’), but even more difficult to accept was that it seemed as if male chimpanzees joined together to raid and kill neighbouring males collectively and deliberately. This behaviour of the Gombe chimpanzees was immediately classified as unnatural, pathological or site specific by many researchers. However, since these first observations similar incidents have been documented in many other chimpanzee populations (e.g. Nishida 1979, Nishida et al. 1985, Goodall 1986, Wilson et al. 2004, Mitani & Watts 2005, Watts et al. 2006, Boesch et al. 2007, 2008, Mitani et al. 2010), indicating that killing of conspecifics is a regular part of the chimpanzees’ behavioural repertoire. Terms like ‘killer apes’ and ‘chimp war’ found their way through the media. For instance, on June 21, 2010 The New York Times published an article with the headline: “Chimps, too, wage War and annex Rival Territory”. Of course, it should also be noted that, although lethal aggression has been observed in most chimpanzee populations which have been studied over a longer period, such behaviours, as in

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humans, are relatively rare, and recurrent peaceful encounters also occur (Goodall 1986, Kawanaka & Nishida 1974, Watts et al. 2006, Boesch et al. 2008). But why was it so shocking for behavioural biologists that male chimpanzees apparently kill conspecifics of neighbouring groups? Was it the challenge of the notion that only humans are capable of doing such violent and merciless acts? Was it the loss of hope that at least nature is ‘good’? The occurrence of such violence within chimpanzees might indicate that the potential for violence and war are not culturally acquired traits of humans, but have a biological root in our primate legacy. Until the 1970s the prevailing paradigm, which was championed by Konrad Lorenz, said that animals with dangerous weapons, such as lions, wolves or even chimpanzees with their disproportionate strength and long, sharp canine teeth, possess an innate inhibition to kill conspecifics. (Lorenz 1955, 1963 [1966]). Animal behaviour at that point was considered almost exclusively in terms of how it evolved for the ‘good for the species’ or, at the very least, the ‘good of the group’ (e.g. Lorenz 1963 [1966], Wynne-Edwards 1962), making any interpretation of lethal aggression among conspecifics as anything but pathological impossible. Even non-lethal aggression was seen as, not pathological, but “favourable to the future of a species if the stronger of two rivals takes possession either of the territory or of the desired female” (Lorenz 1963 [1966]: 27). It was further believed that human aggression was unique and humans had lost the natural inhibition when they started to make and use weapons. Only through cultural evolution did man become a dangerous enemy for his own kind (Mead 1940, Kempf 2003, Sussman & Marshack 2010). This view was impressively depicted in Stanley Kubrick’s science fiction film, 2001: A Space Odyssey. In an arid and harsh savannah, a small horde of australopithecines tries to seize the water place of another horde. The leader of the strange horde is batted to death by one of the locals using a bone as a weapon. The manslayer tosses his weapon triumphantly into the sky. A cut takes place and in the following scene one can see a spaceship, an advanced product of cultural evolution, which began with that first use of a weapon. This view started to be challenged in the 1970’s, when a number of field researchers began to report back observations of lethal aggression among their study animals. In 1975, Edward O. Wilson noted in his influential book on socio-biology that “(t)he evidence of murder and cannibalism in mammals and other vertebrates has now accumulated to the point that we must completely reverse the conclusion advanced by Konrad Lorenz in his book On Aggression, which subsequent popular writers have proceeded to consolidate as part of the conventional wisdom”.

Since then, many additional cases of animals killing conspecifics have been reported for a variety of species. Because of the overwhelming evidence, the killing of conspecifics is no longer regarded as bare ‘pathological behavioural aberrations’ or ‘accidents’ induced, for instance, by the anthropogenic alteration of the natural habitat or population structure of certain species, but as part

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of the natural behavioural repertoire of many species. According to the frequency of fatalities during intraspecific contests, species can be grouped into three categories (Wrangham 1999): Species where killings of adult conspecifics are rare (accounts for less than 1% adult mortality). Most species fall into this category. Species where this phenomenon occurs more often (often more than 10% adult mortality; Enquist & Leimar 1990). All cases, however, occur only in the context of dyadic interactions in direct confrontations (e.g. red deer: Clutton-Brock et al. 1982, pronghorn: Byers 1997, wedge-capped capuchins: Miller 1998, mountain gorillas: Watts 1989). Fighting in these species is dangerous and risky (contestants possess dangerous weapons, such as antlers or long, sharp canines; Fig. 1). Killings of adult conspecifics occur relatively often but, in contrast to category two, killing is polyadic (coalitional). The killing of conspecifics by coalitions of multiple individuals is known only in ants (Wallis 1962, Adams 1990, Hölldobler & Wilson 1990), a few social carnivores (African wild dogs: Creel & Creel 2002, wolves: Mech & Boitani 2003, lions: Grinnell et al. 1995) and a few primate species (e.g. Western red colobus: Starin 1994, Diana monkeys: McGraw et al. 2002, white-faced capuchins: Gros-Louis et al. 2003, mandrills: Setchell et al. 2006, chimpanzees: Wilson & Wrangham 2003).

Fig. 1: Fight between Guinea baboon males for access to a fertile female. (photo Peter Maciej)

Among chimpanzees, intraspecific killing sometimes occurs in a peculiar manner that is remarkable in its similarity to a behaviour commonly seen in human intergroup conflicts: groups of males, sometime accompanied by a smaller number of females, from one community will collectively make an unprovoked ‘raid’ into the range of a neighbouring community, search until members of that community are located, attack them (often fatally), and then return to their own territory (reviewed in Wrangham 1999). The victims of these raids are

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normally males, but females have also been observed to be targeted in these aggressive encounters. Such raids are associated with ‘border patrols’, in which groups of males travel quietly in or directly adjacent to the ranges of neighbouring groups, paying especially close attention to any evidence that other chimps have recently been in the area, including the presence of night nests or recently used fruit trees (Watts & Mitani 2001). If an individual from a neighbouring group is detected, the raiding group will approach stealthily prior to attacking, a behaviour which is exceptional and seen only on one other occasion - when they engage in cooperative hunting of red colobus monkeys or other primates (Boesch 1994). These behavioural patterns during border patrols and attacks bear a strong resemblance to the planned and highly coordinated assaults by humans in both large- and small-scale warfare, which lead Richard Wrangham to conclude that “(c)urrent evidence supports the hypothesis that selection has favored a hunt-andkill propensity in chimpanzees and humans, and that coalitional killing has a long history in the evolution of both species” (Wrangham 1999: 1).

Wrangham views aggressive behaviour of chimpanzees and humans explicitly in an evolutionary, Darwinian framework, a perspective which we will present in this chapter in more detail. Specifically, we will consider the following questions: (1) What is aggression and what adaptive function does it serve? (2) Is coalitionary aggression towards conspecifics, as observed in chimpanzees and humans, unusual in the animal kingdom? (3) Is there continuity in the evolutionary history between this form of chimpanzee behaviour and particular forms of aggression in humans?

II. What is Aggression? The Problem of Definition Although almost everyone has an idea of what aggression is, it is hard to find a generally accepted, cross-disciplinary definition. Among humans, aggression can range from simple gestural or verbal behaviours to wars with millions of victims and may in some cases be institutional. Thus it is understandable that different fields of research, for example social and developmental psychology, psychiatry, sociology or political science, will apply different definitions. Even within the field of animal behaviour the term has proven difficult to define (Archer 1988). Thus although it is a very basic concept, examples of aggressive behaviour can vary greatly between species, considering that it can be found in animal species ranging from some of the most ancient phyla whose members possess very simple nervous systems, to those with the most complex neural mechanisms. Hand (1986: 216) defined aggression as “actual attacks, threats of attack or encroachments“, and also considered signals which suggest that attack may occur to be ‘implied aggression’. At the same time, Hand (1986) rejected including what psychologists may call ‘passive aggression’, as this requires a psychological assessment of the animal that is in most cases difficult if not im-

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possible to achieve. Most authors also distinguish between aggression specifically and agonistic behaviours more generally, the latter which includes not only aggression, but also the related concepts of threats (Hand’s ‘implied aggression’), retreats and submissive behaviours (King 1973). Discussion of aggression in animal behaviour is also typically limited to interactions only among individuals of the same species. Interspecific interactions which fall under the broad definition of aggression are most often predator-prey interactions (Davis 1964), but in some cases occurs among species competing for access to the same resource (e.g. Rose et al. 2003). But what of the case of cannibalism, which by definition is intraspecific and certainly involves behaviours that would fall under most definitions of aggression, but bears strong resemblance to predator-prey interactions? A number of definitions based in evolutionary biology define aggression based on an economic cost-benefit approach, wherein the currency is fitness, which is measured by the number of offspring an individual produced over its lifetime (e.g. Wilson 1975, Markl 1982). While aggressive interactions do have some reproductive consequences, which may range from negligible to severe and be long term or immediate, not all behaviours that decrease the target’s fitness fall into what most would consider aggression (e.g. tactical deception: Byrne 1997). Considerations such as these contribute to the difficulty in defining aggression. For the sake of simplicity, we will generally refer to aggression in terms of Hand’s (1986) definition provided above. Examples of aggression can be found throughout the animal kingdom. Solitary sea anemones, for example, 'fight' over space if they come too close together (Knowlton 1996), sometimes resulting in the loser becoming detached from the substrate, which is highly risky for a sessile organism. In some species severe aggression among siblings occurs regularly, and can result in the death of one offspring (‘siblicide’). This behaviour is most prominent in birds of prey (Mock 1984, Mock & Parker 1997, Simmons 2002) but occurs also in spotted hyenas (Frank et al. 1991, Golla et al. 1999). In social organisms, aggression between groups, like that described above for chimpanzees, is common. Ant colonies, for example, are aggressive toward each other and colony 'warfare' both within and between species is a common phenomenon (Wilson 1975). Particularly striking are fights among males when they vigorously compete over fertile females, for example in rhinoceros beetles, pythons, red deer, musk oxen, sea elephants, lions or baboons (e.g. Palmer 1978, Clutton-Brock 1982, Greene 1997). During such fights males often make use of particular morphological structures, such as horns and antlers in ungulates, and this can result in injury or even death of one or both actors (Drews 1996). In most primates, males are larger than females and have longer and sharper canine teeth, which are potentially lethal weapons occasionally employed in aggressive contests (Plavcan & van Schaik 1994). These structures are shaped by what Darwin referred to as ‘sexual selection’ (that is, selection resulting from competition for reproductive opportunities, contrasting with ‘natural selection’, which refers to competition for survival) and are typically only found in males or are other-

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wise much larger in males than in females. This results from the fact that competition among males for access to females is typically much more intense than vice versa, because females tend to be the ‘limiting sex’ in that their number has a much stronger limiting effect on the number of offspring that can be produced in a given population (Darwin 1871, Andersson 1994). In a wide range of mammals and birds, sexual selection has also favoured aggression by adult males towards individuals that are not their direct competitors, but rather are the infant offspring of their competitors (Hausfater & Hrdy 1984, van Schaik & Janson 2000). This form of infanticide is in most cases committed by males after they have taken over another male’s access to a female or group of females, killing the females’ dependent offspring sired by the male predecessor (van Schaik 2000a).

III. The Evolutionary Perspective Beside humans, the primate order contains by some counts more than 400 species, among them the great apes (the group to which humans belong, also including gorillas, chimpanzees, bonobos and orang-utans), lesser apes (gibbons and siamangs), Old World monkeys of Asia and Africa including cercopithecines (e.g. baboons, macaques and guenons) and colobines (colobus monkeys, langurs and odd-nosed monkeys), New World monkeys of Central and South America (e.g. marmosets, capuchins, spider monkeys and howler monkeys), tarsiers of Southeast Asia, lemurs of Madagascar, and lorises and galagos of Africa and Asia. The vast majority of primates live in complex social groups in which the same individuals repeatedly interact with one another. Without any known exception, all primate species show some degree of overt aggression against conspecifics, either within their social groups, between social groups or against solitary non-group members. In some species, aggression is extremely rare (e.g. muriquis of South America: Strier 1992), and even in species where it is relatively common it is most often subtle: for example, a baboon threatens another baboon simply by raising its eyebrows or one macaque supplants a second from a feeding location by the approach of the first, with or without further signals of threat and submission. Aggressive interactions are simply part of the daily life of most primates! The interesting question is therefore not whether a primate or any other animal shows aggression, but why?

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Fig. 2: Play-fighting among juvenile Japanese macaques. Such play fights sometimes escalate into aggressive fights, which can cause injuries like those seen on the individual on the left. (photo Barbara Tiddi)

IV. Tinbergen’s Four Questions or the Level of Analysis There are several different perspectives one can take when asking why an animal engages in any particular behaviour. These perspectives, referred to as ‘Tinbergen’s Four Questions’ after Nikolaas Tinbergen who first proposed them in 1963, allow us to consider behaviour from both developmental and causative perspectives, and in terms of both the individual employing the behaviour and its evolutionary history. Taking a Tinbergian approach, we might thus ask why a male baboon is aggressive in the following ways:

IV.1 What are the Proximate Causes of Aggression? Here proximate internal and external factors and mechanisms which control the behaviour in the short term are of interest. Which stimuli elicit the behaviour pattern and what are the underlying neurobiological, psychological or physiological mechanisms regulating the animal’s behaviour? For example, perhaps the baboon is in his prime and naturally experiences increased testosterone levels, hormones which increase the likelihood that he will behave aggressively (Wingfield et al. 1990, Cavigelli & Pereira 2000). An external proximate cause may be that a second male approaches a fertile female which elicits threats and an attack by the first male.

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IV.2 How did Aggressive Behaviour develop over the Lifetime of the Individual? Asking behavioural questions from this perspective generally requires investigation of the internal and external factors that influence the way in which a behaviour develops during ontogeny and how the developmental processes work. What is the interplay between the individual and its physical and social environment during the maturation of its behaviour? For example, the baboon is aggressive because he has learned through experience that aggression often leads to desired results (e.g. access to food or mates). Likewise, the maturation of fighting behaviour from play-fighting to escalated fighting during the baboon’s ontogeny might also provide such an ontogenetic explanation for the behaviour (Fig. 2).

IV.3 How did the Behaviour evolve? From this phylogenetic perspective, factors involved in shaping the behaviour over the course of evolutionary history are of interest. For example, perhaps a male baboon shows his canines as part of aggressive displays because, during the evolution of the species, this behaviour evolved from a non-aggressive cue (yawning) into an aggressive signal (threat yawning with presentation of the sharp canines) (Fig. 3). It seems likely that the normal form of yawning (without the purpose of showing the canines) appeared first and evolved into a threat gesture during phylogeny.

IV.4 What is the Ultimate Cause or Adaptive Function of the Behaviour? The ‘ultimate’ perspective is interested in how a particular behavioural strategy is adaptive for the survival and reproduction of an individual in its physical and social environment. That is, how does the behaviour contribute to the individual’s fitness, if it does at all? An example would be that a male baboon is aggressive because, if it fights and defeats a rival and gains access to a fertile female, then this increases his chances to produce offspring. Such ultimate questions regarding the evolution of social behaviour form the basis of the field of socio-biology.

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Fig. 3: Threat-yawning of a male hamadryas baboon (above) and an open mouth display during a male-male coalitionary threat in tufted capuchin monkeys (below). The long sharp canines are clearly visible. (Photos Dietmar Zinner and Brandon Wheeler)

The evolutionary perspective on aggression is largely focused on the ultimate function of a behaviour. In this framework, aggression is considered a behaviour which occurs during conflicts among individual within or between groups. Darwin recognized that competition and conflict are fundamental elements of the evolutionary process (Darwin 1859, Silk 1993). Two individuals with similar physiological or psychological requirements compete over a limited resource, such as a piece of food. One contestant supplants the other and uses the resource for itself. If such aggressive acts result in better survival and

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higher reproduction, natural selection will favour individuals showing such behaviour. If access to critical resources is determined by the outcome of competitive encounters, aggression is likely to become an integral part of the behavioural strategy of many organisms. Aggression can be regarded as an evolved solution to an ecological problem. In this sense aggression is useful and is not only destructive, something that Konrad Lorenz emphasised in his book ‘Das sogeannte Böse’ (1963) by citing J.W. v. Goethe’s Faust: “(I am) Part of that Power which would // The Evil ever do, and ever does the Good” (Faust I, translated by George Madison Priest). Under a Darwinian approach behavioural patterns are treated similarly to physiological or morphological traits, which is to say that they are subject to natural and sexual selection (Darwin 1859, 1871, Dawkins 1976, Andersson 1994). Assumptions for an evolutionary approach are: (1) Reproduction is limited by the availability of essential resources, resulting in competition among individuals; (2) As in morphological traits, behaviours or predispositions to engage in particular behaviours must have a genetic basis which is heritable from parent to offspring; (3) As in morphological traits, there must be variation among individuals in terms of their behaviour. This variation provides the base for selection. The physical and behavioural traits of some individuals of a population will lead them to be more successful in the competition for the limited resources and, as a result, to pass more copies of their genes on to the subsequent generation. The term ‘fitness’ refers to the contribution of an individual to the gene-pool of the next generation relative to the contributions of other individuals of the same population (Darwin 1859, Williams 1966, Dawkins 1976). Over the course of evolutionary time, the frequencies of particular genes within a population are adjusted by natural and sexual selection, favouring those genes that award individuals with higher fitness, causing the corresponding phenotypes (i.e., the physical expression of the genes) to tend towards adaptation to the current environment. If the environmental conditions change, other individuals with a different genetic make-up and resulting phenotype will become more successful. In many primate groups the dominant male will sire the most offspring (Cowlishaw & Dunbar 1991, Ellis 1995, Rodriguez-Llanes et al. 2009, Muniz et al. 2010, Weingrill et al. 2011) and dominance is most often achieved by fighting and defeating other males in the group (Carpenter 1942, de Waal 1982). If aggressive behaviour leads an individual to increase its fitness, then this behaviour will spread in subsequent populations. However, aggression also incurs costs, including higher energetic expenditure and risk of injuries during fighting (Figs. 1 & 4), stress, reduced time for other important activities (e.g., foraging or vigilance for predators), and suppression of the immune system due to elevated testosterone levels (Beehner et al. 2006, Muehlenbein & Watts 2010). Accordingly, an individual who engages in a certain amount of aggression, and employs it in the appropriate circumstances, will likely be

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more successful than a conspecific which adopts a strategy of too much or too little aggression, or employs it in inappropriate circumstances. Functional hypotheses about fitness consequences of aggression are often embedded in formal models based on economic cost-benefit analyses or game-theoretical approaches (Maynard Smith & Price 1973, Maynard Smith 1974, 1982, Krebs & Davies 1978, Lewontin 1979).

Fig. 4: Risk of fighting: An injured male baboon. (Photo: Julia Fischer)

V. Economy of Aggression Most direct conflicts between conspecifics arise if two individuals meet and compete for the same resource. Although fights between conspecifics can be very fierce, most conflicts cease before severe injuries occur. In many species, the fight will be superseded by ritualized displays. Why do conflicts not escalate in every case into a life-or-death struggle? Why do individuals of many species show ritualized forms of aggression and others not? Why do closely related species sometimes differ in their level of aggression (e.g. capuchins: Janson 1986, macaques: Petit et al. 1997, Thierry 2011, chimpanzees and bonobos: de Waal 2005, Hare et al. 2012)? A possible reason for these differences may actually be the magnitude of the ‘economic’ cost-benefit relationship of aggression, in terms of how it affects the actors’ fitness. In cases in which the costs of aggression outweigh the benefits, there is a net decrease in fitness (that is, it is not adaptive) and the behaviour is therefore selected against. The optimal form and degree of aggression is that in which the benefits of the behaviour outweigh the costs by the greatest amount (i.e. produces the greatest fitness ‘profit’). This can be illustrated by a simple optimality graph (Fig. 5),

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wherein the gain in fitness is depicted as a function of the level of aggression. The optimal level of aggression is achieved if the difference between costs and benefits is at its maximum. The optimal level is different in different species and in different social and ecological environments. Above the optimal aggression level individual fitness cannot be further increased by more aggression, but decreases by the additional costs.

Fig. 5: Optimality model of aggression. The benefits of aggressiveness (dashed curve) are assumed to increase asymptotically. At low levels, slight increases in aggressiveness entail rapid increases in the benefits. At higher levels, further increases of aggressiveness result in relatively small fitness increases. The costs are assumed to increase linearly with increasing aggressiveness (solid line). In all cases, natural selection will favour a level of aggressiveness (optimal level of aggressiveness = ‘O’) where the difference between benefits and costs are maximised (mf = maximum fitness) (Modified from Barash 1977). In part B the costs of aggressiveness are higher compared to part A. Thus, the optimal level of aggressiveness OB is pushed to smaller values.

The cost-benefit relation is influenced by several physiological, ecological cognitive and social factors. A resource may not yield the same fitness value for every individual. For example, a hungry animal most likely has a higher motivation to fight for a piece of food than does a conspecific which is full (Janson & Vogel 2006). Likewise, costs may vary. For a larger individual or an individual in a larger group, it might be not as risky to fight as for a smaller conspecific or an individual in a smaller group (Scarry & Tujague In Press). Animals should therefore be selected to assess the power and fighting motivation of a contestant in relation to their own abilities. Assessment of each other’s ‘resource holding potential’ (RHP, Parker 1974) defines for each contestant a critical probability of winning, above which fighting or escalation is the favoured strategy, and below which withdrawal, flight or submission is the better, more economic alternative. Escalation of a fight ensues when both contestants assess their probability of winning as positive. This economic view of animal conflicts provides a framework in which we can also explain why many species have evolved ritualized contests. In such rituals, each contestant tries to demonstrate its power and intimidate the oppo-

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nent: for example, a male gorilla stands up bipedally to appear larger and beat his chest (Schaller 1963, Yamagiwa 1987) while a male chimpanzee rushes furiously through the forest screaming and breaking branches and small trees (Goodall 1986). In other species it is the size of the weapons (horns or antlers, Geist 1966) or endurance in ‘vocal contests’ (red deer: Clutton-Brock & Albon 1979; chacma baboons: Kitchen et al. 2003) that demonstrate an individual’s strength. Strength may also be conveyed through demonstration of physical feats, such as the ‘high jump’ displays of male Indian gray langurs which are believed to signal RHP towards potential competitors (Sommer 1985). If such displays allow for reliable assessment of the RHP of an opponent prior to an escalation, this helps to lower the risks and save the costs of aggression, which might be in the interest of both contestants. The opponent with the lower RHP may retreat and avoid the risks of a direct, physical contest (which it would probably lose anyway), and the other gets access to the contested resource without the added cost of escalated aggression. The conditions under which behaviours such as agonstic displays can evolve have been studied primarily through models based in economic game-theory (Hammerstein 1981, Maynard-Smith 1982).

VI. Game Theory or, watching what the Others are doing Various models based in economic game theory have been proposed to explain the behaviour of animals involved in conflicts. According to such models, players can adopt one of two or more behavioural ‘strategies’, and the best strategy for any one individual to play depends to large extend on the strategies chosen by other individuals in the population. The most famous of these is Maynard Smith’s (1976) ‘Hawks and Doves’ model, wherein two animals have a contest for a hypothetical resource, and the winner takes all. In this game, it is assumed that it is essential to get the resource, so the animal should be willing to take some risks to get it, but there are just two strategies an individual can play. 'Hawks' always fight until the opponent flees or dies, though in the process they will risk injury or death themselves. 'Doves' hate risks and simply display until the opponent flees, but they will never engage in a physical fight and will flee themselves if the opponent fights. The winner of a contest scores +50, and the loser 0, while the cost of a serious injury is -100 and the cost of wasting time in a display is -10. Each individual reproduces in proportion to their pay-offs (i.e., the pay-offs are a measure of fitness). Now, we have to do some simple calculations in a two by two matrix with the average pay-offs for the four possible types of encounter (Table 1).

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Hawk Dove

Hawk ½ (50) + ½ (-100) = - 25 0

Dove +50 ½ (50 - 10) + ½ (-10) = +15

Table 1: Two by two matrix of encounters between Hawks and Doves (after Maynard Smith 1976, Maynard Smith & Parker 1976). Pay-offs: Winner +50 Injury -100 Loser 0 Display -10 Hawk plays Hawk: When two Hawks meet, it is assumed that each individual has a 50% chance of winning unscathed and a 50% of losing and suffering an injury. The average gain for a Hawk in these contests is thus: (50/2) + (-100/2) = -25. Hawk plays Dove: Hawks always beat Doves. The average gain of the Hawk is: +50. Dove plays Hawk: Doves always immediately retreat against Hawks without injury and without displaying. The average gain of the Dove is: 0. Dove plays Dove: We assume that there is always a display and it wins on half the occasions. The average gain of the Dove is: ((50-10)/2) + (-10/2) = +15

We begin by considering a population that consists exclusively of Doves. Every contest is between two Doves and the pay-off is on average +15. If a ‘mutant’ Hawk is introduced in the population, it would win and receive a pay-off of +50 for every encounter, resulting in higher fitness than the dove strategy, and Hawks would spread in subsequent generations. But if we consider a population of all Hawks, the average pay-off is -25 and any ‘mutant’ Dove introduced in the population would have an average pay-off of 0, better than the Hawk average of -25 in that population, and we would thus see the dove strategy spread in subsequent generations. Thus it’s best to be a Dove when Doves are rare and a Hawk when Hawks are rare, leading to unstable ratios of the two strategies within the population from one generation to the next. The ratio of Hawks to Doves in a population becomes stable only when the average pay-offs for a Hawk are equal to the average pay-offs for a Dove. If we consider the proportion of Hawks in a population to be h, then the proportion of Doves must be 1 - h. The average pay-off for a Hawk (H) is the pay-off for each type of fight multiplied by the probability of meeting each type of contestant (Hawk or Dove). Therefore, H = -25h + 50(1 - h). Similarly, for Dove the average pay-off will be D = 0h + 15(1 - h). At a stable equilibrium H is equal to D, that is -25h + 50(1 - h) = 0h + 15(1 - h). Solving this equation gives the proportion of Hawks as h = 7/12, and the proportion of Doves as 1 - h = 5/12. This particular mixture of the two strategies could be achieved in two distinct ways: First, the population could consist of individuals who play pure strategies. Each individual would be either Hawk or Dove and we would find 7/12 of the population being Hawks and the rest being Doves (5/12 of the population). Second, the population could consist of individuals who all adopt a mixed strategy. Each individual plays Hawk with a probability of 7/12 (58%) and Dove with a probability of 5/12 (42%), choosing at random which strategy to play in each contest. This is a simple model, but it gives us an idea of why not all individuals behave aggressively in all encounters: the likelihood of success of aggression depends on the likelihood that your opponent will choose a

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particular strategy. Under the conditions described above, the proportion of 58% Hawk and 42% Dove is called an evolutionarily stable strategy (ESS) because any deviation from this proportion (for example due to genetic mutations or migration of individuals into the population) will quickly return to the stable ratio via natural selection. However, the evolutionarily stable ratio can increase or decrease simply by changing the costs and benefits for each of the four match-ups. If a Hawk for instance gains a higher benefit than +50 or if the costs of an injury are less than -100, then the proportion of Hawks will rise and that of Doves fall, a situation depicted in Fig. 6.

Fig. 6: Simplified model of an evolutionary stable equilibrium of two alternative behavioural strategies, ‘Hawk’ and ‘Dove’. The x-axis represents the proportion of the population that plays a particular strategy (or the percentage of cases an individual plays a particular strategy). * = frequency of members of the population playing this strategy (or proportion of occasions an individual plays this strategy) with the same mean fitness of both strategies, respectively (after Voland 1993). This particular mixture of both strategies is assumed to be an evolutionarily stable strategy (ESS).

The costs and benefits associated with different behavioural strategies depend on the environmental (social, biotic and abiotic) conditions of the population. A change in any of these factors may trigger a change in the cost-benefit balance of certain behaviours, including aggression and may change the optimal level of aggressiveness. In a simple model we can assume a certain distribution of aggressiveness within a population with the optimal (adaptive) level contributing to a maximum fitness gain (Fig. 7A). If for instance the environment changes and competition for food becomes more severe, natural selection may favour individuals which show a higher level of aggression. After some generations the distribution of aggressiveness is pushed towards higher aggression levels (Fig. 7B). Other environmental changes may select for individuals that are less aggressive. A change of the environment entails a change in the cost-

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benefit relation of an aggressive strategy and thus a change in the adaptive level of aggression (Maynard Smith & Harper 1988).

Fig. 7: Relationship between aggression and reproductive success (fitness) under different environmental conditions. In environment A animals showing a certain level of aggression (optimal level) achieve the highest fitness. Animals which are more or less aggressive have lower fitness. Following a change of the environment towards condition B, animals which are slightly more aggressive compared to the optimal level in A achieve the highest reproductive success. After several generations under condition B the optimal aggression level is on average higher than under condition A.

With the economic view on aggression it becomes relatively straightforward to understand how considerations of costs and benefits and an assessment of the RHP predict the level of aggression in conflicts and also differences in aggressive behaviour between sexes and individuals.

VII. Behavioural Contexts of Intraspecific Aggression in Primates VII.1 Competing for Resources Understanding the economics underlying the evolution of aggression, we can now consider conflicts and aggression in primates from this perspective. According to socio-ecological theory, the distribution of essential resources is a prime factor determining the extent and form of competition and cooperation among individuals. In general, the reproductive success of male mammals is closely tied to their access to fertile females, while the reproductive success of females, because of the high energetic demands of lactation and gestation, is most constrained by access to food (e.g. Trivers 1972, Emlen & Oring 1977, Wrangham 1980, Ims 1987, Sterck et al. 1997). As such, male competition is expected to occur primarily in relation to mating opportunities, while female competition is expected to occur primarily in relation to feeding opportunities. Of course, this doesn’t preclude males competing for food (e.g. Janson 1985), who need sufficient food to build up and maintain their RHP, or females competing for mates (Doran-Sheehy et al. 2009, Smuts 1987). In addition, many

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primates also face competition for access to water (e.g. Wrangham 1981), safe sleeping sites (e.g. Radespiel et al. 1998) and a safe position within the group (e.g. Ron et al. 1996). Since most primates are gregarious, entire groups may compete with other groups for access to quality home ranges (Isbell 1991). Aggressive behaviour is one of several means to stand up to competitors and to maximise fitness. Competition refers to the negative effects which one organism has upon another by consuming or controlling access to an essential limited resource (Keddy 1989). In other words, if there is enough for all, competition does not occur and aggression is not necessary. For instance, animals rarely compete for air to breathe, or for a super-abundant food resource. Cows on a meadow will seldom compete aggressively for a mouth full of grass. However, severe fights may occur for scarce resources, such as fruit trees or mating partners. Competition among conspecifics is particularly high because animals of the same species have highly similar ecological requirements, much more than members of two different species. Food is the most important resource for most animals, because it provides the essential energy and nutrients for growth, maintenance and reproduction. In primates, competition for food is a major cause of conflict within and between social groups (e.g. Janson 1985, Zinner 1993, Koenig et al. 1998, Harris 2006, Wheeler 2009). The availability of food and its distribution in space and time impact the form and intensity of competition (Wrangham 1980, van Schaik 1989, Isbell 1991, Koenig 2002). If relatively low quality foods are evenly distributed over an area that exceeds the spread of the group, then it is not ‘economical’ to fight for any given food item; the payoff is low and one can simply achieve the same result at a lower average cost by quickly moving on to obtain an adjacent item. If, however, food is of high quality and is densely clumped, then it may pay to monopolise such a bonanza, even if one has to fight. The distribution of food is therefore thought to be a primary ecological factor shaping the evolution of primate social systems, affecting not only the patterns of aggression over access to food, but also the way that females distribute themselves in the environment, which in turn affects the ways that males compete for reproductive opportunities (Fig. 8).

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Fig. 8: Socio-ecological model of evolution of primate social systems (e.g. Wrangham 1980, van Schaik & van Hoof 1983, Terborgh & Janson 1986, van Schaik 1989, Sterck et al. 1997, Isbell & Young 2002, Koenig 2002). Ecological factors including predation risk, disease risk, and/or the distribution of food resources favour group-living among females. The distribution of food relative to the distribution of females affects the cooperative and competitive strategies, including aggression, which females employ to maximise food intake. The distribution of females itself affects the ways males distribute themselves in the environment and their competitive strategies to maximise reproductive opportunities. The conflicts and need for agonistic support that arise within and between the sexes as a result of reproductive competition, including competition for mating opportunities, male coercion and infanticide risk, further affect of the distribution of both males and females. Ultimately, these factors shape the social system that characterises a given population of primates, including its social organization (i.e., demographic makeup), social structure (i.e., the patterns of social relationships and interactions, including aggression) and mating system (e.g., polygynous, monogamous, etc.) (Kappeler & van Schaik 2002).

Competition for food occurs both within and between the sexes and, in most primate species, males dominate females because they have large bodies, longer canines and are physically stronger (e.g. hamadryas baboon: Kummer 1968, chimpanzees: Goldberg & Wrangham 1997, Stanford 1998), traits that have been favoured due to male-male mating competition (Andersson 1994). However, there are exceptions. In most Malagasy lemurs, females are dominant over males (Kappeler 1993), and female bonobos tend to be dominant to the males in their groups (White & Wood 2007). Among lemurs, females tend to be slightly larger than males, and female dominance has been argued to have evolved as a way to ensure that females obtain sufficient resources for gestation and lactation from the relatively food-limited conditions characteristic of Madagascar (Jolly 1984, Wright 1999). In contrast, bonobo males maintain the size advantage over females that is seen among their (male-dominant) chim-

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panzee cousins, and female dominance is primarily found in feeding situations, but not other competitive contexts. This is suggested to result from male deference to females, which may be a behavioural strategy on the part of males to encourage females to choose them as a mating partner (White & Wood 2007). In cases in which the quality and distribution of food makes monopolizing access to particular food items or patches of food an economically viable strategy, the competition to do so can be either direct (e.g. a conflict arises over a single food item) or indirect. Such indirect competition for contestable resources can be achieved within groups by establishing a clear rank order or between groups by establishing a territory which contains essential resources. Animals may therefore compete for dominance status or a territory, which can in turn result in differential access to resources without necessarily competing directly for such access (Silk 2002a, Vogel 2005). In primates and other groupliving species where individuals regularly and repeatedly associate with the same individuals, group members often establish dominance hierarchies, which in turn regulate access to resources (Rowell 1974, Kaufmann 1983). Dominant animals have privileged access to scarce resources or reproductive partners (Hausfater 1975, Ellis 1995, Johnstone 2000, Stahl & Kaumanns 2003, Vogel 2005). But subdominant individuals also benefit from the establishment of hierarchies. After the dominance relationships are established, the dominant and subordinate contestants do not have to take the risk of injuries or spend time and energy in fights for every contested resource. Thus although engaging in an aggressive contest may be an economically beneficial strategy for the winner, as in the Hawks and Doves example given above, a firmly established rank order makes the likely outcome of such a contest known before it even happens, and so the likely loser should cut its losses and avoid the fight altogether. Likewise, the position of an individual within the rank order can be signalled by gestures and postures, which are less costly than physical aggression (Rowell 1974, Deag 1977, Lu et al. 2008). In general, a dominance hierarchy is established as a result of the outcome of previous aggressive encounters, which include the initial encounter and subsequent fights or threats (Drews 1993). In at least some primates, support by high ranking relatives may lead to a high dominance status of an animal without any fighting, for instance if a high ranking mother supports her daughter (e.g. macaques: Kawai 1958, Bernstein 1969, Missakian 1972; baboons: Hausfater 1975; green monkeys: Cheney et al. 1981; capuchin monkeys: Robinson 1981). The effect of such nepotistic support is that daughters of high ranking mothers also occupy high ranks; daughters ‘inherit’ the rank from their mothers. The resulting dominance relationships between matrilines of related females can remain stable for years (Hausfater et al. 1982, Samuels et al. 1987). Whether a given primate species lives solitarily, in pairs or larger groups, individuals of all species live in home-ranges which can be used exclusively or overlap the home-ranges of neighbouring individuals or groups (Clutton-Brock & Harvey 1977; e.g. sportive lemurs: Zinner et al. 2003; ring-railed lemurs: Jolly

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et al. 1993; tufted capuchins: Di Bitetti 2001, Scarry & Tujague In Press; langurs: van Schaik et al. 1992; macaques: Saito et al. 1998, Zinner et al. 2001, Cooper et al. 2004; gibbons: Leighton 1986; chimpanzees: Williams et al. 2004, Wilson et al. 2012). Encounters between neighbouring individuals or groups range from peaceful to aggressive and can vary over time, often influenced by the current availability of resources, including food and the number of fertile females (Harrison 1983, Kitchen et al. 2004). During group encounters, it is common that different members of each group have divergent immediate goals and behave in different ways. The adult male(s) of the group may be interested in herding his group’s females away from non-group males or chasing non-group males away. At the same time, he might be interested in an oestrous female of the neighbouring group. The females of the group with young infants might be more likely to stay behind, while young and oestrous females may be interested in the males of the ‘opposite camp’. Hence, many intergroup-encounters are not purely aggressive or purely friendly (e.g. lion-tailed macaques: Zinner et al. 2001, guerezas: Fashing 2001, chacma baboons: Kitchen et al. 2004, greycheeked mangabeys and redtail monkeys: Brown 2011, white-faced capuchins: Meunier et al. 2012).

VIII. Intra-Sexual Competition Generally, there is an imbalance between female parental investment and male parental investment. A female produces larger gametes, which in some cases contain stored food for the embryo (e.g. bird eggs). Among mammals, females provide offspring with nutrients during pregnancy and lactation. Males in contrast, generally have to produce just small, and relatively cheap, spermatozoa and it pays a male more to leave the female after copulation and seek additional females than to put in effort caring for the young (Trivers 1972). It is this imbalance that makes females a limited resource for males, and leads males to be more likely to compete for females than will females for males (Andersson 1994). Indeed, the fiercest fights among primates tend to be among males competing for access to fertile females (Fig. 1). A male, which might have only once chance in his life to reproduce, will more likely adopt a risky all-or-nothing strategy and compete vigorously, than will a female that has a much more likely chance to reproduce year after year. Darwin (1871) recognised male-male competition as one part of the driving force for sexual selection, the other being female choice. While female choice results in the evolution of such features as the plumage of male peacocks (Andersson 1994), male-male competition results in the evolution of morphological and behavioural characteristics like a large body (Crook 1972, Clutton-Brock et al. 1977, Plavcan & van Schaik 1997, Lindenfors 2002), specialized weapons (e.g. antlers and horns in ungulates: Geist 1966, 1978, canines in primates: Clutton-Brock & Harvey 1984, Plavcan & van Schaik 1994), and higher aggressive-

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ness in males (Fig. 9). The form of male-male sexual competition, much like feeding competition, is influenced by the distribution of conceptive females in space and time (Fig. 8). If females have to concentrate in certain places (e.g. to defend patches of food from other groups of females; Wrangham 1980), then a male would gain reproductive benefits if he is able to monopolize that group of females. To take a non-primate example, female elephant seals gather together in groups on certain parts of beaches along the South American Atlantic coast in order to give birth and to mate. Competition between males is pronounced, because a successful male can hold a group of about 40 females (LeBoeuf 1974). Similarly, in primates many species live in stable ‘one-male groups’, consisting of one adult male, several adult females and their offspring, in which the one male monopolises access to an entire group of females (e.g. many Asian colobines: Kirkpatrick 2011, most guenons: Jaffe & Isbell 2011, geladas and hamadryas baboons: Swedell 2011, mountain gorillas: Robbins 2011). In contrast, if the females are solitary and widely dispersed, a male has no chance to monopolise a group of females (e.g. sportive lemurs: Hilgartner et al. In Press, orangutan: Rodman & Mitani 1987, Knott & Kahlenberg 2011). Sexual competition among females may also occur, and in some cases lead to female-female aggression for access to a male (geladas: Dunbar & Sharman 1983, hamadryas baboons: Zinner et al. 1994, chacma baboons: Huchard & Cowlishaw 2011).

Fig. 9: Mean rates of aggression per hour (± SE) by male and female chimpanzees in Kanyawara (N=11 males and 10 females) (Data from Muller 2002). Similar sex differences can be observed in many primate species.

Infanticide is another form of aggressive sexual competition, in which males fight not directly with their opponent, but direct lethal aggression towards the offspring of its competitor. Although female primates also sometime kill infants (Digby 2000), it is far more often performed by males (van Schaik & Janson 2000). Not only is infant-killing inherently less risky for males in species in which sexual selection has favoured dimorphism in body and canine size, but

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the potential benefit for doing so is much greater in males that in females. In primates, as in other mammals, females normally enter a phase of postparturition amenorrhoea due largely to the high energetic costs associated with lactation. If a male takes over the position as the primary sire of infants in a group (i.e. the alpha position or the sole adult male in the group), normally by winning a fight with the male that formerly held that status, he will sometimes kill the unweaned offspring in order to terminate lactation. This causes the female to return to oestrous and the new males may thus sire his own offspring without waiting until the previous male’s infants are weaned. The function of infanticide is to speed up the reproduction of the infanticidal male, hence to increase his fitness. Under these conditions male infanticide has an adaptive value (Borries et al. 1999a). Mothers, fathers (if they survive the takeover), and sometimes even other group males who have a relatively small possibility of being the father (due to having mated at least once with the mother) often try to prevent the killing of the offspring using counter-aggression (Borries et al. 1999b, Palombit et al. 2000, Nguyen et al. 2009, Henzi et al. 2010, Huchard et al. 2010). The relatively long periods of infant dependency characteristic of primates (van Schaik 2000b) has favoured the evolution of infanticide by males in many primates (lemurs: Jolly et al. 2000; capuchin monkeys: Manson et al. 2004, Ramirez-Llorens et al. 2008; hanuman langurs: Hrdy 1974, Sommer & Mohnot 1985; baboons: Palombit 2003, Swedell & Tesfaye 2003; gorillas: Watts 1989; chimpanzees: Watts et al. 2002, Murray et al. 2007;), but is also common among lions (Bertram 1975, Packer & Pusey 1984), many species of rodents (Blumstein 2000), and some avian taxa (Veiga 2000, Møller 2004). In some primate species infanticide is the main cause of infant mortality, accounting for up to 30% of infant deaths (van Schaik & Janson 2000, Cheney et al. 2004, Kitchen et al. 2009, Henzi et al. 2010, Janson et al. 2012). For females, the benefit of committing infanticide is not as obvious. The main potential benefits for infanticidal females seem to be to reduce potential future competitors of herself or her own offspring (Stockley & Bro-Jørgensen 2011), and to increase the proportion of her genes represented in the subsequent generation, not by leaving more copies herself, but by reducing the number of copies left by others.

IX. Sexual Conflict Although the adaptive benefit for a male in committing infanticide falls into the realm of intra-sexual competition, infanticide clearly has a dramatic impact on the fitness of the mother. Economically speaking, with the death of her infant she loses her prior investment in mating, gestation and lactation. Infanticide by males thus also contributes to inter-sexual conflict (Stumpf et al. 2011). Although the final goal of female and male strategies is the same – to maximise individual fitness by reproduction, and each sex needs the other to accomplish this – the more immediate strategies of the sexes often differ greatly and are at

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times incompatible, providing the basis for sexual conflict (Sparks 1999, Arnqvist & Rowe 2005). Because of the general imbalance between the reproductive physiology of females and males, females invest more into each offspring and carry the larger burdens of reproduction. In most mammals, including the vast majority of primates, the primary contribution of a male is the transfer of sperm (Smuts 1987). Therefore, a female should be more careful than should a male in terms of if, when and with which partner she will reproduce; she has much more to lose than does a male for reproducing with the ‘wrong’ partner or at the ‘wrong’ time. Mate choice should thus be more pronounced in females than in males (Darwin 1871, Small 1989, Andersson 1994, Paul 2002, Manson 2011). In species where males are physically stronger than females, as in most primate species (Clutton-Brock et al. 1977, Plavcan & van Schaik 1997, Lindenfors 2002), males sometimes use aggression towards reluctant females to assert their immediate reproductive goals (Fig. 10). Sexual coercion and forced copulations by males are widespread among primates, thus reducing the females’ ability to exercise mate choice and, as a likely consequence, her fitness (Smuts & Smuts 1993, Clutton-Brock & Parker 1995a, Muller & Wrangham 2009). For instance, male hamadryas baboons physically coerce females into semipermanent social units (Kummer 1968, Swedell & Schreier 2009). They associate closely with and mate guard their females at all times and threaten and harass females whenever they stray. Subordinate male orangutans will aggressively force females into copulation against their fierce resistance (Mitani 1985, Knott & Kahlenberg 2011). The conflict between male and female orangutans arises because the female strategies include choosing the best male (normally the dominant resident male, who has proven his physical prowess and longevity). They therefore try to avoid copulations with sub-adult or subdominant males, but are highly vulnerable to sexual coercion by these non-preferred males because males are on average twice as heavy as females, and their mainly solitary lifestyle means that they have no group mates to rely on for coalitionary support. In other species as well, sexual coercion is primarily a strategy of less-preferred males, although evolved female counter-strategies may allow them to mitigate the fitness costs associated with the male tactic (e.g. chimpanzees: Stumpf & Boesch 2010).

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Fig. 10: Sexual coercion. A female chacma baboon is attacked by a male and pushed under water. (Photos Marc Stickler)

X. Parent-Offspring Conflict In addition to conflicts between mates or potential mates, some degree of conflict between parents and infants is nearly universal, especially among primates and other mammals. This results from the fact that the offspring’s fitness is best served by additional investment from the parents, while the parent’s fitness would be best served by diverting those resources to producing additional offspring (Trivers 1974). For example, in many primate species, mothers react aggressively toward their own offspring during weaning, when they no longer want to nurse the infants but the infants continue to attempt to do so. Mothers threaten their infants, push them away, and even slap and bite them. Infants in all primate species are also highly dependent on the mother (or other caregiver) for transportation for at least the first several weeks of life, and simi-

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lar conflicts arise when caregivers attempt to carry infants less often (Nicolson 1987, Maestripieri 2002).

XI. Spite and Punishment The question of whether spiteful behaviours, that is behaviours which are costly for both the actor and recipient, can even evolve has long been a question in evolutionary biology (Knowlton & Parker 1979). Any behaviours which reduce the actor’s fitness, including both spite and altruism, require special explanation because the genes underpinning such behaviours would be expected to drop out of a population through natural selection. Although rare, spite (like altruism) can evolve under some specific conditions (Johnstone & Bshary 2004, Jensen 2010, West 2010). In contrast, punishment can be more easily explained in an evolutionary framework because punishment can shape the motivation of others to behave in a way that increases the punisher’s fitness. For example, hamadryas baboon males will aggressively herd a female who strays too far from the male back into the group, conditioning the female to maintain cohesion with the rest of the male’s harem (Kummer 1968). Parental aggression of infants that attempt to nurse after the mother has begun weaning may also be seen as an example of punishment (Clutton-Brock & Parker 1995b). Evidence of punishment to enforce cooperation or group norms is scarce, especially among nonhuman primates (Jensen 2010), but is an essential aspect of cooperation in human societies. One possible example of the use of punishment to enforce cooperation among primates may be the higher rates of aggression that are received by individuals that do not engage in the apparently altruistic behaviour of alerting others to the presence of food in some primates, as has been argued for rhesus macaques (Hauser 1992, but see Gros-Luis et al. 2004 for an alternative explanation).

XII. Reconciliation and Cooperation While the extent and form of aggressive behaviour is determined by ecological and demographic factors, one factor which limits the degree of aggression is the need to cooperate and coordinate. This may be especially limiting for species like primates that live in stable social groups, because such groups form due to the fitness benefits that the individuals in the group gain via coordination and cooperation with one another (e.g. protection against predators or infanticidal males, cooperative defence of resources: Bertram 1978, Sterck et al. 1997, Krause & Ruxton 2002). However, in order to take advantage of the fitness benefits of group living, animals face the challenge to coordinate group movement and synchronise their behaviour within a group (Fischer & Zinner 2011). Coordination can range from simple decisions such as ‘Should we rest or

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move?’ to collective hunts and border patrols in chimpanzees (Goodall 1986, Boesch 1994, Mitani & Watts 2005). But such cooperation may be difficult to achieve if social relationships are disturbed. Within group aggression may act as a disruptive force that causes groups to fission or the emigration of individuals, reducing the advantages of group life (e.g. Scarry & Tujague In Press). In a number of social species, particularly in primates, behavioural mechanisms have evolved to diminish the disruptive effects of aggression within the group, and to repair social relationships after a conflict (de Waal 1986, Kappeler & van Schaik 1992, Aureli & de Waal 2000, Aureli et al. 2002, Silk 2002b, Fraser & Bugnyar 2011). One mechanism of conflict management in primates are the mainly friendly post-conflict reunions (also called ‘reconciliation’) in which former opponents exchange affiliative behaviour soon after an aggressive conflict. Studies have shown that this affiliation reduces post-conflict rates of attacks between opponents, which are otherwise higher than baseline rates (Aureli & van Schaik 1991, Cords 1992, de Waal 1993, Watts 1995, Silk et al. 1996, Castles & Whiten 1998b, Koyama 2001, Kutsukake & Castles 2001). Postconflict affiliation may consist of several behaviours including body contact, embracing and, most prominently among primates, grooming (Arnold et al. 2011). Grooming is generally regarded as a behavioural mechanism that strengthens social bonds and helps to reduce social tension (Dunbar 2010) (Fig. 11), and this may explain why grooming sometimes occurs more often in association with higher rates of aggression due, for example, to feeding competition (e.g. baboons: Zinner 1993, Barrett et al. 1999). In tufted capuchins, dominant individuals seem to direct less aggression towards those subordinates that provide them with greater amounts of grooming (Tiddi et al. 2011).

Fig. 11: Male chacma baboon grooming a female. (Photo Dietmar Zinner)

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XIII. Back to the ‘Chimp War’ Does the ‘chimp war’ discussed at the beginning of the chapter make sense when examined from an evolutionary perspective? What are the fitness consequences of this intergroup-violence? Goodall (1986) found clear evidence that the primary function of the raids is the extension of territory, gaining access to more females and elimination of extra-group competitors. She wrote that among the male chimpanzees of Gombe, "territoriality functions not only to repel intruders from the home range, but sometimes to injure or eliminate them; not only to defend the existing home range and its resources, but to enlarge it opportunistically at the expense of weaker neighbours; not only to protect the female resources of a community, but to actively and aggressively recruit new sexual partners from neighbouring social groups" (Goodall 1986: 528).

Territorial expansion was later also found to be an effect of intergroup hostilities among chimpanzees in Uganda’s Kibale National Park (Mitani et al. 2010). For males, expanding the size of their territory could be adaptive either because it provides access to additional feeding resources, additional females or both. Among the Gombe chimpanzees, males defend a feeding territory for their resident females and protect them from sexual harassment by extra-group males, but they do not seem to directly acquire access to the females that formerly used their newly acquired territory (indeed they often attack even the females during raids!) (Williams et al. 2004). This does not preclude, however, that the possession of larger territory with greater resources will, with time, attract more females. Regardless of whether the expansion of territory increases access to food, access to females or both, it is clear that chimps that wage war on their neighbours stand to make substantial gains in fitness by doing so (Nishida et al. 1985, Goodall 1986, Williams et al. 2004, Watts et al. 2006, Mitani et al. 2010). But this also raises the question of what makes chimpanzees particularly vulnerable to lethal intergroup aggression? Why is it not more widespread among primates and other animals? A well supported hypothesis assumes that it is their ecology, in particular the spatial distribution of food resources (mainly fruiting trees). The food distribution favours living in a fission-fusion society, wherein the community regularly fissions into smaller parties (even into single solitary males or females), to exploit the resources in a certain area optimally, but comes together to feed in larger groups when the conditions allow it. The splitting into small fractions, however, provides possibilities for larger groups of neighbouring males to raid such small parties when they join forces temporarily to patrol the borders of their home-range (imbalance-of-powerhypothesis, Manson & Wrangham 1991). Such lethal attacks would be much more risky for the aggressors when attacking larger groups. Interestingly, raids by groups of males into the territories of neighbouring groups have also been reported among Central American spider monkeys (Aureli et al. 2006), a spe-

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cies which, although a distant relative of chimpanzees, shows remarkable convergence with them ecologically, morphologically, and in other aspects of their social behaviour: spider monkeys are highly dependent on ripe fruits and also live in fission-fusion societies in which males have a high degree of relatedness with other males in their group. Although no lethal attacks were documented during the relatively few spider monkey raids that were observed, the encounters with neighbouring groups that occurred during the raids were highly aggressive. The documentation of a similar behaviour in a distantly-related but socially and ecologically similar species supports explanations based on ecology imbalances-of-power.

XIV. Biological Roots of Human Warfare? Aggression is a near-universal biological phenomenon and it appears very prominently in our closest living relatives, the monkeys and apes. Given our close evolutionary relationship and the fact that aggressive behaviours in humans are driven proximately by similar proximate mechanisms (i.e. physiological and environmental factors) associated with aggression in other primates (e.g. increased testosterone: Archer 2006, contests for limited resources: Blanchard & Blanchard 2003), it is highly unlikely that aggression and violence in humans do not have the same biological roots as in other primates. Indeed, Wrangham et al. (2006) found similar rates of fatal violence in chimpanzees and human hunter-gatherer and subsistence farmer societies, although the rates of non-lethal aggression in chimpanzees are significantly higher than those in the human societies. The similarity between chimpanzees and humans with respect to the coalitional killing of conspecifics suggests that this behaviour may have already been present in the common ancestor we share with chimpanzees, estimated to have lived some 7.5 million years ago (Wilkinson et al. 2011), and that it was preserved during the evolution of both species. However, this hypothesis needs to be tested further (Crofoot & Wrangham 2010, Wrangham 2010). Another possibility is that the tendency for lethal aggression was absent in our common ancestor and evolved independently in both lineages. Given that humans are equally related to the relatively peaceful bonobo (‘pygmy chimpanzees’) as we are to the more aggressive ‘common chimpanzee’, it is difficult to distinguish between these alternative hypotheses. However, it seems more likely that aggression has decreased along the bonobo line (Hare et al. 2012), especially given that aggression is also common our next closest relative, the gorilla (Watts 1994). But even if lethal aggression did appear independently in humans and chimpanzees, identifying similarities in the factors underlying these behaviours can still provide keen insights into their evolution in both lineages. Indeed, the lethal intergroup encounters in chimpanzees do show a number of similarities with primitive warfare of humans (Goodall 1986, Manson & Wrangham 1991, Boehm 1992, van der Dennen 1995,

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Wrangham & Peterson 1996, Gat 1999, 2000). In both species, males regularly patrol boundaries of their territory cooperatively and systematically, and violently kill adults from neighbouring groups, sometimes leading to the extinction of whole communities. Of course, as mentioned above, primate social relationships are defined by cooperative interactions as much as they are by aggressive ones (Kappeler & van Schaik 2006, Sussman & Garber 2011). Still, the frequency and scope of cooperation in intragroup social behaviour in humans is not matched by any other primate species (Ridley 1996, Gurven 2004, Silk & Boyd 2010). It is hypothesised that human altruism and cooperation has evolved as a reaction to pressure from intergroup competition and conflict (Bowles 2006, O’Connor 2006, Crofoot & Wrangham 2010). Individuals who communicate and cooperate better either for attack or defence would have an advantage in intergroup competition and in turn gain higher fitness. But the evolutionary origins of human altruism are subject to much debate and will undoubtedly be the focus of much research in the years to come.

XV. Acceptance of the Evolutionary Perspective Since the publication of E.O. Wilson's influential book Sociobiology in 1975, the evolutionary perspective on human behaviour has been often criticized from a number of angles, ranging from biological arguments that most aspects of human behaviour are not the result of natural selection (e.g. Gould 1991) to political arguments that the application of socio-biological principles to human behaviour serves to justify the malevolent goals of social Darwinists (for an overview see Sommer 1992). Although criticisms of the underlying science are normally constructive, injecting politics into science rarely (if ever) is. But it is indeed important to consider non-adaptive explanations for the evolution of behaviour as well as adaptive ones, and we should be careful not to assume without strong evidence that similar behaviours in different species are driven by the same proximate and adaptive factors (Sussman & Marshack 2010). In the current case, the evidence that a better understanding of aggression in nonhuman primates can have important implications for understanding aggression in humans is indeed strong. But that’s not to say that culture does not also shape human aggression in ways that that aren’t experienced in other species (Sussman & Marshack 2010) resulting in patterns of social relationships between groups of humans that are not observed in chimpanzees or other primates (Alexander 1989, Knauft 1991, Foley 1995, Kempf 2003, Otterbein 2004, Kelly 2005). But just as we shouldn’t ignore cultural influences on human aggression, ignoring the evolutionary biology of human aggression would also be a mistake. Although objections are still raised in some quarters (e.g. MacKinnon & Fuentes 2005, Sussman & Marshack 2010), the acceptance of the evolutionary perspective on human aggression and violence has grown in recent

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years. Evolutionary thinking, in particular the ‘Darwinian Theory of Conflict’, has resulted in fruitful discussions in the social sciences and criminology and to the formation of new hypotheses and interpretations (Sanderson 2001, Eisner 2001, Meleghy et al. 2008, Pinker 2011, Schlomer et al. 2011, Wood 2011). Today the evolutionary perspective is accepted as one of several equitable approaches to understanding the causes, mechanisms and functions of aggression and violence in humans and other animals. But it is in no way suited to infer or justify any moral or ethical rules for the behaviour of humans. It is about understanding the biological basis of aggression and violence and its adaptive significance. Such an understanding is critical to the goal of reducing the prevalence of these behaviours in society. To say it in Peter Singer’s (2000: 38) words: “To be blind to the facts about human nature is to risk disaster”.

XVI. Acknowledgments DZ is indebted to Henning Kortüm and Jürgen Heinze for inviting him to present the evolutionary perspective at their symposium in Regensburg. We also thank the other participants of the symposium for their discussions, comments and suggestions.

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Explaining Aggression: The Ultimate-Proximate Problem CLAIRE EL MOUDEN “A TEENAGER was left bruised and bloodied after being set upon by two youths in an unprovoked attack. The incident happened at around 9pm on Tuesday night on a footpath near Marlborough Way. Two youths approached a 19-year-old man and asked him for a lighter. When he said he didn't have one, they punched him to the floor and then kicked and stamped on him as he lay on the ground. After the attack, the injured man managed to make his way to a relative's house and raise the alarm. He suffered bruising to his body and cuts and bruises to his head and face and was treated by paramedics.” 1

Whenever I read news reports like this, the evolutionary biologist in me starts asking questions. Why is unprovoked violence so common in humans? Why is it particularly common in young men? Are we unique, or are other animals this violent, too? From crazed gunman to football hooligans and from playground bullies to the youths in Tamworth, our society seems to be full of people willing and wanting to harm and even kill each other. Why? If you expect to learn the evolutionary explanation for specific violent behaviours, then you are going to be disappointed in this chapter. Evolutionary biology is rarely able to explain specific behaviours like the attack in Tamworth and the aim of this chapter is to explain why. Part one summarises the ‘evolutionary explanations’ for aggression, and outlines the distinction between proximate and ultimate explanations. Part two considers why evolutionary explanations for specific behaviours are not possible, and discusses the widespread inaccurate evolutionary interpretations in the human literature. This chapter shows that the power of evolutionary theory lies not in its ability to explain every detail of a specific event (for I hope to convince you it cannot), but in the fact it provides a general framework for enquiry which we can use to ask questions that help us develop a better understanding of our behavior.

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Report from a local British newspaper, the Tamworth Herald, May 25th 2012. Tamworth Herald (2012).Youths punch and stamp on teen in unprovoked attack. Available: http://www.thisistamworth.co.uk/Youths-punch-stamp-teen-unprovoked-attack/story16184642-detail/story.html. Last accessed 3rd July 2012.

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Part I I.1 Evolutionary Theory The fundamental goal of evolutionary biology is to explain the presence of adaptation in nature i.e. the fact that organisms appear designed (Williams 1966, Leigh 1971, Maynard Smith 1982, Gardner 2009). This fact was first recognized by the eighteenth century Christian philosopher William Paley, who argued in Natural Theology that organisms appear designed because they possess many parts that are all contrived for a common purpose (Paley 1802). For example, one cannot understand the function of an eye’s retina in isolation unless we first appreciate that it is contrived, together with the cornea, lens, muscles and nerves for the common purpose of seeing. Paley argued the presence of adaptation was inexplicable, unless one accepted the existence of a godlike designer. Inspired by his work, fifty years later Darwin’s Origin of Species (1859) solved Paley’s ‘problem’ differently, by showing that complex design could arise via a natural process. Darwin considered two distinct issues when developing his theory of natural selection. First, what is the process that generates these adaptations? Second, what is the apparent purpose of the resulting adaptation i.e. what is it that organisms appear designed to do? Darwin’s (1859) key insight was that over time, heritable traits that are associated with greater reproductive success will tend to accumulate in a population, and that those associated with reduced reproductive success will tend to disappear. The result of this process, Darwin argued, is that organisms will appear increasingly well designed to maximize their reproductive success. Darwin’s verbal arguments were shown to be mathematically correct by Fisher (1930, Provine 2001). In his seminal work The Genetical Theory of Natural Selection, Fisher showed first that the process of natural selection works through changes in gene frequencies and second that genes associated with a greater individual fitness would be predicted to accumulate in natural populations, which lead him to conclude that organisms would appear designed to maximize their Darwinian fitness. Thus Fisher confirmed that Darwin had solved Paley’s ‘problem’ in two ways: first, showing how complex design (adaptation) could result from a natural process and second, by identifying what natural design was ultimately for  to maximize Darwinian fitness (Gardner 2009). Since Darwin, the only person to significantly extend his ideas has been Hamilton. He showed that the purpose of natural design was not to maximize ‘Darwinian fitness’ but ‘inclusive fitness’, which incorporates the indirect effect on an individual’s fitness of relatives who share their genes (Hamilton 1964). Formal studies have confirmed that inclusive fitness is the fitness that organisms should appear designed to maximize, so this is the purpose of adaptation (Grafen 2002, 2006, 2007). By correctly identifying what natural selection max-

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imizes, Hamilton’s insight allowed better models of animal behavior to be constructed that made more accurate, testable predictions. This helped behavioural ecology flourish as a discipline. In particular, inclusive fitness theory was instrumental in understanding the diversity of sociality in the natural world, as it explained the presence of cooperative behaviours which are selected, at least in part, for their beneficial effect on others (West et al. 2007, Hamilton 1996).

I.2 The Proximate-Ultimate Distinction To fully understand any behavior, such as aggression in young men, we must establish what both the proximate and ultimate explanations are (Mayr 1961, Tinbergen 1963). The distinction between proximate and ultimate explanations is absolutely fundamental and anyone who studies behavior must be absolutely clear which type of explanation they are seeking (Scott-Phillips et al. 2011). Proximate explanations concern the causal mechanisms that underlie a behavior i.e. how it occurs. A complete picture would include, inter alia, understanding how the behavior is underpinned by the genes that control brain development and hormone expression, how the behavior is changed by different economic, social, cultural and environmental conditions and how these internal and external factors interact. By contrast, ultimate explanations concern the biological fitness consequences of a behavior by explaining why it is (or is not) favoured by natural selection i.e. why it occurs. Given that natural selection maximizes an individual’s inclusive fitness, the ultimate explanation must show why the behaviour yields the greatest fitness benefits compared to the set of possible alternatives. To illustrate the difference between proximate and ultimate explanations, consider how we might explain the observation that fighting is particularly common between young men. The ultimate explanation may be that fights help establish the adult social dominance hierarchy. This is an ultimate explanation because it concerns the biological fitness benefits of the behaviour: males which gain a higher position in the hierarchy can dominate access to fitness-enhancing resources such as food or females (i.e. it explains why winning fights increases a male’s inclusive fitness). The proximate explanations include the external triggers for aggression such provocative language or gestures, factors such as the number, age and relationship between potential aggressors and aggresses, their cultural background and childhood experiences. Proximate explanations also include the internal triggers for aggression such as levels of testosterone and serotonin, and the activity in the brain’s hypothalamus, prefrontal cortex and amygdala. A further level of proximate explanation is genetic, such as understanding how the MAO-A gene codes for proteins that affect aggression and how different genes interact with environmental factors. These diverse explanations are all proximate because they describe the causal triggers of aggression.

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As this example illustrates, proximate explanations encompass many disciplines. However, one class of proximate explanation is not more important, or more ‘right’ than another. Furthermore, no proximate explanation can answer an ultimate question (the answer cannot provide a solution to the other). Indeed a fuller understanding of behaviour is gained by considering the ultimate and full range of proximate explanations. There is often a synergy between the explanations (i.e. a better understanding of a proximate explanation will improve our understanding of the ultimate explanation, and vice versa). Examples of the successful interplay between proximate and ultimate explanations can be found in any introductory textbook on evolution and behavior (e.g. Alcock 2009, Davies et al. 2012) or evolutionary psychology (Nettle 2009). Recently, disciplines as diverse as agriculture, clinical medicine, microbiology, conservation biology, international relations, criminology and behavioural economics have become interested in the other explanations for the behaviours they study (Davies et al. 2012). This has included interest in both the other proximate and ultimate explanations. For example, behavioural economists are investigating the proximate explanations that neuroscience, psychology and genetics can offer and are proposing ultimate explanations for the sociality they observe (Rustichini 2009, Kosfeld 2005, de Quervain et al. 2004, McDermott et al. 2009). Such ‘silo-busting’ interactions are of course a good thing. However, interdisciplinary work is notoriously difficult, as misconceptions are common at the interface of multiple disciplines, as it is impossible for researchers to know all the literatures completely (West et al. 2011).

I.3 Ultimate (Evolutionary) Explanations for Aggression Over the past 40 years, an extensive literature has developed within evolutionary biology to explain how natural selection may favour the evolution of aggressive or violent behaviours (Enquist & Leimar 1990, Innocent & West 2006). Aggression is a proximate description of a behavior, and it results from an evolutionary conflict where the fitness interests of two or more individuals clash (Maynard-Smith & Price 1973). Evolutionary conflicts are extremely common and arise in many situations, when there is competition between individuals for food, mates or territory (Davies et al. 2012). Familial conflicts, where the fitness interests between mates, parents and offspring or siblings are not aligned are also common and include males coercing females to mate (Barash 1977), chicks throwing their siblings from the nest (Mock & Parker 1997) and gestational diabetes, which is caused by the conflict between mother and fetus over resources (Haig 1993). Conflict also lies at the heart of sociality, due to the tension between individual and group interests (Hamilton 1996). It can manifest in the communal defense of group resources (Brown 1982), the repression of social cheats via ostracism, punishment, policing or sanctions (Clutton-Brock & Parker 1995), and in spiteful acts, which indirectly benefit kin by

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hurting unrelated competitors (West et al. 2007, West & Gardner 2010). For example, lazy meerkats that do not feed pups are attacked (Clutton-Brock et al. 2005), if worker bees lay eggs (instead of rearing the queen’s), they get eaten (Ratneiks & Visscher 1989) and bacteria will engage in spiteful suicidal chemical warfare (Gardner et al. 2004). Conflict is extremely common, but aggression and fighting is not (Enquist & Leimar 1990). This is because, the fitness costs of fighting are very high (due to the risk of injury or death), which means aggression is not usually adaptive. Instead, animals minimize the fitness costs of conflict by evolving peaceful solutions, such as turn-taking, displays or ritualized behaviours that allow opponents to assess each other safely (Parker 1974). To determine whether or not aggression and fighting is adaptive requires careful accounting of the costs and benefits incurred in terms of individual fitness (Innocent & West 2006). There are numerous factors which influence this calculation including the number, density, quality and social rank of competitors, the rarity and value (both now and in the future) of the contested resource, the relatedness between competitors and the individual’s assessment of their fighting ability relative to the opponent (Innocent & West 2006, West et al. 2002). Thus when we observe aggression, we must ask ourselves why a more peaceful form of conflict resolution was not adaptive. An important point here is that one ultimate explanation can underpin behaviours that lie on a spectrum from totally peaceful to extremely violent. For example, the ultimate explanation of male-male mate competition over access to females, explains both the peacefully resolved competition amongst male lion coalitions and the extremely violent fights between male elephant seals. Examining the fitness costs and benefits of aggression in each case enables us to understand the difference. Elephant seal females are crowded on beaches so strong males can control large harems, meaning a minority of the strongest males dominate reproduction (Le Boeuf 1974). If gaining control of a harem is the only way to breed, a fully-grown male has nothing to lose and everything to gain, which makes extremely violent fights adaptive. By contrast, male lions resolve conflict peacefully because they are normally brothers, they conflict only over access to a single female and there are multiple opportunities to mate in the future (Packer & Pusey 1982). This discussion illustrates why it is necessary to understand both the fitness costs and benefits and the type of evolutionary conflict and individual faces, when considering the appropriate ultimate explanation for an aggressive act. However, as part two illustrates, care is needed as ultimate explanations are not possible for every observed behavior, and inaccurate claims are often made about the ultimate explanations for human aggression.

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Part II II.1. The Ultimate Explanation for Specific Cases cannot be known This section explains why evolutionary explanations for specific behaviors like the Tamworth attack are not possible. Evolutionary theory may offer ultimate explanations for general behaviors, such as increased aggression between young men, but it cannot provide an adaptive justification for every specific event. The reason for this is tied up with the fact that ultimate explanations are only possible when a behavior maximizes fitness and specific behaviors may not do this. It is useful to divide the following discussion into four points: first, natural selection is not the only evolutionary force; second, natural selection acts on the average fitness consequences of a behavior; third, environmental change can lead to mistakes; and fourth, in the trade-off between accuracy and ease, natural selection tends to favour quick and cheap, not precise and complex decision-making rules. First, there is more to evolution than natural selection (Fisher 1930). Environmental effects such as unpredictable floods, droughts, fires and storms, population movements and genetic mutation will affect an individual’s probability of surviving and reproducing (i.e. fitness). These are all evolutionary forces because over time, they cause changes in the population average gene frequencies. The difference between these forces and natural selection is they do not maximise anything, they have no ‘purpose’ so they degrade the appearance of design produced by natural selection. In other words, non-selective evolutionary forces divert phenotypes away from the fitness-maximising optimum. As non-selective evolutionary forces are often the dominant cause of gene frequency change, there is no expectation that individuals will reach the goal of optimality, or even be anywhere near it, just that natural selection will direct genetic variation in that direction. Thus we expect irrational behavior, which does not maximize anything, to be commonplace. This point matters, because it means if we behave irrationally, while it may seem unsatisfactory, ‘weF not expected to be rational all the time’ may be the best ultimate explanation (El Mouden et al. 2012). Second, natural selection acts upon the average fitness consequences of a particular behaviour. A behaviour is favoured when the fitness cost/benefit analysis is favourable, which means it may spread even if it is associated with some fitness costs (Davies et al. 1992). These costs could cause the odd individual to make a very costly error, or for many people to incur occasional costs. Of course, natural selection will minimize these errors, but they cannot be eliminated if they are inextricably linked to the benefit (e.g. it may not be possible have imaginative, intelligent minds without the risk of developing psychiatric disorders). These behaviours are not adaptive, they are best understood as costly side effects, and hence an explanation for them in isolation is not possible. This means that if an aggressive behaviour represents an isolated case,

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there may be no specific evolutionary explanation for it (but it may be understood as a costly side effect of an otherwise adaptive behavior). Third, in the evolutionary trade-off between precision, time and effort, natural selection tends to favour quick and cheap adaptations. This explains why human subconsciously rely on simple rules-of-thumb, when making decisions (known in psychology and economics as heuristics or cognitive biases; Kahneman & Tversky 1972, Tversky & Kahneman 1974, Gilovich et al. 2002, Gigerenzer & Gaissmaier 2011). Rules-of-thumb are extremely important for humans as they enable reliable, fast decisions to be made in commonly encountered situations with minimal cognitive effort. However, when we are placed in unfamiliar environments, or are faced with complex calculations, they can cause us to behave irrationally, albeit often in predictable ways (Gilovich et al. 2002, Pohl 2004, Haselton et al. 2005). Natural selection will minimize the costs of such mistakes, but it may not be economic to eliminate them. For example, we have a hard-wired bias to be altruistic towards in-group members (Burton-Chellew & West 2012, Burton-Chellew et al. 2010). This was adaptive during our evolutionary past, as our ingroup represented our extended family and long-term friends, whereas our out-group contained potentially dangerous competitors. However today, this hard-wired bias helps fuel xenophobia, gang violence and football hooliganism. Fourth, as the third point implies, novel environments lead to non-optimal behaviour. No matter how clever, or behaviorally flexible an organism is, if its decision-making is influenced by rules-of-thumb, it cannot respond optimally to conditions outside the range of natural variation it or its species has encountered before. As the physical and cultural environment humans live in today is so radically different from the one we evolved in, it is unsurprising that we sometimes exhibit behaviours that are not adapted for our current environment. In humans, irrational behaviour due to changed environments is variously known as the ‘mismatch hypothesis’ (Hagen & Hammerstein 2006), artifact biases (Haselton et al. 2005) and, in cooperation research, the ‘big mistake hypothesis’ (Boyd & Richerson 2005). This issue is particularly relevant when studying conflict because demography and population structure often determine whether a conflict exists, and will influence what sort of behaviours evolve as a result. Thus aggression between unrelated youths in a quiet town like Tamworth may not be adaptive, but the proximate triggers that led to the behavior, such as social pressure, or the production of testosterone may have been adaptive if the environment in which they evolved was one where the cost/benefit analysis meant aggression was the appropriate response.

II.2 Proximate Explanations do not answer Ultimate Questions Earlier, the discussion of proximate and ultimate explanations made clear they are different and explained why an answer to one cannot solve the other. Un-

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fortunately, this distinction is not well appreciated in the human literature, and the result is that proximate mechanisms are frequently presented as solutions to an ultimate question (Scott-Phillips et al. 2011). For example, one suggestion is that if people fail to cooperate, anger motivates us to punish them ‘spitefully’ (i.e. at a cost to us and them) and that this explains the origins of human cooperation (Marlowe et al. 2011). However, this behavior and its emotional motivator are descriptions of psychological phenomena – they explain (if the hypothesis is correct) how humans behave cooperatively. They do not explain the origins of cooperation i.e. why this behavior is present in the first place. It may be, for example, the case that individuals who get angry and punish people who do not cooperate with them have lower inclusive fitness than those who do not get angry and do not punish, or those that control their anger and punish more selectively  in which case indiscriminate ‘spiteful’ punishment would be selected against. To explain why such ‘spiteful’ punishment exists in the first place entails reference to inclusive fitness effects and consideration of the set of alternative behavioural strategies which natural selection had to ‘choose’ from. Behavioural studies are, of course, extremely valuable for evolutionary biologists as they can help reveal how fitness benefits are gained. Indeed, ultimate explanations are usually developed via an iterative process of model making, empirical testing and refinement that necessitates close collaboration between empiricists and theoreticians (Davies et al. 2012). In the human literature, the confusion between proximate and ultimate explanations seems to occur because the difference between defining costs and benefits in terms of time, money or effort and costs compared to lifetime average inclusive fitness is not appreciated (El Mouden et al. 2012). This problem is neatly illustrated by a recent article published on pschologytoday.com entitled “The evolution of spiteful behaviour”, concerning car drivers using their horns: “when I drive on the streets of California and I ignore a stop sign or a red light other drivers honk at me even though they are not directly affected by my actions. These drivers are willing to risk getting a ticket for honking to increase the chance that I will get a ticket for violating another traffic rule. Evolutionarily, behavior that carries a cost to the actor and a cost to the recipient is not selfish: it’s spiteful.”2

Here, the author quantifies the cost of honking and being honked (in terms of risk of getting a ticket) and assumes they are equivalent to fitness costs. This problem is extremely common in behavioural economics where authors frequently equate money profit or loss with fitness costs and benefits, probably due to the superficial similarities between economic rational choice models (which assume utility or profit is maximized in real time) and evolutionary models (which assume lifetime average inclusive fitness consequences are maximized). _____________ 2

Maestripieri, D. (2012). The Evolution of Spiteful Behavior; Available: http://www.psychology today. com/blog/games-primates-play/201206/the-evolution-spiteful-behavior. Last accessed 5th July 2012.

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The error here is one of degree, not kind. In principle, there is nothing wrong with using proxy measures for fitness and behavioural ecologists normally use proxies such as number of offspring, number of mates, food intake or body size in their experiments. However, they know that animals will not appear to maximize a bad proxy measure (i.e. not closely correlated with inclusive fitness), so when an animal appears not to behavior rationally, one of the first questions is whether the proxy measure is appropriate for the question being asked. Thus economic models which assume people are designed to maximize their income suffer from the same problem as models that assume a stag is designed to maximize its antler length or a bee its nectar intake; they will only be accurate insofar as income, antler length or nectar intake are accurate proxies for inclusive fitness. It seems highly unlikely that the risk of getting a traffic ticket could ever be a suitable proxy. Furthermore, as isolated behaviours may not be adaptive (i.e. not maximizing anything), car honking, is too specific, too isolated, too insignificant in terms of lifetime fitness costs and too far removed from the environment we evolved in to be given a sensible ultimate explanation. This does not mean evolutionary theory has nothing useful to say about such behaviours. Indeed this example raises questions ripe for evolutionary investigation: why does natural selection favour humans that experience anger when others break culturally accepted norms of good behavior? Furthermore, why does natural selection favour individuals that still try to break them? And why were these cultural norms favoured in the first place? This last question touches on an important issue: the status of cultural evolution.

II.3 Culture as an Ultimate Explanation A discussion on the proximate-ultimate issue and humans would be incomplete without mentioning the debate concerning cultural evolution (Laland et al. 2011, Scott-Phillips et al. 2011). People who study cultural evolution (by assuming cultural change may be modeled as an evolutionary process, driven by natural selection acting on cultural norms; Boyd & Richerson 1985, 2005) argue that ultimate explanations are also needed for culturally inherited adaptations (Laland et al. 2011). For example, consider a society where young men establish their social dominance hierarchy by rituals such as the Maasai’s adumu, jumping dance. The ultimate genetic explanation is that jumping high increases a male’s inclusive fitness, by increasing his chances of attracting the best mates. A ritualized jumping dance is beneficial as it allows males to resolve their conflict peacefully, which avoids the fitness costs of violence. However the dance is a cultural tradition, so its origins require a cultural explanation that shows why the Maasai’s adumu dancing persisted and spread rather than other cultural norms to limit aggression. If cultural evolutionary processes

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can explain the persistence of such norms, it suggests that they are a complimentary part of the ultimate explanation. However, many disagree with this perspective (Scott-Phillips et al. 2011, Sperber & Claidiere 2008). Culturally inherited norms impact an individual’s biological fitness, therefore natural selection will favour individuals that can identify and adopt ideas that increase their biological fitness. Thus, the counterview is that culture is part of the proximate explanation and the ultimate explanation should focus on explaining why our cognition and learning abilities have enabled us to identify and retain ideas that maximize biological fitness. While cultural change may be described using evolutionary models, these authors would argue that it is human minds that drive the change, not natural selection. To some degree, the debate is a matter of perspective. What is clear is that whether one views cultural change as a product of a separate process of natural selection or simply clever minds, the knowledge we acquire from others is a crucial component of our fitness and must form an important part of any ultimate explanation for aggression in humans (West et al. 2011).

Part III: Conclusion Conflict is pervasive in the natural world, so consequently, natural selection has found many ways to resolve it. As intensely social animals, humans are exposed to many conflict scenarios, so evolutionary biology has a great deal to offer those interested in studying aggression. By understanding how we were shaped by the environment we evolved in, we can understand why we rely upon particular rules-of-thumb, why the balance between aggression and peaceful resolution of conflicts has been set as it has, and broadly, why we experience the emotions that we do, when we do. However, given how hard it is to test model predictions in humans (for very obvious reasons, we are not as easy to experiment on as bacteria, insects or birds), there is the risk that ultimate explanations become little more than ‘adaptationist storytelling’. To avoid this trap, we must use our understanding of evolutionary theory to ask the right questions: what are the direct and indirect costs and benefits of the proximate behavior? Who performs it? Is it abnormal? Is this behavior closely linked to a more general class of behavior? Does it concern a decision with real fitness consequences? Does it occur in familiar environments? What am I assuming the person is maximizing? Is this a good proxy for fitness? Ultimate explanations make us take a step back from the action and think about big underlying questions that influence our everyday behaviour and when used appropriately, the benefits of an evolutionary approach are immense.

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References Alcock, J. (2009). Animal Behavior. Sunderland, MA: Sinauer Associates. Barash, D. (1977). Sociobiology of Rape in Mallards (Anas platyrhynchos): Responses of the Mated Male. Science, 197 (4305), 788-789. Boyd, R. & Richerson, P. J. (1985). Culture and the Evolutionary Process. Chicago: Chicago University Press. Boyd, R. & Richerson, P. J. (2005). Solving the Puzzle of Human Cooperation. In S. Levinson & P. Jaisson (Eds.), Evolution and Culture (pp. 105-132). Cambridge, MA: MIT Press. Brown, J. L. (1982). Optimal Group Size in Territorial Animals. Journal of Theoretical Biology, 95, 793-810. Burton-Chellew, M. N. & West, S. A. (2012). Pseudo-Competition among Groups increases Cooperation in a Public Goods Game. Animal Behaviour. In Press. Burton-Chellew, M. N., Ross-Gillespie, A. & West, S. A. (2010). Cooperation in Humans: Competition between Groups and Proximate Emotions. Evolution and Human Behavior, 31, 104-108. Clutton-Brock, T. H. & Parker, G. A. (1995). Punishment in Animal Societies. Nature, 373, 209-216. Clutton-Brock, T. H., Russell, A. F., Shape, L. L. & Jordan, N. R. (2005). ‘False Feeding’ and Aggression in Meerkat Societies. Animal Behaviour 69 (6), 1273-1284. Darwin, C. R. (1859). The Origin of Species. London, UK: John Murray. Davies, N. B., Hatchwell, B. J., Robson, T. & Burke, T. (1992). Paternity and Parental Effort in Dunnocks Prunella Modularis: How Good are Male Chick-Feeding Rules? Animal Behaviour, 43, 729-745. Davies, N. B., West S. A. & Krebs, J. R. (2012). An Introduction to Behavioural Ecology, Third Edition, 4th Edn. Oxford, England: Blackwell Scientific Publications. de Quervain, J. F., Fischbacher, U., Treyer, V., Schellhammer, M., Schnyder, U., Buck, A. & Fehr, E. (2004). The Neural Basis of Altruistic Punishment, Science 305, 1254–1258. El Mouden, C., Burton-Chellew, M., Gardner, A. & West, S. A. (2012). What do Humans maximise? In S. Okasha & K. Binmore (Eds.), Evolution and Rationality: Decisions, Cooperation and Strategic Behaviour (pp. 23-49). Oxford: Oxford University Press. Enquist, M. & Leimar, O. (1990). The Evolution of Fatal Fighting. Animal Behaviour, 39, 1-9. Fisher, R. A. (1930). The Genetical Theory of Natural Selection. Oxford, UK: Clarendon. Gardner, A. (2009). Adaptation as Organism Design. Biology Letters, 5, 861-864. Gardner, A., West, S. A. & Buckling, A. (2004). Bacteriocins, Spite and Vilence. Proceedings of the Royal Society London Series B, 271, 1529-1535. Gigerenzer, G. & Gaissmaier, W. (2011). Heuristic Decision Making. Annual Review of Psychology, 62, 451-458. Gilovich, T., Griffin, D. & Kahneman, D. (2002). Heuristics and Biases: The Psychology of Intuitive Judgment. Cambridge: Cambridge University Press. Grafen, A. (2002). A First Formal Link between the Price Equation and an Optimization Program. Journal of Theoretical Biology, 217, 75-91. Grafen, A. (2006). Optimization of Inclusive Fitness. Journal of Theoretical Biology, 238, 541–563. Grafen, A. (2007). The Formal Darwinism Project: A Mid-Term Report. Journal of Evolutionary Biology, 20, 1243–1254.

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Ratnieks, F. & Visscher, P. K. (1989). Worker Policing in the Honeybee. Nature, 342, 796797. Rustichini, A. (2009). Neuroeconomics: What have We found, and what should We search for? Current Opinion in Neurobiology, 19, 672-677. Scott-Phillips, T. C., Dickins, T. E. & West, S. A. (2011). Evolutionary Theory and the Ultimate/Proximate Distinction in the Human Behavioural Sciences. Perspectives on Psychological Science, 61, 38-47. Sperber, D. & Claidière, N. (2008). Defining and explaining Culture. Biology and Philosophy, 23, 283-292. Tinbergen, N. (1963). On Aims and Methods of Ethology. Zeitschrift für Tierpsychologie, 20, 410-433. Tversky, A. & Kahneman, D. (1986). Rational Choice and the Framing of Decisions. Journal of Business, 59 (4), S251-S278. West, S. A. & Gardner, A. (2010). Altruism, Spite and Greenbeards. Science, 327, 13411344. West, S. A., El Mouden, C. & Gardner, A. (2011). Sixteen Common Misconceptions about the Evolution of Cooperation in Humans. Evolution and Human Behavior, 32, 231-262. West, S. A., Griffin, A. S. & Gardner, A. (2007). Evolutionary Explanations for Cooperation. Current Biology, 17, R661-R672. West, S. A., Pen, I. & Griffin, A. S. (2002). Conflict and Cooperation – Cooperation and Competition between Relatives, Science, 296, 72-75. Williams, G. C. (1966). Adaptation and Natural Selection. Princeton: Princeton University Press.

Human Sex Differences in Aggression from the Perspective of Sexual Selection JOHN ARCHER

I. Sexual Selection Sexually reproducing organisms are characterized by the production of two types of germ cells (gametes), eggs (ova) and sperm (spermatozoa), the first specialized for storing food for the developing offspring, and the second for providing only genetic material. In humans and most other vertebrates, eggs and sperm are located in two different individuals, males and females. In addition to possessing different-sized gametes, males and females show many other differences in form that are unconnected with reproduction. The term sexual dimorphism was first used by Charles Darwin (1859) in On the origin of species, following the use of the similar terms “secondary properties” and “secondary characters” by Hunter (1780: 528, 531) – also referring to features that differed between the sexes and were not directly connected to reproduction. Whereas natural selection refers to selection from any part of an organism’s environment, sexual selection refers to selection in relation to an individual’s mating opportunities. It is clear that males and females do not mate at random, but have preferences for some individuals over others: the result of this process is that some are more successful than others, leave more offspring in future generations. It was the role of male competition in this process that was first noticed. Hunter (1837: 45) stated that “The males of almost every class of animals are probably disposed to fight, being, as I have observed, stronger than the females”. He went on to note that males of many species had body parts specifically adapted for fighting. Darwin (1859/1911: 77) similarly described the presence of natural weapons in males of many species, and developed this further, noting the “struggle between the males for possession of the females”, which he termed “sexual selection” to distinguish it from natural selection. This was greatly elaborated it in The descent of man (Darwin 1871/1901), where chapter after chapter described the secondary sexual characteristics of lower invertebrates, insects, fish, birds and mammals, the last two occupying six chapters. Darwin’s discussion, like that of Hunter before him, emphasized male competition. He wrote: “It is certain that amongst all animals there is a struggle between the males for possession of the females. This fact is so notorious that it

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would be superfluous to give instances” (Darwin 1871: 326 from 1901 edn.). Although Darwin emphasized male competition, his concept of sexual selection also involved a second process, female choice, which he first viewed as arising from male competition. He viewed females as selecting “vigorous and well armed” males. His subsequent elaboration stated that female choice can be independent of this, when females select males with “ornaments or other attractions”. The third part of The descent of man brought together the previous two sections, on human evolution and sexual selection. Here Darwin used both the principles of male competition (“the law of battle”) and female choice to seek to understand the origins of human sex differences, both physical and psychological. Darwin was in no doubt as to the origin of human sex differences in aggression and other psychological characters. He wrote: “Man is more courageous, pugnacious and energetic than woman” (p. 847)1. The implications of this statement for the psychology of sex differences have either been elaborated (Archer 1996, 2009, Geary 2009) or played down (Hyde 2005) in recent accounts. Although there were many theoretical advances in the elaboration of sexual selection in the century following Darwin’s writings, it was Trivers (1972) who was most influential in advancing the understanding of the evolution of behavioural sex differences, human and non-human. Put simply, they derive from the initial specialization of females and males for producing, large foodstoring gametes and small motile gametes respectively. Eggs involve a high initial investment in time and energy whereas sperm do not. There is therefore an intrinsic limitation to a female’s reproductive output that does not apply to the male. His reproductive output is therefore likely to be limited extrinsically, via the two processes of sexual selection, female choice of certain males, and inter-male competition for access to females or to resources necessary for this access. Trivers’ theory is known as the parental investment theory of sexual selection, and it holds that sex differences in form and behaviour arise not from any inherent aspect of being male or female but what is likely to follow from specializing in producing one of two types of germ cell. This specialization typically leads to the sex that has initially invested more (the female) subsequently continuing to invest more than the male, unless there are selection pressures to counter this process. Further theoretical models examined which sex was likely to desert the offspring first under different conditions (e.g. Lazarus 1990, Maynard Smith 1977). In many bird species it is the number of offspring that can be fed that is the main limitation on successful reproduction: under these conditions, it pays neither parent to desert, resulting in a monogamous mating system with minimal sexual dimorphism (Maynard Smith 1977). In some cases, _____________ 1

I have omitted the remainder of this sentence, which read “and has a more inventive genius”, out of sensitivity to modern thinking on this issue. But see Baumeister (2010) on this issue.

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such as in wading birds such as phalaropes, the male makes a larger initial investment and continues caring for the eggs and young, resulting in a polyandrous mating system (one male and several females), with reversed sexual dimorphism (Jenni 1974). Where conditions are such that the offspring are viable when cared for only by the female, as it is in most mammals as a consequence of lactation (Maynard Smith 1977), polygyny will be the norm. In terrestrial mammals, a polygynous mating system is associated with a number of sexually dimorphic features (Andersson 1994), which are the consequence of greater inter-male competition: (1) males show more escalated aggression to other males than females show to other females; (2) these sex differences tend to be more pronounced in young adulthood; they are accompanied by (3) males having natural weapons and showing aggressive displays, and (4) greater male than female size and strength. As a consequence of these features, there will be: (5) higher male than female mortality rates at all ages, which is accompanied by a greater male than female conception rate; (6) later sexual maturation in males than in females; and (7) greater male than female variation in reproductive success, which can be viewed as an index of effective polygyny.

II. Sexual Selection in Humans Before examining whether each of these applies to humans, I consider briefly whether humans have evolved in a different direction, towards a monogamous mating system. Monogamy does occur in some primate species, although it is usually associated with male territoriality (Clutton-Brock & Harvey 1977). In humans, males and females do show the prolonged attachments associated with the evolution of paternal care (Eastwick 2009, Fraley et al. 2005, Geary 2000), so that their relationships involve the social pair bonding associated with a monogamous mating system. A comparative analysis of both mammals and anthropoid primates by Fraley et al. (2005) indicated that paternal care was clearly associated with monogamy, and the authors suggested that one prepared the way for the other in evolutionary history. Mechanism originating for parent-to-offspring bonding became co-opted for bonding between sexual partners. Likewise, Eastwick (2009) argued that pair-bonding and attachment were built upon an earlier more polygynous pattern. According to these analyses, we would expect some lessening in indicators of sexual selection in humans compared to their nearest living relatives, and compared to earlier hominins. Other analyses have emphasized a polygynous mating system as forming the basic ancestral pattern of humans. Alexander, Hoogland, Howard, Noonan, and Sherman (1979) argued that the apparently widespread existence of monogagmy found in the world today is culturally imposed on a basic pattern of polygyny, or at least a flexible system that allows polygyny. The exception is monogamy that is found under harsh ecological conditions. Based on a histori-

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cal analysis, Betzig (1986, 1992) has argued that where there is great inequality between men, as occurred throughout history, the mating system was in effect polygynous, even if the only legally-sanctioned marriage was monogamy. Throughout history, powerful men have had a disproportionate number of sexual partners and have left a disproportionate number of offspring (Betzig 1986, 1992, Daly & Wilson 1988). We therefore have two views of the human mating system, and they probably emphasize different aspects of the behaviour that is possible in human males, which probably predominated in different individuals and under different conditions in human pre-history and history. The present chapter contributes to this debate by first assessing whether the pattern of sex differences in human aggression is one that fits an explanation in terms of sexual selection; and second whether other differences between men and women are those we should expect in a sexually-selected species.

III. Sex Differences in Human Aggression Psychologists recorded sex differences in human aggression in the 1920s, and narrative reviews were published from the 1950s onwards, culminating in a synthesis by Maccoby and Jacklin (1974). Systematic meta-analyses began in the 1980s, and have continued to the present time, providing a much more precise analysis of the evidence. During this time, the forms of aggression that have been examined have widened, to include indirect (or “relational”) forms (Archer & Coyne 2005). Here I briefly describe the findings from meta-analyses published between 1986 and 2004 (Archer 2004, Bettencourt & Miller 1996, Eagly & Steffen 1986, Knight et al. 2002). These cover various ages, from early childhood to adulthood (but rarely the older end of the lifespan), and include the following: (1) experimental social psychological methods (typically involving North American university students); (2) observations, teachers’ reports, and peer reports of children; and self-reports by adults, including questionnaires, single-item responses and scenarios. Only the most recent meta-analysis (Archer 2004) included indirect forms of aggression: these are typically shown more frequently by girls than boys, using peer-reports, observations or teachers’ reports (Archer 2009: Table 22). No sex differences are found in adulthood, although this could be a function of the self-report methodology used at these ages. Using experimental methodology, Hess and Hagan (2006) did find more indirect aggression by women than men, and Vaillancourt and Sharma (2011) demonstrated the social context in which this form of aggression is pronounced among women. In contrast to this, both forms of direct aggression, verbal and physical, are more frequent among _____________ 2

In this table, the headings for the two right hand columns are transposed.

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men than women: this applies across ages and data-collection methods (Archer 2009: Table 2). In all cases the magnitude of the sex difference is higher for physical than for verbal aggression. One other relevant finding is that there is no sex difference in self-report measures of anger (Archer 2004). Taken together, the findings from psychological studies show the following pattern for sex differences in aggression: they are absent for overall arousal to anger; there is more frequent indirect aggression by females, at least at younger ages; there is more verbal aggression by males, with considerable overlap; and more physical aggression by males, with a greater difference than for verbal aggression. For physical aggression, the effect sizes (Cohen’s d, a standardized measure of difference) range from .40 to .91, medium to large considering social science findings in general (Cohen 1977). There is an indication therefore that the sex difference becomes larger in the male direction as the form of aggression changes from indirect – where immediate retaliation is unlikely – to physical aggression – where it is. The explanation for this is that males are more likely than females to escalate aggressive encounters to the more dangerous levels (Archer 2009, Campbell 1999). To examine further the proposition that there is a trend for more dangerous forms of aggression to show a larger sex difference in the male direction, we need to examine measures indicative of more escalated forms of aggression. One of these concerns those arrested for aggravated assault, which involves serious forms of assault. The FBI figures for 2001 indicate that 79.9% of those arrested were male and in 2011 the figure was 77.6%. To make this comparable with the psychological studies described above, the figures were converted into Hedges’ g (a very similar effect size measure to Cohen’s d) using Comprehensive Meta-Analysis. Hedges’ g from these figures was .76, somewhat higher than the measures of physical aggression used in psychological studies. Another index of potential willingness to escalate aggression to dangerous levels was referred to in my 2004 meta-analysis: it involved surveys of weaponcarrying among American teenagers. One of these (Brener et al. 1999) collected data from a US school-based national Youth Risk Behavior Survey (YRBS), which included a question on carrying a weapon. The survey covered four years, every other year from 1991-1997: the percentages of those carrying weapons that were male were 68, 79, 79, and 80, for the four years of the survey. Effect size values for sex differences for the four years for weaponcarrying were .94, .90, .89 and .90, all higher than is typically found for more inclusive measures of physical aggression (comparable values for being involved in a physical fight, from the same sample, were .36, .45, .37, and .48). Another study involving a US High School students (Singer & Flannery 2000) produced values of g = 1.04 for sex differences in endorsing the item “shot at someone”. Physical aggression that results in death is the most escalated form of aggression in terms its outcome. We can therefore examine statistics for same-sex homicides to assess whether the sex difference is even more pronounced for

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these. Figures presented by Daly and Wilson (1988: 147) indicate that in 34 out of 35 studies the proportion of male-to-male killings is over 90%. Restricting these to non-relatives, Daly and Wilson (1990: 85) listed 20 studies: aggregating these, the proportion of male-to-male killings was 97.1%. If we take the first set of figures from this table, those for Chicago from 1965-1981, the proportion is 97.4%, i.e. it is typical of the whole data set. Converting these percentages into effect sizes requires numbers for the reference population, and these can be obtained from census data (http://www.infoplease.com/ipa/A0922422.html)3. The value is 2.10, much larger than for any of those described above. A similar difference can be found for FBI homicide figures from 1976-2004: of those adults killed by a member of the same sex, 96.4% were male (Fox & Zawitz 2012). Thus homicide values are consistent with the view that the sex difference becomes larger with the degree to which the aggression is carried to a dangerous level. A summary of the increasing sex difference as the form of aggression entails greater physical risk is shown in Figure 1.

Sex differences (d values) with increasing risk of injury from aggression

Fig. 1: Sex differences (d values) with increasing risk of injury from aggression, from (left) indirect aggression, to verbal, physical, assault, weapon-carrying, and homicide (right).

These figures were all obtained from modern states which operate a generally effective rule of law. Historical analyses indicate a much higher rate of homicide when and where this effective rule of law and associated codes of behaviour are absent (Pinker 2011: 59-128). Eisner (2003) examined a large historical data-set of homicides in a wide range of European countries from the thirteenth century onwards, and documented a general decline, which occurred at different times in different countries. He also presented data for the proportion of women in the homicide figures: this was similar to the pattern found in to_____________ 3

Although the effect size remains stable irrespective of the size of this unless the proportion of male to female homicides changes.

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day’s figures, between 5 to 12 per cent across a range of locations and dates. He stated that the sex difference has been constant for 800 years. Of course, this also means that the decline over this time is largely a reduction in men killing other men. Criminological statistics do not include the differential part played by the two sexes in all forms of coalitional aggression, from gangs and raiding parties through to pre-state warfare up to larger-scale wars between nation states. Keeley (1996) estimated that in non-state societies, inter-group warfare occurred more or less continuously, and involved the mobilization of up to 40% of the population. Throughout human history warfare has been a common and in many times and places a continuous state of affairs (Pinker 2011), and has in nearly all cases involved men fighting other men (Goldstein 2001). It has also been argued that there are evolved adaptations for inter-group competition among men (Daly & Wilson 2001, van Vugt et al. 2007). One further prediction about these sex differences, if they are the result of sexual selection, is that we would expect them to co-incide most strongly with the main ages of reproductive competition, which in most mammals is in young adulthood. However, in many polygynous mammals, males have to continue competing beyond this time, further into their adult lives, in order to keep their status, which is related to reproductive success (Daly & Wilson 1990). Although most studies of age and violent crime find the peak to be in the men’s 20s (Daly & Wilson 1990, Eisner 2003, Hirschi & Gottfredson 1983, Quetelet 1833/1984), in a review of violence in non-state societies, Nivette (2011) found that the peak of violence continued beyond this time, particularly where older men took second and third wives.

IV. Weapons and Male Threat Displays Thus the pattern of human sex differences in aggression is that we would expect if it is the result of sexual selection. Sexually-selected males show a number of other features, either directly or indirectly associated with male aggressive competition, or reflecting the consequences of such competition. In this section, I consider weapons and male threat displays, which tend to accompany male aggression, and are pronounced in sexually selected species. The observation that human males do not have the large canines characteristic of the males of other polygynous primates, including the Great Apes, has sometimes been viewed as evidence against sexual; dimorphism in humans (e.g. Eagly & Wood 2009). It is, however, a single feature that is missing from humans, among several others that all indicate sexual selection. The fossil record indicates that large male canines were lost early in the evolution of the hominin line (Plavcan & van Schaik 1997b). Pinker (2011: 40) suggested that the small jaws of hominins (which were present from early in their line) do not open widely enough for large canines to be practical for inter-male fighting.

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Since the hands would have become free at a similar time this would have enabled these to be used as alternatives. Another suggestion comes from an analysis of other primates with sexual dimorphism in bodily size but no enlarged male canines, where an alternative sexually-dimorphic feature has been found. This is facial width, or more precisely higher male than female facial width-to-height ratio (Weston et al. 2004, Weston et al. 2007), which is found during puberty under the influence of testosterone (Verdonck 1999). Carré and McCormick (2008) found that facial width-to-height ratio correlated positively with aggressiveness in three samples of young men. Subsequent studies have found that men with wider faces are more likely to exploit others for personal gain in an experimental situation (Stirrat & Perrett 2010), and that former US presidents with wider faces were characterized by greater achievement drive (Lewis et al. 2012). Intriguing as these findings are, it is not clear how facial width could act as an effective substitute for canine teeth, and why it is found as a signal (presumably of male dominance striving) only in primates without sexually dimorphic canines. The functional significance of male facial width is made even more puzzling by the finding that it does not correlate with body weight, and therefore it presumably does not convey information about bodily size and strength (Weston et al. 2004). Yet the male face can convey reliable information about a man’s size and strength, via two other sexually dimorphic features, the brow ridges and the size of the jaw. Studies by Sell and his colleagues showed that ratings of bodily strength (across 17-18 raters) correlated highly (r = .51 and .58) with actual strength assessed on five weight-lifting machines (Sell 2005, Sell et al. 2009). Sell (2006) argued that the size of the brow ridges and the jaw are the main features used to reliably assess the person’s strength. Both are influenced by testosterone at puberty and both are accentuated in the angry face (Sell et al. 2009). Two other testosterone-dependent features have also been implicated in displaying aggressive intent or competence. These are beard growth and deep voices. Facial hair is found among the males of non-human primate species, where it is associated with threat displays (Andersson 1994: 345, Guthrie 1970). Two studies have found that college students rated male faces with beards and other forms of facial hair as more aggressive, dominant and stronger than male faces without hair (Addison 1989, Muscarella & Cunningham 1996). This supports Darwin’s (1871/1901) suggestion that beards are the result of inter-male competition, that they are features used in threat displays. Vocalizations are important for male threat displays throughout the vertebrates, and depth of voice is generally indicative of bodily size and strength (e.g. Clutton-Brock & Albon 1979, Davies & Halliday 1978, Mager 2007). This applies to human males, as several studies have found that depth of voice conveys information about bodily shape and size (Evans 2006, Sell et al. 2010).

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Men with deeper voices are rated by other men as having better fighting ability than those with higher voices (Puts et al. 2006, Puts et al. 2007).

V. Size and Strength Although other selection pressures may increase female size (Ralls 1976), sex differences in size and strength are generally regarded as an indication of past sexual selection. This is especially the case if the sex difference involves greater strength and musculature. In humans, men are around 7 to 8% taller than women (Alexander et al. 1979, Brennan et al. 1997), based on cross-cultural surveys. Where it is possible to calculate effect sizes from comparable smallerscale studies, these values are large, for example d = 1.14, 2.24 and 2.67, from Canadian, American and Chinese samples (Archer 2009: 260). In a range of studies men are typically about 25% heavier than women, with effect sizes ranging from.60 to 1.79 (Archer 2009: 260). However, when fat-free mass or strength is measured the differences are larger. In Swiss and US samples, men had around 40% more fat-free body mass than women (Lassek & Gaulin 2009, Kyle et al. 2005), with d values ranging from 2.06 to 3.46 (Archer 2009: 260). Pheasant (1983) reviewed 112 studies, finding that men’s mean strength was 64% greater than women’s: the effect sizes are very large, from 1.45 to 3.09 (Archer 2009: 260). The study by Lassek and Gaulin (2009) involved a US national survey from 1988 to 1994, and also included measures of upper arm muscle volume, which was 78% more in men than in women (d = 2.50), and thigh muscle volume, which was 50% more (d = 1.86). That such features are still subject to sexual selection is indicated by another finding from Lassek and Gaulin’s study: as muscularity increases, so do the number of sexual partners, and the calorific intake. However, there is decrease in immunity. Overall, these findings are what we would expect of a sexually selected species, although they are smaller than in primate species that show pronounced polygyny, where the ratio of male-to-female body mass is around 1.5 or more (Plavcan 2000: 331, Plavcan & van Schaik 1997a). Reconstructions of extinct hominins are based on fragmentary remains (Plavcan & van Schaik 1997b, Larsen 2003), and are not able to indicate the crucial variables of fat-free mass, and upper body muscle and strength.

VI. Maturation Rates and Mortality One consequence of male competition is a sex difference in maturation rates, males taking longer than females (Andersson 1994). The important feature seems to be a delay in reaching sexual maturity, so that this occurs later in species where size is important for inter-male competition. Boys take around two

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years longer than girls to reach sexual maturity, and the difference in maturational rate is apparent as early as prenatal life (Tanner 1989). Another feature that is apparent in prenatal life, starting with conception, is an unequal ratio of males to females. Estimates are that this is between 110 and 160 males for every hundred females at conception. Thereafter boys are more prone to conditions leading to spontaneous abortion and stillbirth. Throughout life the mortality of males is greater than that of females, so that there is approximate equality in young adulthood. The unequal ratio at birth can be viewed as reflecting a long history of selection reflecting the greater male than female mortality before this time. It is well known that women tend to live longer than men, but this is seldom seen as reflecting the greater reproductive competition among males than females. Throughout life human males shows greater mortality than human females, reflecting both internal influences such as greater male vulnerability to disease and stress, and external factors, such as greater inter-male violence and risk-taking. The ratio of male-to-female deaths in the US was found to be highest in young adulthood, when there are roughly 3 times more male than female deaths (Kruger & Nesse 2006).

VII. Polygyny and Variance in Reproductive Success A consequence of the greater male competition is more inequality among males in their reproductive success. We might imagine that in the absence of any sexual selection all males in a population would be likely to leave the same number of offspring, and that any deviation from this would indicate the operation of sexual selection. In practice, the usual measure is the degree to which there is inequality among males relative to the variation among females. Clearly, some females will leave more offspring than others but generally the variance would be less than that for males that are subject to inter-male competition. Thus, the degree to which the variance in male reproductive success exceeds that of the females provides index of inter-male competition, or effective polygyny, in a population. In pre-industrial societies, this index (VR) ranges from 1.664 for Aka pygmies (Hewlett 1988) to 4.69 for Yanomamö (Chagnon 1979). Brown, Laland and Borgerhoff Mulder (2009) extended this analysis to list 18 studies including those of pre-industrial societies (including huntergatherers), but also one survey from the United States and some historical Scandinavian sources. Some of these are from societies where there was imposed monogamy, with differing degrees of coercion (see below). Despite this, which would operate against a trend for larger male than female variance and reproductive success, I calculated the overall VR to be 1.98 (weighted by the _____________ 4

This was wrongly calculated as 2.76 in a previous article (Archer 2009: 259).

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reciprocal of the variance in each sample, which controls for more reliablysampled data-sets). These analyses are limited by the populations for which such data are available, and also because they involve current or historically recent samples. DNA studies provide a further source of evidence, one that is without these limitations. In three diverse current human populations, Wilder, Mobasher and Hammer (2004) estimated that the most recent common ancestor was twice as long ago for women as for men. They derived these estimates from mitochondrial DNA, which is only transmitted through the female line, and the noncombining part of the Y-chromosome, which is only transmitted through the male line. The inference from these findings is that the current human population is descended from around twice as many women as men, reflecting a historical pattern of greater male than female competition for mates. This is found despite the majority of humans living relatively recently, when the marriage system in much of the world was monogamy (Baumeister 2010: 63-64). Ethnographic and historical sources support the view that there is greater inequality in the reproductive success of women than men. Murdock’s (1967) Ethnographic Atlas covered 862 pre-industrial societies, of which only 16% were exclusively monogamous: the remainder allowed polygynous marriages, although most men were monogamous, polygyny being restricted to those men with a larger share of resources. The current widespread pattern of monogamous marriages is regarded as being enforced on an older pattern of facultative polygyny (Alexander et al. 1979). The only exception is in societies where harsh ecological conditions preclude the accumulation of riches necessary for polygyny (see Section 8 below). As indicated in the introduction, historical analyses also show that throughout history, powerful men had very high numbers of sexual partners (Betzig 1986, 1992) a trend that is continued today. There is also striking evidence from a genetic analysis of the disproportionate number of descendents left by one family group who were successful in armed conquest in a particular area. Zerjal et al. (2003) found a high frequency of one particular Y-chromosome lineage in 16 populations inhabiting the area once occupied by the Mongolian empire. Their origin was traced back to around a thousand years ago, and from historical sources it is likely that the origin was Genghis Khan and his male relatives, who killed the men and raped the women in the areas they conquered (see Pinker 2011: 196). We can safely conclude from these different sources of evidence that the human species shows evidence of an ancestral pattern of effective polygyny, reflecting sexual selection over a long period, and that this pattern is maintained today to varying degrees in different societies, despite the imposition of monogamy as the only officially-recognized pattern of marriage in much of the world. Nevertheless there is also evidence for variability in human mating systems, which can be understood from a sexual selection framework. I consider this in the following section.

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VIII. Sources of Variability In mammals and birds it is clear that mating systems vary with the ecological conditions. For example, in the small European passerine bird, the dunnock or hedge sparrow, the mating system varies from polyandry (two or three males and one female) to monogamy (pairs) to polygyny (one male and two females), and this variability depends on the ability of males to monopolize access to females (Davies & Lundberg 1984, Davies & Hartley 1996).This in turn depends on the female range size, which is subject to the food distribution: where this in dense patches, female ranges are small, leading to easy monopolization by males, and the greater likelihood of polygyny. Where food patches are scarce and food is widely distributed, female ranges are large and it is difficult for males to monopolize them, leading to the greater likelihood of polyandry. This example illustrates a more general principle, set out by Emlen and Oring (1977), that where resources necessary for reproduction are concentrated in one place, polygyny will be more likely, owing to greater male competition for females. Where resources are widely distributed, monogamy is more likely, since males are less able to monopolize females. Alexander et al. (1979) applied these principles to humans, noting that among traditional societies, those with monogamy tend to be found either in more marginal habitats, or in complex societies where monogamy is culturally imposed (see previous section). It is clear from historical and ethnographic evidence that polygyny occurs where certain men are able to monopolize a disproportionate share of the resources necessary for reproduction. At the other extreme, human polyandry occurs in a specific set of circumstances, as an adaptation to low-resource mountainous conditions (Crook & Crook 1988). We can therefore see that human mating systems follow general principles set out by Emlen and Oring (1977) and illustrated in the example of the dunnock. Associated with the broad principle that inter-male competition is accentuated where it is possible for a minority of the male population to obtain a disproportionate share of the resources, we might expect a link between the distribution of income in a society and the degree of inter-male competition. Unequal access to resources can be operationalized as the Gini index, a measure of a nation or region’s inequality of income, and inter-male competition can be operationalized as the homicide rate. The rational for the second is that most of the variation between regions or nations in homicide rates is derived from inter-male homicides (Leyton 1995: 22, Pinker 2011: 63-64). As part of an extensive cross-national analysis, Wilkinson and Pickett (2009) found a positive association between the Gini index and homicide rates. Similar findings were reported for US states (Wilkinson & Pickett 2009) and different neighbourhoods of one American city (Daly & Wilson 1997). Emlen and Oring (1977) also proposed that there was a more fundamental principle underlying the associations between resource-distribution and mating systems. This was the Operational Sex Ratio, the ratio of sexually active

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males to fertilizable females at any particular time and place. As expected, male competition is greater when this is high. Historical analyses by Hudson and Den Boer (2002, 2004) show that there is more violence and there are more expansionist military policies at times when there are more surplus males, i.e. when the OSR is high. In the reverse case, where the OSR is low, there is more intense competition between women for a few resource-rich men, leading to greater physical aggression among women (Campbell 1995, Schuster 1983, 1985). These are just two examples of how human variability can be encompassed within an evolutionary framework. In recent years this has been applied to the impact of different degrees of parasite stress on human cultural values in geographical locations (e.g. Thornhill et al. 2009), following earlier cross-cultural analysis linking polygyny with greater parasite stress (Low 1988, 1990). The overall framework used in these studies is that of evoked culture, where aspects of human cultural systems are viewed as adaptive responses to ecological conditions (Gangestad et al. 2006).

IX. Conclusions In this chapter I have shown that human sex differences conform to the pattern that is predicted from an origin in sexual selection, first described in detail by Darwin (1871/1901). Thus the magnitude of the sex difference increases from less escalated, more indirect, to more escalated, more physically-dangerous forms of aggression. The highest rate of dangerous inter-male aggression occurs in young adulthood, consistent with expectations from sexual selection. Human males also show threat display features that are found in other primates and more widely in vertebrates. These include facial hair, lower-pitched vocalizations, and facial features indicating bodily strength. Other aspects of human sex differences fit the pattern expected in a sexually selected species. First, there is greater male than female strength, particularly upper body strength, and associated size dimorphism, adaptations for intermale physical competition. One consequence of greater male risk-taking and competition, together with the physiological effects of testosterone is greater male than female mortality, which in relative terms is greatest in young adulthood. This is associated with a higher male than female conception rate, which indicates a long evolutionary history of sexual selection. Humans also show sex differences in maturational rate, culminating in later puberty in boys than girls. The final test of the operation of sexual selection is the greater male than female variation in reproductive success, which is shown in analyses of reproductive output in current populations, and of genetic material that is specific to male or to females. Altogether, therefore, humans show a number of features that can be regarded as an adaptive complex indicating the operation of sexual selection.

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This does not mean that the behaviour of males and females is fixed by our evolutionary heritage. The human mating system, like that of other animals, is subject to variability induced by the environment. Where resources are scarce or widely distributed, inter-male competition is attenuated, so that the mating system will tend towards monogamy; where a few men can monopolize resources necessary for reproduction, inter-male competition will be accentuated, so that the mating system will tend towards polygyny. Where the sex ratio is distorted so that there are more males than females, inter-male competition is accentuated, with higher rates of physical aggression. The consequences of human’s sexually selected heritage are widespread, and yet have been minimized in psychology and the social sciences, where feminist influences tend to provide the dominant discourse. Acceptance of humans as a sexually selected species would provide a different starting-point for considering men’s and women’s behaviour and biology to that based solely on the historical oppression of women by men.

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Schuster, I. (1985). Female Aggression and Resource Scarcity: A Cross-Cultural Perspective. In M. Haug, D. Benton, P. F. Brain, B. Olivier & J. Mos (Eds.), The Aggressive Female (pp. 185-207). Montreal: Eden Press. Singer, M. I. & Flannery, D. J. (2000). The Relationship between Children’s Threats of Violence and Violent Behaviors. Archives of Paediatric and Adolescent Medicine, 154, 785-790. Stirrat, M. & Perrett, D. I. (2010). Valid Facial Cues to Cooperation and Trust: Male Facial Width and Trustworthiness. Psychological Science, 21, 349-354. doi: 10.1177/ 0956797610362647. Tanner, J. M. (1989). Foetus into Man: Physical Growth from Conception to Maturity. (2nd Ed.). London: Castlemead. Thornhill, R., Fincher, C. L. & Aran, D. (2009). Parasites, Democratization, and the Liberalization of Values across Contemporary Countries. Biological Reviews, 84, 113-131. doi: 10.1111/j.1469-185X.2008.00062.x. Vaillancourt, T. & Sharma, A. (2011). Intolerance of Sexy Peers: Intrasexual Competition among Women. Aggressive Behavior, 37, 568-576. doi: 10.1002/ab.20413. van Vugt, M., de Cremer, D. & Janssen, D. (2007). Gender Differences in Cooperation and Competition: The Male Warrior Hypothesis. Psychological Science, 18, 19-23. doi: 10.1111/j.1467-9280.2007.01842.x. Verdonck, A., Gaethofs, M., Carels, C. & DeZegher, F. (1999). Effect of Low-Dose Testosterone Treatment on Craniofacial Growth in Boys with Delayed Puberty. European Journal of Orthodontics, 21, 137-143. doi: 10.1093/ejo/21.2.137. Weston, E. M., Friday, A. E., Johnstone, R. A. & Schrenk, F. (2004). Wide Faces or Large Canines? The Attractive versus the Aggressive Primate? Proceedings of the Royal Society of London B, 271, 416-419. doi: 10.1098/rsbl.2004.0203. Weston, E. M., Friday, A. E. & Lio, P. (2007). Biometric Evidence that Sexual Selection has shaped the Hominin Face. PLoS ONE, 2, e710, 1-8. doi: 10.1371/journal.pone. 0000710. Wilder, J. A., Mobasher, Z. & Hammer, M. F. (2004). Genetic Evidence for Unequal Effective Population Sizes of Human Females and Males. Molecular Biology and Evolution, 21, 2047-2057. doi: 10.1093/molbev/msh214. Wilkinson, R. & Pickett, K. (2009). The Spirit Level: Why more Equal Societies almost always do better. London: Allen Lane. Wilson, M. I. & Daly, M. (1997). Life Expectancy, Economic Inequality, Homicide and Reproductive Timing in Chicago Neighbourhoods. British Medical Journal, 314, 12711274. doi: 10.1136/bmj.314.7089.1271. Zerjal, T., Xue, Y., Bertorelle, G., Wells, R. S., Bao, W., Zhu, S. & Tyler-Smith, C. (2003). The Genetic Legacy of the Mongols. American Journal of Human Genetics, 72, 717-721. doi: 10.1086/367774.

The Logic of Aggression Reciprocal Altruism, Psychological Games and the Persistence of Conflict LUCIANO ANDREOZZI Game Theory sees social conventions as shared behavioural regularities that coordinate individual’s choices. In the presence of diverging interests, the existence of these regularities reduces the frequency and intensity of conflicts. Little attention has been paid to another function of social conventions, as coordinators of social emotions like (reciprocal) altruism, anger and spite. Recent results from psychological game theory show that, paradoxically, such social emotions may hinder the emergence of socially beneficial conventions that reduce conflicts. This paradox illustrates the “dark side” of social emotions which is rarely acknowledged in the economic literature.

Introduction Two cars drive in opposite directions on a windy mountain road. Although each driver can only see a limited portion of the coming road, they both drive with confidence at the highest speed allowed by the frequent narrow corners they encounter. When they finally meet, they pass safely one next to the other on the opposite sides of the road. The two drivers had never met each other but they could easily predict the other’s choice because they obey the same convention, which dictates that cars must keep to the right hand side of the road. We are so accustomed to this type of spontaneous coordination that we rarely, if ever, pause to reflect on the rather amazing fact that the behaviour of millions independent drivers gets spontaneously coordinated by simple norms like these. Even more amazing is that this convention is entirely self-polishing. You would drive on the same side of the road as anybody else even in the absence of police forces, hidden cameras and speeding devices.1 Here is a less obvious example. Two drivers, A and B, are approaching a cross-road. A is slightly closer to the cross-road than B and is slightly faster. However, he stops, waits for B to pass and then moves again. This would look _____________ 1

The classical game-theoretic analysis of social convention is due to the philosopher David Lewis (Lewis 1969). More recent accounts are Sugden (1986) and Binmore (2005).

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amazing to anybody unaccustomed to modern automobile traffic but is dead simple for the rest of us: A stopped because he had a red traffic light. Again, coordination is obtained by simply following a norm: stop on red, go on green. This example may look less amazing than it is, because apparently the traffic light is not really self-polishing. One might argue that A only stops because he fears the police’s sanctions if he crosses on the red light. To see why this is incorrect, assume there is no police and consider A’s decision whether to stop. He knows that if he has the red light, then it must be the case that B has a green light. Also, he knows that B expects him to stop. (How does he know? The answer is that they belong to the same society, and in this society drivers usually stop on the red light at cross-roads.) Since B expects A to stop, he will fail to stop or even to slow down. This means that if A crosses on the red light he is likely to bump B’s car, which is definitely worse than waiting for a few seconds at the traffic light. What distinguishes the two examples is that at the cross-road there is room for some conflict of interest. While A and B are presumably indifferent between driving on the right or on the left-hand side of the road, in the cross-road example they both prefer to be the first to pass. The traffic light resolves this conflict by giving the green light alternatively to one of the two roads. Notice that although A and B face the same ex-ante probability of having a red light, in this particular instance it is A who has to wait. And even if A does not like to give priority to B, this is what his self-interest dictates for him in this particular circumstance. This example illustrates the role of social conventions in reducing the occurrence of conflicts in society.2 Convention obtains these results by simply creating correct mutual beliefs: one motorist is expected to cross, the other to wait. Once these beliefs are in place, efficient social outcomes characterized by perfect coordination are reached even in the absence of the coercive power we tend to consider necessary for a viable social order. Also, and this is a crucial point as we shall see, individuals will respect a social convention even if they are perfectly egoistic. No altruism is required to stop on the red light, just like no altruism is required to drive on the correct side of the road. Our final example illustrates something a convention cannot do. A and B are on the same windy mountain road as in the first example, but A is slowed down by a very slow lorry driving in front of him. A is tempted to overcome it, although this implies an increased risk of an accident with a car coming in the opposite direction. A is willing to run this risk. If he manages to get home on time, he can take his son to the park, something he really cares for. So he overtakes the lorry and narrowly misses B’s car. Was this decision rational? From A’s point of view it was: the larger risk of the accident was compensated by him being able to spend time with his son. From B’s point of view it was not: _____________ 2

In biology, John Maynard Smith used this type of game theoretic reasoning to explain the low incidence of conflicts in animal behavior. (See for example Maynard Smith 1982).

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he pays the cost of A’s decision (he also has a larger risk of a serious accident) without any compensating benefit. This example illustrates a true social dilemma. A and B would be better off if they both drove more carefully, but each of them prefers the other to refrain from hazardous overtaking, while he overtakes any time he finds it useful to do so. In social dilemmas of this type it is not sufficient to generate correct expectations on the behaviour of others to avoid socially inefficient outcomes. Driver A will overtake the slow lorry even if she believes that B will drive carefully. It follows that A and B will drive carefully only in the presence of a police force that punishes hazardous overtaking, or if each of them also considers, when making a decision, the welfare of the other player. Either coercion or a degree of concern for the social welfare is required to overcome the dilemma. So far we have assumed that the only motive behind individuals’ choice is pure self-interest. Taken literally, this assumption is plainly absurd. Nobody is ready to violate the norms that regulate the life of their community whenever he or she stands to gain a few dollars, or save a couple of minutes. Pure egoism can be a reasonable first approximation in some parts of economics, for example, in explaining consumer’s behaviour, but it is clearly inadequate when social norms are at stake. Not surprisingly, the experimental evidence accumulated in the last two decades show that individuals playing simple games with small monetary rewards often make choices that cannot be reconciled with the simple pursuit of material gains.3 As a response to this evidence, there is now a large body of literature in economics that argues that people are quite good at solving social dilemmas, because they have a degree of concern for non-selfish motives such as reciprocity (Rabin 1993), inequality aversion (Fehr & Schmidt 1999, Bolton & Ockenfels 2000) pure altruism (Andreoni 2002) and guilt aversion (Battigalli & Dufwenberg 2007). The catch-word for all these deviations from pure selfishness is “social preferences.” Part of the appeal of the “social preferences” literature is that its main tenet agrees with common sense. Who would deny that society would be utterly impossible without a minimum concern for the wellbeing of others, or for virtues such as honesty and truthfulness? In this paper we present some recent results in game theoretic literature on reciprocal altruism to draw a less optimistic picture. Our informal presentation of the social dilemmas highlighted that self-interest can sustain social norms that solve the type of bargaining problem A and B encounter at a cross-road, but they cannot solve the type of dilemmas they face when they have to decide whether to try an hazardous overtake. On my draw the conclusion is that non_____________ 3

See Cooper & Kagel (2010) for a recent overview of this large literature. Camerer (2003) is now a classic reference. There is room for (serious) disagreement on the degree to which the experimental literature has proved that individuals are unselfish and whether there is a coherent theory of the non-selfish behavior than better accommodates the existing evidence. For a skeptical overview of this topic see Binmore & Shaked (2010).

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selfish motivations can only have positive effects on the general welfare of a society. Altruistic individuals will be able to cope with the type of social dilemmas that selfish individuals are unable to solve. We shall show that this is in fact incorrect, at least in all the cases in which non-selfish motivations have the form of reciprocal altruism (Rabin 1993). A reciprocally altruistic individual will be altruistic if, and only if, he expects the other people to be altruistic as well. He may become spiteful (i.e. negatively altruistic) if he expects a spiteful behaviour form others. Spiteful attitudes imply that an individual is willing to pay in order to reduce another individual’s wellbeing. We show that this form of reciprocal altruism has a paradoxical effect on the emergence of the norms of social cooperation. In particular, in the presence of games of conflicting interests, it destabilizes efficient conventions that would be stable if individuals were perfectly selfish. People driven by reciprocally altruistic motives will be less prone to respect this type of social convention, and may be trapped into inefficient equilibria in which they mutually punish each other. Reciprocally altruistic individuals will be able to solve the type of problems we have dubbed social dilemmas. However, we shall show that this solution is fragile. A small number of deviation will suffice to bring the spontaneous cooperation down. We shall proceed as follows. Section two introduces the necessary game theoretic formalism and the toy models that we shall use in the rest of the paper. Section three introduces the idea of psychological games and the notion of reciprocal altruism. Section four discusses how reciprocal altruism influences the selection of equilibria in the two basic forms of social dilemmas we consider: bargaining games and Prisoner’s dilemmas. Section five presents some final thoughts on the role of social preferences in generating, or attenuating, social conflicts, drawing on the well-known example of the killing of prisoners of war during the two World Wars.

II. Game Theory and Social Conventions Consider the game below. It is played by a Boy and a Girl who must decide whether to spend the night at a football match or at the Ballet house. Since they love each other, they have a strict preference for spending the night at the same place, which is why they both get zero if they choose different places. If they go to the same show, their payoff is positive, although the boy obtains a larger payoff at the football match and the girl at the Ballet house. In game theory parlance this game contains two Nash equilibria (in pure strategies), one in which the boy and the girl attend the football match, another in which they go to the Ballet. They have opposite preferences over them: while the boy prefers the former, the girl prefers the latter. This game is formally identical to the situation discussed in the Introduction, in which two motorists decide whether to speed up at a cross-road. While each of them prefers to be the first to pass,

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they both prefer to wait if they expect the other to pass, because an accident is the worst outcome for both. (In the example at hand, the boy and the girl are going to two different places would be the equivalent of two motorists having an accident.) So there are two Nash equilibria, one in which A passes and B waits, another in which B passes and A waits. Boy Girl

Football Ballet

Football 1,2 0,0

Ballet 0,0 2,1

This is the simplest example of a bargaining game. In a bargaining game there are several efficient outcomes and the individuals involved have conflicting preferences over them. In this sense they differ from coordination games, in which individuals are indifferent among the equilibria. In the Introduction we encountered a coordination game, in which motorists had to decide on which side of the road to drive. Bargaining games can be solved through direct negotiations between the parties involved. The boy and the girl can spend hours discussing about how to spend the evening, and the outcome of this negotiation is hard to predict. But this is not the only possibility. Norms and social conventions can solve bargaining problems making actual negotiations redundant. If the same type of bargaining situation takes place over and over between individuals belonging to the same group, or society, a conventional way of solving it becomes the norm. For example, the motorists may give priority to the cars approaching crossroads from the right. When such a conventional solution is in place, each individual can easily predict what the other person will do, and what other people expect him to do. This leads to see the norms regulating social life as analogous to traffic rules.4 The coordinate people’s expectations by dictating who gets priority, or a larger share of a pie. Standard examples of conventional solutions to bargaining problems are sharecropping contracts. Young & Burke (2001) proved that the conventional division of the harvest in fixed shares between the landlord and tenant is independent from the relative bargaining powers of the parties. When a specific conventional division is in place (for example ⅔ to the landlord ⅓ to the tenant), each participant to the game knows that he can easily solve the bargaining problem by simply proposing the conventional division.5 Conflicts over the division of the harvest will only arise if one of the parties involved claims a larger share than the conventional contract allows. _____________ 4 5

The classical treatment of the social conventions in analogy to traffic rules is contained in Hume (1739). For an analysis of conventional contracts of this type see Young & Burke (2001).

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This type of norms dictates behaviour in less obvious examples as well. Consider again our Battle of the sexes game. In a traditional society in which men usually get their way, the girl can easily predict that the boy will insist in going to his favourite show. The boy will predict that the girl will be ready to concede to his own will. Also, the girl knows that the boy expects her to concede. How are these beliefs formed, before the boy and the girl ever meet to play this specific game? The answer is that they have seen countless situations similar to this, in which the will of the man prevailed. Consider now the game below. It is the familiar Prisoner’s Dilemma and illustrates the other type of social dilemma we discussed in the Introduction. Here A and B must decide whether to Cooperate or Defect, for example whether to respect a norm that is socially beneficial. In our example, A and B cooperate if they do not overtake slow lorries and defect if they do. Clearly, cooperation is better than defection for both, because they are both earning 3 rather than 1. However, even if A expects B to play Cooperate, it will be in his best interest to play Defect, because 4 is larger than 3. Since the same is true for B, the only equilibrium of the game is the one in which cooperation is absent. B A

Cooperate Defect

Cooperate 3,3 4,0

Defect 0,4 1,1

How to generate cooperation out of a state of universal defection is the central problem of political philosophy since Thomas Hobbes. While many different solutions have been proposed for this type of social dilemma, three have attracted most attention. First, a political authority is set, which changes the nature of the game by punishing those who play Defect. If the punishment is sufficiently severe, and defectors are discovered with a sufficiently high probability, playing Cooperate can become a rational choice, at least when one expects the other to play Cooperate as well. The second solution requires that the game is repeated over time, by the same pair of individuals. This is the type of cooperation that is observed among neighbours. Each individual keeps a cooperative behaviour, because he anticipates that the others will stop cooperating with him tomorrow if he defects today.6 Both of these solutions are costly. On the one hand, running a coercive state requires resources and has dangerous collateral effects. An all-powerful king ruling on unharmed citizens is a constant risk for their own freedom and safety. On the other hand, the type of cooperation based on repeated interaction is only viable in small group of individuals. It would not explain the type of cooperation we observe in our large and anonymous societies. _____________ 6

Axelrod (1984) popularized the idea that cooperation observed in society is based on this type of mechanism.

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The third solution does not suffer from these limitations. It is based on the simple idea that people may not be entirely selfish. Maybe people do not play Defect in a Prisoner’s Dilemma because this is morally wrong. The concern for “doing the right thing” may override monetary considerations, at least when they are not too large. A morally motivated individual may renounce to a few extra dollars he could get by playing Defect, because he wants to choose in a way that maximises social welfare, if everybody does the same. This solution to the dilemma is costless and works equally well among neighbours and strangers. In the next section we shall see that this type of solution may have its costs as well. Cooperation can only be sustained by individuals who are conditionally cooperative. They are altruistic when they believe that other people are altruistic, otherwise they become spiteful. Spiteful behaviour is particularly dangerous in bargaining games: reciprocally spiteful individuals may easily get trapped into hating each other.

III. Moral Emotions: The Logic of Reciprocity In a seminal paper published two decades ago, Matthew Rabin (1993) showed how game theory could be used to make sense of the idea of reciprocity.7 Consider again the Prisoner’s Dilemma and suppose that the numbers in the matrices are monetary amounts. These numbers will thus represent a player’s preferences only if he were perfectly egoistic (and risk-neutral). Suppose now that A believes that B is going to play Cooperate. How would such belief influence A’s preferences for cooperating and defecting? If he were completely selfish, this would have no effect, because Defect yields a larger monetary payoff than Cooperate regardless of the other player’s choice. But what if A’s preferences are not completely egoistic? For example, A may be inclined to cooperate now that he knows that B will cooperate as well, but he would be ready to switch to defection if he believed that B would play Defect. A’s preferences would reveal a degree of reciprocal altruism. A cares about B’s payoff: if B behaves nicely towards A, A will feel altruistic towards him. If B behaves spitefully towards A, A will feel spiteful. Notice that reciprocity is fundamentally different from pure altruism. In fact, in choosing Cooperate rather than Defect, A reduces his payoff of one dollar and increases B’s payoff of three dollars (if B plays Cooperate) or four dollars (if B plays Defect). So if A’s behaviour is motivated by the desire to increase B’s payoff (as it would be the case with pure altruism) he would either always Cooperate (if he is sufficiently altruistic) or never cooperate (if is he is sufficiently egoistic), but he will never show the type of belief-dependent preferences that are typical of reciprocity. _____________ 7

Recent contributions to this literature are Battigalli & Dufwenberg (2007), Segal & Sobel (2007, 2008).

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There is a further complication. A’s evaluation of B’s choices depends clearly on what he believes B expected him to do. To see this, consider how A’s evaluation of B’s decision to defect changes if believes that B expected A to defect rather than cooperate. B’s decision to defect when he believes that A will defect as well is in line with reciprocity. B is simply returning the type of unkind behaviour he expects from A. Compare this with B’s decision to defect when he believes that A will cooperate. In this case B’s decision is clearly morally wrong. B defects even if he believes that A will cooperate. The logic of reciprocity dictates in this case that A becomes spiteful towards B. If he could, A would be willing to pay to reduce B’s payoff, in order to punish him for his unkind behaviour. This example illustrates the importance of what game theorists call second order beliefs, that is A’s beliefs concerning what B think A will do. The importance of second-order beliefs is probably easier to illustrate in the context of a bargaining game like the Battle of the Sexes. Again, imagine that the number in the boxes represent monetary amounts, and suppose that the boy expects the girl to go to Ballet. What should he do? The answer is simple if the boy is purely egoistic, because in this case he should go to Ballet as well. But suppose he is also motivated by reciprocity. What should he make of the girl’s decision to go to Ballet? The answer crucially depends on what the boy believes the girl expected him to do. In fact, if she expected him to go to Ballet as well, going to Ballet is the choice that maximizes the boy’s payoff and hence is perceived as altruistic. On the other hand, if the girl believed that the boy was going to football, in going to Ballet she is intentionally reducing the boy’s payoff, from 2 (which is the payoff he would get if she went to Football as well) to 0 (which is the payoff he gets if she goes to Ballet.) It follows that if the boy is sufficiently motivated by reciprocity, he will go to Ballet when he believes that the girl will go to Ballet under the belief that he was going to ballet as well, because he feels altruistic towards her. He will go to football if he believes that the girl went to Ballet despite the fact that she believed he was going to football, because he feels spiteful towards her. Including these type of reciprocal preferences changes the nature of the games in a radical way, and in many cases make them closer to common sense. First, one needs a more sophisticated notion of equilibrium that Rabin dubs psychological equilibrium. The strategies chosen by the two players form a psychological equilibrium if two conditions are met. In the first place, first order and second order beliefs must be correct. The boy and the girl of our previous example must be able to anticipate the other player’s behaviour, and also must have correct beliefs about what the other expects them to do. Second, each player uses his best strategy, given his beliefs. In the Prisoner’s Dilemma, this implies the existence of a psychological equilibrium in which the two players cooperate. This is not difficult to see. Suppose A and B believe that they will both cooperate and suppose these beliefs are correct. This means that they will both feel altruistic towards each oth-

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er and hence they will prefer to cooperate rather than defect. Notice that this is not the only equilibrium: mutual defection is an equilibrium as well. If A believes that B will not cooperate, he will not feel altruistic towards him and hence will not cooperate. The same applies to B. It follows that if the players have the “wrong” beliefs, A and B will fail to cooperate, despite the fact that they are reciprocally altruistic. Notice that this conclusion does not depend on the fact that players are not sufficiently motivated by non-selfish motivations. A reciprocally altruistic person will never cooperate with somebody who does not cooperate, no matter how important these non-selfish motivations are for him. Reciprocal altruism has even more surprising consequences for bargaining games. In our Battle of the Sexes game, if the boy and the girl are sufficiently motivated by reciprocity there is an equilibrium in which they both go to their favourite show and hence no coordination takes place. To see this, suppose that the girl believes that the boy goes to the football match knowing that she will be going to Ballet. She will (correctly) believe that the boy wants to minimize her payoff and hence will feel spiteful towards him. And the only way of hurting him is to go to Ballet, because this is what minimizes his payoff. On the other hand, the boy believes that the girl will go to Ballet even if she knows that he is going to go to football. He will have hard feelings towards her, which will induce him to minimize her payoff by going to football. This is a rather counterintuitive result. Inefficient outcomes that would not be equilibria if the players were purely selfish become equilbria because of reciprocally altruistic preferences. If the boy and the girl would only care about their own payoffs, the only equilibria would be those in which coordination, either on Ballet or on football, takes place. When they are motivated by reciprocal altruism, they may end up trapped into a vicious circle of mutual punishment. The girl goes to Ballet to punish the Boy for going to football, which the boy does just to punish her for going to Ballet! Rabin’s model illustrates an important point. Reciprocal altruism does not necessarily facilitate social coordination and cooperation. Its effects on social equilibria are rather subtle. In social dilemmas like the Prisoner’s Dilemma reciprocal altruism creates socially efficient cooperative equilibria that would not exists among purely egoistic individuals. In bargaining games like the Battle of the Sexes reciprocal altruism create equilibria characterized by conflict and mutual punishment that would not exist if players would only care about their own material payoffs.

IV. Learning Fairness Psychological games are entirely static. In a psychological equilibrium individuals have correct (first order and second order) beliefs, but there is no a theory concerning the way in which these beliefs can be generated. The traditional

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game theoretic literature has produced many models that explain how players may come to have correct beliefs. In these models it is assumed that individuals that are boundedly rationally learn over time to play a given game, and converge to a conventional solution to it. 8 Evolutionary models of learning assume that instead of being played only once by two perfectly rational players (as it is usually assumed in standard game theory) a game is played repeatedly by pairs of agents drawn from two large populations.9 Suppose for example that the Battle of the Sexes is played repeatedly by pairs of boys and girls drawn from two large populations. Individuals within each population are not assumed to be rational, but to learn from experience. Each of them adopts a strategy for playing the game, which is not necessarily the strategy that maximizes their payoff given the choices of the individuals in the other population. Occasionally, individuals are given the opportunity to revise their strategy, and they exploit this occasion to revise their beliefs about how individuals in the other population play the game. After they have updated their beliefs, they revise their strategy by choosing the one that maximise their payoff, given their (now correct) beliefs. This defines a stochastic process in which individuals keep adjusting their beliefs concerning the way in which the game is to be played and their behaviour. A steady state of this process is a state in which agents have correct beliefs and play an optimal strategy against these beliefs. When two populations have reached a steady state, agents who are given the opportunity to revise their beliefs and strategies will have nothing to update. It is not difficult to see that the steady states of this process can only be Nash equilibria of the game the agents are playing. When a Nash equilibrium has been reached, a conventional way of playing a given game has been selected. This is the process through which a population of motorist will spontaneously select the convention in which everybody drives on the right hand side of the road, or the convention that gives priority to the vehicles approaches cross-roads from the right. If individuals are selfish, in the Battle of the Sexes there are only to steady states: one in which boys and girls go to ballet, and another in which they go to football. Notice that when everybody goes, say, to ballet, there is no point in going to football, even if you are a boy and football is your favourite show. Populations are assumed to be sufficiently large that a single individual’s choice will make no difference. An individual who decides to violate the current social convention will only pay a cost for his own choice. States that do not correspond to Nash equilibria cannot be stable, because by definition the individuals of one of the two populations will increase their payoff by choosing a different strategy. The state in which all the boys go to football and all the girls go to ballet cannot be a steady state, because, when they are given the opportunity, both the boys and the girls will prefer to switch _____________ 8 9

This section is based on Andreozzi (2012). A complete treatment of these models of learning in games is contained in Young (1998).

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to the other strategy. These changes will eventually lead populations to converge towards one of the two Nash equilibria. The analysis is usually completed by the hypothesis that individuals occasionally make mistakes. When this happens, instead of choosing the strategy that maximises their own payoff, they choose a strategy at random. This randomness produces occasional jumps from one equilibrium to another. If enough girls, by mistake, choose Ballet instead of football, doing Ballet can become the optimal strategy for the boys, and this will trigger a jump from the equilibrium in which everybody plays football to the one in which everybody does Ballet. The introduction of noise allows to discriminate among equilibria. Intuitively, the two populations will tend to gravitate around those equilibria that are more robust in the presence of this type of noise, which are the equilibria that can be overturned by a larger number of mistakes. Since mistakes are unlikely, the more mistakes are required to leave an equilibrium, the longer the system will spend at that equilibrium. These models can be applied to psychological games with minimum changes. The main difficulty comes to the fact that in psychological games the players’ optimal choices depend upon their first order beliefs (that is, what they expect the other players will do) and their second order beliefs (that is what they expect the other players expect them to do) while in standard game theory only first order belief matters. This problem has a relatively easy solution, because an individual can update his second order beliefs by simply observing how the individuals in his own population play the game. If he is a boy, he can guess what a girl expects him to do by simply observing the way in which boys play that game. He updates his second order belief just like girls update their first order beliefs: by observing the behaviour of boys. The same applies, mutatis mutandis, for the girls. This sets the stage for a learning model that can be applied to the selection of an equilibrium in psychological games. Two populations repeatedly play a game (the Prisoner’s Dilemma or the Battle of the Sexes or any other game) and individuals form (first order and second order) beliefs concerning the way in which the game is to be played. A steady state of this process is a characterized by all individuals having correct first and second order beliefs and playing their best strategies against these beliefs. Just like in standard game theory, a state can only be an equilibrium if it corresponds to a (psychological) equilibrium in the underlying game. The stability of each equilibrium can be assessed by introducing mistakes: with a small, but positive, probability each agent does not play the strategy that maximizes his own payoffs, but picks a strategy at random. This allows the possibility that the two populations occasionally jump from one equilibrium to another. The system will spend most of the time at the equilibrium that is more difficult to displace. The results of this analysis are somewhat counterintuitive, both for the Prisoner’s Dilemma and for the Battle of the Sexes. Consider the Prisoner’s Dilemma first. We know from the static analysis that if the individuals are suf-

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ficiently motivated by reciprocity there is a cooperative equilibrium in which they cooperate. The dynamic analysis reveals that the corresponding steady state is never the equilibrium at which the two populations will spend most of the time at. And this is independent from the importance of reciprocal altruism for the individuals. The non-cooperative equilibrium always turns out to be the more stable one. The intuition behind this result is relatively straightforward. To switch from the non-cooperative equilibrium to the cooperative one is required a critical mass of individuals, who switch from Defect to Cooperate. Similarly, the cooperative equilibrium can be destroyed by a critical mass of individuals who switch from Cooperate to defect. The more stable equilibrium is the one that requires the smallest critical mass of “mistakes” to be reached from the other one. It follows that the fate of cooperation is determined by an individual’s choices when the other population is split exactly in halves between cooperators and defectors. Consider the decision of a reciprocally minded individual in this situation. If reciprocity was the only motivation for his behaviour, he would be indifferent between the two strategies. He would like to cooperate, to reciprocate the co-operators’ good behaviour, and to defect to punish those who defect. When the number of co-operators equals exactly the number of non co-operators, these non-selfish motivations cancel out, so the choice of the individual is dictated exclusively by the self-interest part of his payoff. Since the game is a Prisoner’s Dilemma, his self-interest dictates that he defects. This proves that the state in which a population is evenly split between co-operators and defectors will inevitably evolve towards universal defection. In turn, this implies that the fraction of co-operators that need to switch from Cooperate to Defect in order to unsettle the cooperative equilibrium is smaller than half of the population. By a symmetric argument, it takes more than half of the population to unsettle the non-cooperative equilibrium. This is a rather depressing conclusion: even if reciprocity can sustain an equilibrium in which individuals cooperate in Prisoner’s Dilemmas, this equilibrium is always fragile. A small number of defectors will destroy it, and recreating it from universal defection will always require a larger critical mass of co-operators.10 The results for bargaining games are even less promising. From the static equilibrium analysis we know that if individuals are sufficiently motivated by reciprocity, an equilibrium emerges in which there is no coordination: boys go to football, girls to Ballet. The dynamic analysis reveals that if these reciprocity concerns are sufficiently large, upsetting this equilibrium always requires more mistakes than upsetting the efficient (but unfair) equilibria in which there is coordination. If the game is played between reciprocity minded individuals, the populations will spend most of the time at the inefficient (psychological) _____________ 10

This is consistent with the existing experimental literature on Public Good games. Although most people seem to have reciprocally altruistic preferences, the level of cooperation in these games always decline over time approaching almost zero. See Ledyard (1995) for an early survey and Fischbacher & Gachter (2010) for a more recent result in this vein.

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equilibrium in which there is no coordination than to the efficient one in which boys and girls coordinate on a way to spend the evening. Boys will hate girls for going to ballet, and girls will have boys for going to football. They will be trapped into a vicious circle of mutual punishment which prevents them from reaching a more efficient equilibrium in which their choices are coordinated.

V. Conclusions The examples in the previous sections involving groups of boys and girls fighting over the evening show may appear trivial, but the results of our analysis are meant to be applied to more serious arguments. Historian Niall Ferguson has told the tragic story of the killing of the prisoners of war during the First and the Second World War at several occasions (Ferguson 2006). He quotes from the memories of a German soldier who fought on the Eastern Front in WW I: Sometimes one or two prisoners might emerge from their hideout with their hands in the air, and each time the same tragedy repeated itself. Kraus killed four of them on the lieutenant’s order […] We were mad with harassment and exhaustion … we were forbidden to take prisoners … We knew that the Russian didn’t take any … [that] it was either them or us, which is why my friend Hals and I threw grenades … at some Russians who were trying to wave a white flag (444).

This example illustrates the type of perverse effect of reciprocally altruistic preferences we have modelled by using game theory. The soldiers killing or mistreating prisoners of war were not monsters. They were normal people like me and you. In the vast majority of the cases, they had no intrinsic preference for killing other soldiers who were trying to surrender. If they only listened to their purely selfish preferences, they would have not inflicted unnecessary hardship on their prisoners. It was each soldier’s “moral sense” to tell him that the prisoners needed to be punished for what they would have done if roles had been reversed. The key passage of the quotation above is that German soldiers killed Russians prisoners, because they believed (correctly) that Russians did not take prisoners. And the reason for Russians to kill German surrenders was the (correct) belief that they were responsible for a similar treatment of Russian soldiers. Our analysis shows that this type of equilibrium, which is characterized by reciprocal punishment, can be very stable indeed. Ferguson’s analysis reveals that the practice of abusing captured soldiers could only be stopped by the external intervention of the Allied high command, which realized (too late) that the systematic abuse of prisoners was one of the factors that were prolonging the war. Fearing to be killed if being captured, German and Japanese soldiers usually fought with extreme determination. In his influential work on the emergence of cooperation, Robert Axelrod has used some examples taken from World War One, to show that cooperation can emerge even across the trenches of enemy armies. Our analysis reveals that

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this is indeed a possibility. If reciprocal altruism is sufficiently strong, the possibility exists that mutual cooperation emerges. However, our analysis also reveals that this type of cooperation is fragile. A few mistakes are sufficient to destroy it, and once destroyed, it will be difficult to rebuild it. This maybe the reason why we recall wars mostly for their act of brutal inhumanity than for the positive examples of enlightened cooperation.

References Andreoni, J. & Miller, J. (2002). Giving according to GARP: An Experimental Test of the Consistency of Preferences for Altruism. Econometrica, 70 (2), 737-753. Andreozzi, L. (2012). Learning to be fair. Forthcoming Journal of Economic Behaviour and Organization. Axelrod, R. (1984). The Evolution of Cooperation. New York: Basic Books. Battigalli, P. & Dufwenberg, M. (2007). Guilt in Games. American Economic Review, 97 (2), 170-176. Battigalli, P. & Dufwenberg, M. (2009). Dynamic Psychological Games. Journal of Economic Theory, 144, 1-35. Binmore, K. G. (2005). Natural Justice. Oxford: Oxford University Press. Binmore, K., Samuelson, L. & Young, P. (2003). Equilibrium Selection in Bargaining Models. Games and Economic Behavior, 45 (2), 296-328. Binmore, K. (2007). Does Game Theory Work? The Bargaining Challenge. Cambridge and London: The MIT Press. Binmore, K. & Shaked, A. (2010). Experimental Economics: Where Next? Journal of Economic Behavior & Organization, 73 (1), 87-100. Bolton, G. E. & Ockenfels, A. (2000). ERC: A Theory of Equity, Reciprocity, and Competition. American Economic Review, 90 (1), 166–193. Camerer, C. (2003). Behavioral Game Theory: Experiments in Strategic Interaction. Princeton, NJ: Princeton University Press. Charness, G. & Rabin, M. (2002). Understanding Social Preferences with Simple Tests. Quarterly Journal of Economics, 117 (3), 817-869. Cooper, D. & Kagel, J. H. (2010). Other-Regarding Preferences: A Selective Survey of Experimental Results. In A. Roth & J. H. Kagel (Eds.), Handbook of Experimental Economics, vol. 2. Princeton, NJ: Princeton University Press. Dufwenberg, M. & Kirchsteiger, G. (2004). A Theory of Sequential Reciprocity. Games and Economic Behavior, 47, 268-298. Falk, A. & Fischbacher, U. (2006). A Theory of Reciprocity. Games and Economic Behavior, 54 (2), 293-315. Fehr, E. & Schmidt, K. M. (1999). A Theory of Fairness, Competition, and Cooperation. Quarterly Journal of Economics, 114 (3), 817-868. Ferguson, N. (2006). The War of the World: History’s Age of Hatred. London: Allen Lane. Fischbacher, U. & Gachter, S. (2010). Social Preferences, Beliefs, and the Dynamics of Free Riding in Public Goods Experiments. American Economic Review, 100 (1), 541556. Gibbard, A. (1992). Wise Choices, Apt Feelings: A Theory of Normative Judgment. Cambridge: Harvard University Press.

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Hume, D. (1739). A Treatise of Human Nature. The Clarendon Edition of the Works of David Hume. Oxford: Clarendon Press. Kandori, M., Mailath, G. J. & Rob, R. (1993). Learning, Mutation, and Long Run Equilibria in Games. Econometrica, 61 (1), 29-56. Ledyard, J. O. (1995). Public Goods: A Survey of Experimental Research. In J. Kagel & A. E. Roth (Eds.), The Handbook of Experimental Economics (pp. 111-194), Princeton, NJ: Princeton University Press. Lewis, D. K. (2002). Convention: A Philosophical Study. Oxford: Blackwell. Maynard Smith, J. (1982). Evolution and the Theory of Games. Cambridge: Cambridge University Press. Rabin, M. (1993). Incorporating Fairness into Game Theory and Economics. American Economic Review, 83, 1281-1302. Schelling, T. C. (1960). The Strategy of Conflict. Cambridge: Harvard University Press. Segal, U. & Sobel, J. (2007). Tit for Tat: Foundations of Preferences for Reciprocity in Strategic Settings. Journal of Economic Theory, 136, 197-216. Segal, U. & Sobel, J. (2008). A Characterization of Intrinsic Reciprocity. International Journal of Game Theory, 36 (3), 571-585. Sobel, J. (2005). Interdependent Preferences and Reciprocity. Journal of Economic Literature, 43 (2), 392-436. Sugden, R. (2005). The Economics of Rights, Cooperation and Welfare. Houndsmill and Basingstoke: Palgrave MacMillan. Young, H. P. (1993). The Evolution of Conventions. Econometrica. 61 (1), 57-84. Young, H. P. (2008). Social Norms. In S. N. Durlauf & S. N. Blume (Eds.), The New Palgrave Dictionary of Economics, Second Ed. London: MacMillan. Young, H. P. (1998). Individual Strategy and Social Structure: An Evolutionary Theory of Institutions. Princeton, NJ: Princeton University Press. Young, H. P. & Burke, M. A. (2001). Competition and Custom in Economic Contracts: A Case Study of Illinois Agriculture. American Economic Review, 91, 559-573.

What causes Large-Scale Variation in Homicide Rates? MANUEL EISNER Violence  i.e. the intentional infliction or threat of physical harm against another person  is a pervasive feature of human societies. There is no known human society where the equivalents of assault, rape, robbery, or murder do not occur. The omnipresence of violence amongst members of the human species has led some researchers to argue that violence has evolutionary roots in the development of humankind during the Pleistocene (2 Mio years ago). According to this view violence was not always the dysfunctional ‘disease’ or abhorred crime as which it appears to be in contemporary societies. Rather, evolutionary psychologists argue, violence had a number of uses that increased the likelihood of survival of a person who is sometimes aggressive over somebody who is always peaceful (Buss & Shackelford, Eisner 2009). But while violence seems to be a human universal, there also exists a lot of variation in the amount of violence in any society at a given moment of time. In some societies violent attacks by others account for up to 60% of all deaths, making violence a hugely important factor in one’s chances to survive (Knauft et al. 1991, Robarchek & Robarchek 1998). In other societies lethal interpersonal violence accounts for less than 0.05% of all deaths, meaning that it barely affects the overall life expectancy of a population. This suggests that the extent to which humans primarily display co-operative and caring or antagonistic and violent behaviour depends on social circumstances (Roth 2011). This paper examines homicide, the best documented manifestation of violence. In particular, it examines whether any generalizable conclusions can be drawn from three research traditions that have tried to understand why societies differ in levels of homicide. The three research traditions examined here are a) criminological research on cross-national differences in homicide, b) comparative anthropological research on levels of lethal violence in non-state societies, and c) historical research on the factors that affect long-term variation in homicide rates over time. Many researchers believe that homicide is probably a good lead indicator of overall levels of interpersonal violence in a society. However, the extent to which this assumption is true is not clear, and one should bear in mind that different manifestations of violence may have different distributions across societies and over time. Thus, the large-scale variation in the frequency of rape, robbery, wife beating or infanticide may be partly correlated with the distribution of homicide, but each of these behaviours is probably also influenced by specific factors.

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The first section will examine the extent of variation in homicide across human societies. I then examine the extent to which some characteristics of ‘homicide’ differ between relatively peaceful societies and violent societies. This is important because we need to understand whether peaceful and violent societies simply have more or less of the same problems, or whether homicide in high-violence societies differs systematically from homicide in low homicide societies. In a third section I discuss empirical research in each research tradition on the factors that are systematically associated with variation in homicide levels, both over time and between societies. In a final section I examine some research gaps and strategies for future research.

I. How Much Variation is there? Before we can understand the causes of large-scale variation in homicide rates we need to establish the extent of variation that exists between human societies. To do this I examine data from three research traditions that rely on very different data sources: Data on variation in homicide rates across modern states can now be found for almost every nation of the world. Comprehensive recent tabulations of data along with methodological considerations are reported in the Global Study on Homicide by the United Nations Office on Drugs and Crime, UNODC (United Nations Office on Drugs and Crime 2011). A large proportion of the data are derived from either of two sources: The World Health Organisation compiles annual cross-national mortality datasets based on national mortality statistics, where deaths due to injuries resulting from assault by another person are coded in the ICD (International Classification of Diseases) codes X85-Y09. The United Nations Survey of Crime Trends and Operations of Criminal Justice Systems, on the other hand, is based on the number of police recorded completed homicides as reported to the UNODC through national police agencies. While great progress has been made in the quality and the geographic coverage of homicide data, the quality is generally more likely to be problematic in low-income countries. Also, coverage can be biased in societies with high levels of violent political conflict and civil war, not only because the bureaucratic process of collecting mortality statistics breaks down, but also because the boundaries between homicide and war-related death become blurred. Data on variation in homicide rates across historical time periods are mainly available for Europe and for the United States. Probably the most comprehensive review of historical data on homicide rates is the European Homicide Database compiled by Eisner (2003). This database comprises estimates of homicide rates across Europe over the past 800 years. Estimates for the period before the onset of national statistics (i.e. between the 13th and the early 19th centuries) are based on research findings published by historians of crime, each relating to a specific geographic area and historical period. From about 1800

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the database comprises series of annual data derived from official statistics in 18 countries. The most comprehensive analysis of historical homicide in the United States is the study American Homicide by Roth (2009). It is based on an extensive examination of thousands of homicide cases, based on a variety of primary sources including newspapers, judicial records and mortality data, in every region of the United States over up to 400 years. Homicides estimates based on historical sources are subject to various limitations: An unknown number of cases may not have come to the attention of the authorities; the likelihood of dying from trauma changed over time as medical treatment improved; population estimates are often inaccurate; and often – particularly for periods before 1800  we only have estimates for limited geographic regions or cities, making generalisations to whole countries difficult. Data on homicide levels in non-state societies have been collected by various anthropologists interested in the cross-cultural comparison of violence. Nivette (2011b) has reviewed much of the evidence, collating estimates from a considerable number of studies. Estimates of homicide rates in non-state societies have many limitations, and comparisons with data from bureaucratic nation-states must be made with great caution. First, the estimates from many studies derive from societies that have had contact with Western societies or that had effectively been colonialized (e.g. Bohannan). Evidence from such societies may have been influenced – in either direction  by the consequences of Western cultural, political and economic domination. Second, many estimates for primitive societies relate to small societies of sometimes just a few hundred members and are based on recollections of violent deaths as recorded in ethnographic interviews. Such estimates can be subject to wild fluctuations as a result of small numbers, and it is difficult to determine to what extent they are subject to bias. Third, some studies such as the well-known analysis by Keeley (1996) indiscriminately combine deaths from intra-group and intergroup (war) conflict. While this is a defensible strategy if one is interested in overall levels of lethality from human conflict, it results in a lack of comparability to modern statistics, where deaths resulting from war and deaths due to homicide are clearly separated. Fourth, one needs to bear in mind that in any society without a functioning state it is impossible to clearly distinguish capital punishment as a strategy to restore order from homicide as a transgression of moral rules (Boehm 1984, Chagnon 1988). Thus, many violent deaths in nonstate societies were considered justifiable since they were permissible reactions to a previous harm such as theft or adultery, and hence equivalent to punishment in a state-organized society. Finally, it is important to recognize that all ‘simple’ societies had very limited abilities to treat wounds resulting from injury, meaning that a far greater proportion of traumas resulted in death than is the case in modern societies. All together, these three datasets represent several hundreds of estimates of homicide rates from all parts of the world, different historical periods, and both state-organized and non-state societies. Despite their limitations, they give an

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impression of the variation in homicide rates across human societies. To illustrate this amount of variation I have placed a small selection of estimates from each of the three data sources on a single spine (figure 1). The spine, shown on the vertical axis, is displayed on a logarithmic scale that ranges from 0.1 per 100,000 person-years to 10,000 per 100,000 person-years, meaning that it comprises six orders of magnitude. 10000

Rwanda Genocide (1994)

1000 Monte Rey County (1850s)

Gebusi (New Guinea)

Los Angeles (1860s) Florence (14th c.)

100

Western Apache Honduras El Salvador

Rome (16th c.) !Kung (Namibia)

Stockholm (16th c.) Cologne (15th c.)

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Nigeria Gisu (Uganda) Unites States

Geneva (18th c)

Kreyol (Dominica)

India

Dijoula (Burkina Faso)

1

Switzerland Japan

Tiv (Uganda) Semai (Malaysia)

Sussex (18th c.) Norway (18801900)

0,1 0

1

Historical

2

Anthropological

3

Contemporary

Figure 1: Homicide rates in selected contemporary, historical, and non-state societies

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II. The World Records in Peacefulness Finding the haven of peace where people live harmoniously together has always fascinated philosophers and social scientists. What exactly ‘peacefulness’ means and whether a low likelihood of lethal within-society conflict is an acceptable operationalization, may be open to contention. There was a time when researchers interested in peaceful societies were primarily looking for primitive pre-state societies (Bonta 1996, Mead 1928). However, the illustrative data shown in figure 1 and more comprehensive analyses suggest that some modern, highly developed societies may be as close to the ideal of a peaceful society as any other. The tables of officially recorded homicide rates in modern states such as those compiled for the Global Study on Homicide (United Nations Office on Drugs and Crime 2011) suggest that the lowest national homicide rates found in today’s world are around 0.5 cases per 100,000 inhabitants per year. At such rates homicide becomes a very marginal cause of death, accounting for around 0.04% of all deaths in any given year. According to the UNODC database there existed several contemporary societies with homicide rates of about 0.5 in around 2005-08. This includes Hong Kong (0.5), Singapore (0.5) and Japan (0.5) in Eastern Asia; Bahrain (0.6), Oman (0.7) and the United Arab Emirates (0.8) in the Middle East; and Norway (0.6), Slovenia (0.6), Austria (0.5) and Switzerland (0.7) in Europe. In Africa or the Americas no nation-state is currently recorded with a homicide rate of less than 1 per 100,000. Remarkably, historical data on homicide rates suggest a similar lower boundary during the history of Europe or the United States over the past centuries. For example, the European Homicide Database suggests a cluster of countries including Sweden, Denmark, Norway and the Netherlands in the late 19th and early 20th century that had remarkably few homicides. In Norway, for example, the mortality statistics between 1875 and 1900 report an average of 9 non-infant deaths due to murder or manslaughter each year. In a country with a population of 2 Million people this equals a homicide rate of about 0.45 per 100,000. This is particularly impressive as emergency services in the late 19th century were considerably less effective than they are today and Norway was a relatively poor, sparsely populated and rural country, where trauma victims were unlikely to receive specialist treatment. No exact estimates exist on the impact of emergency services on the mortality risk of, for example, stab wounds. However, Monkkonen (2001) estimates that probably half of all violent deaths that occurred in the mid-19th century could have been prevented with modern technology. This implies that the late 19th century homicide rates of Norway might be equivalent to a rate of around 0.2 per 100,000 in the contemporary world, once progress in medical emergency technology is taken into account. I am not aware of any society at any time with fewer within-group lethal conflicts. However, a very similar range of values found in Denmark and the Netherlands during this period, and I believe these rates may be very close

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to the world records of peacefulness as measured by the rate interpersonal killing within a society. However, low homicide rates are not the exclusive prerogative of people living in well-integrated nation states. The evidence reviewed by Nivette (2011b) suggests a lot of variation amongst ‘simple’ non-state societies. Some of these societies have homicide rates that are very close to those of the most pacified societies found in the historical and contemporary data of Western Europe. For example, the Dioula in Burkina Faso  a group known for their important historical and contemporary role as traders and merchants across the Sahara and the Sahel areas  are reported in the study by Faurie and Raymond (2005) to have an estimated homicide rate of 1.3 per 100,000. Similarly, several societies studied in the contributions to the volume African Homicide and Suicide, published in 1960, were found to have very low levels of homicide. For example, Beattie examined homicide amongst the Nyoro, an ethnic group of about 110,000 people in Uganda. Over the 20-year period from 1935-1955 a total of 34 cases of completed or attempted intentional homicides were recorded by the district court, the coroner, or in Police files. This equals a homicide rate of about 1.5 attempted and completed homicides per 100,000 in a society with hardly any medical services  certainly a remarkably low level, even if the official figures may be incomplete. Finally, there is the famous example of the Semai, a farming people living in small autonomous bands in the rainforest of Malaysia, amongst whom physical violence including fights among adults, domestic violence against wives or the beating of children appears to be extremely rare. According to Dentan (1968) there were two homicides amongst the Semai (one of which was an abandonment of an old person) in the period between 1955 and 1977 (Robarchek & Robarchek 1998). With a population of about 15,000 this equals a homicide of about 0.6 per 100,000, a value that is very similar to what has been found to be the range of what human societies can achieve in contemporary and in historical research.

III. The World Records in Violence It is unclear whether something like an upper limit of intraspecific killing exists in human societies. There are examples of massacres and genocides where humans got as close as possible to wiping out whole societies. To illustrate the issue I have included Rwanda during the year of the genocide against the Tutsi minority in 1994 at the top of the scale of homicide rates in modern societies. Various estimates put the death toll at 5-800,000 people within a few months, equal to about 10% of the population or a homicide rate of 10,000 per 100,000. Of course, the rate would be even higher if we calculated a separate rate for the Tutsi minority, but exact figures are not relevant here. The issue is whether such ‘extreme’ and organized kinds of large-scale killings should be placed on a single spine of violence as suggested in Figure 1. Most criminologists don’t

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consider themselves competent to analyse these kinds of levels of killings, believing that genocides and civil wars are something entirely different from criminal homicide, and better analysed by sociologists or political scientists. I don’t believe this is true. Rather, the single spine used in figure 1 reflects my assumption that there is a continuum from the more individual ‘traditional’ types of pathological murder to the organized elimination of whole population groups. If we ignore such extreme cases as Rwanda in 1994 and focus on homicide in the more narrow sense, we find that the highest rates of recorded homicide in the modern world are between 50 and 100 per 100,000 inhabitants. In the UNODC statistics the countries with the highest homicide rates were Honduras (82.1), El Salvador (66.0), Cote d’Ivoire (56.9), and Jamaica (52.0). In all four countries endemic violence, much of it associated with organized groups, is a major problem and homicide constitutes a major cause of death, especially amongst young men, who are the most highly affected group of victims. Also, the national averages mask a lot of regional variation, meaning that violence levels in some regions will be much higher than the respective national averages. While these four countries certainly suffer from very serious levels of violence, it is possible that UNODC statistics seriously underestimate the values of some other countries. For example, the Global Study on Homicide counts 608 homicides in Iraq in 2008, equivalent to a homicide rate of 2.0 per 100,000. This would make Iraq in 2008 one of the most peaceful places in the world. It is unclear, how this figure was arrived at, but it must exclude a very large number of civilian deaths related to the political instability and sectarian violence in the country. According to the Iraq Body Count project, for example, there were almost 10,000 civilian violent deaths in Iraq in 2008, which would amount to a homicide rate for 2008 of about 30 per 100,000. Even more stunningly, the UNODC report presents a figure of 138 homicides for Somalia in 2008, equivalent to a rate of 1.5 per 100,000 in 2008 and corresponding to a level that would put the homicide rate in Somalia close to that of politically stable and affluent societies in Western Europe. In that year Somalia was in the midst of a cruel civil war with gangs of rival war-lords fighting each other in Mogadishu and other cities. According to the Mogadishu-based Elman Peace and Human Rights Organisation the number of civilian deaths in 2008 was around 7,500 persons, more than fifty times the figure in the homicide statistics. These inconsistencies illustrate two problems that we often observe in societies with high levels of violence: One is that under such conditions the bureaucratic structures that are responsible for collecting information about causes of death stop operating, meaning that data will be incomplete. The other problem is that under such conditions the distinction between ‘criminal’ interpersonal violence, political violence and civil war becomes increasingly blurred. Historical research has uncovered a number of societies with very high rates of homicides. In Europe, for example, some studies find very high rates of

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homicide in some cities in the Middle Ages and the Early modern period – although the notion of general lawless and pervasive violence in the high Middle Ages is certainly wrong (Dean 2001). For example, local studies for 14th century Florence (Becker 1976), 16th century Stockholm (Karonen 2001) or medieval Utrecht (Berents 1976) suggest that the upper limit are homicide rates in the range of 50-150 per 100,000 inhabitants over extended periods of time, although Spierenburg (2001) quotes a homicide rate as high as about 700 per cent 100,000 for Corsica in the early 18th century. In the United States, Roth documents some very high homicide rates during the early phases of European colonialisation. For early 17th century Virginia Roth reports a homicide rate of about 250 per 100,000 (Roth 2009: 37), and a rate of almost 500 per 100,000 for Maryland during the same period. Roth explains that this rate was not solely the result of the clash between settlers and Native Americans: “Non Puritans killed Puritans; the Dutch killed their fellow Dutch; Englishmen killed Frenchmen; and Frenchmen retaliated. Men died in clashes between rival governments and political factions that fought to control trade and territory” (Roth 2009: 27).

Some ethnographic studies have come up with even higher estimates. Amongst these, the estimates provided by Lawrence Keeley in War before Civilization (Keeley 1996) are particularly remarkable, not least because they have been cited prominently in the more recent work by Pinker (2011). Amongst others, Keeley reports rates of violent deaths of about 1450 per 100,000 for the Kato tribe in California during the 1840 or of 970 per 100,000 in the 1920s for the Dinka, an agripastoral group without central political authority in what is now Sudan, who had a long tradition of cattle-raiding and encroachment on grazing lands of their neighbours (Fluehr-Lobban 1976). Similar rates have been documented for a range of hunter-gatherer and horticultural societies across the globe. They imply that in some agropastoral societies up to 1% of the population were killed by violent acts each year, which in turn means that up to 50% of men could expect to die through violence (Schiefenhoevel 1988). However, it should be borne in mind that these and similar data are reported by Keeley and Knauft (Knauft 1987) relate to lethal violence in a much broader sense as they include, besides the conflicts we would identify as homicides, wars with neighbours, conflicts within tribes, and capital punishments for perceived transgressions such as theft, adultery or witchcraft.

IV. Variation of What? Homicide is a judicial category, which lumps together all cases where the intentional infliction of a trauma by a person leads to the death of another person  and the perpetrator is not legitimized by the state to inflict the lethal trauma

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(e.g. in a war or as an executioner). As a behavioural category homicide therefore is a mix of very different situations. This raises the question of whether the characteristics of homicides (and of violence more generally) are broadly similar in societies with much and with little interpersonal violence, the only difference being that there is more or less of the same phenomenon, or whether different kinds of violence dominate in high violence societies as compared to pacified societies. This is an important issue to address if one wants to understand the causes of macro-level variation in violence. The reason is that some types of violence or some groups of perpetrators may be more strongly influenced by contextual conditions related to the likelihood of violence than others. Currently there is limited consolidated knowledge about which contextual aspects of homicide are invariant across societal levels of homicide and which aspects vary in systematic way. However, it seems that for some aspects of homicide different comparative studies converge to similar findings. I briefly review a selection of relevant aspects. It is mostly based on sex of perpetrators: It is clear from empirical studies across the world that the vast majority of homicides are committed by men (Eisner 2003, Nivette 2011b: 11). However, much less is known about the extent of variation in the proportion of male perpetrators. Eisner (2003) has examined samples of assault and homicide over the past 800 years and concluded that the male perpetrator rate fluctuated within a rather narrow range of between 3 and 15%, with no clear trend over time or between societies. Some isolated findings may suggest that the preponderance of male perpetrators is even greater in high homicide societies than in pacified societies. However, no systematic research has yet been done on this issue. Social Disadvantage of Perpetrators: Evidence from many studies suggests that in pacified societies the majority of homicides is committed by people who are socially highly marginalized. Perpetrators often have no or irregular employment and a large proportion suffers from a combination of problems including substance abuse, mental health problems, etc. In contrast, it seems that in highly violent societies the perpetration of murders and manslaughters is much less constrained to disadvantaged groups. Cooney (1997, 1998) has argued that high involvement of the elites in violence was the rule over long historical periods, and that it was not limited to political conflicts but included a range of violent clashes over private matters amongst members of the elites, and ruthless lashing out against people at lower ranks. Eisner (2003) has presented some empirical evidence supporting the idea that historical societies with high levels of homicide often also displayed a high involvement of members of the elites in violent acts. In a similar vein, ethnographic research suggests that in some societies with a very high homicide rate men with a high prowess to fight and kill enjoy a higher social status and have a reproductive advantage over other men (Chagnon 1988, Ember & Ember 2007). Sex of Victims: In most societies the majority of homicide victims are men. However, the proportion of male homicide victims varies considerably be-

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tween societies. According to the Global Study on Homicide (United Nations Office on Drugs and Crime 2011), for example, the proportion of female victims varied between about 50% in Slovenia (53.8%), Korea (51%), Japan (50.0%), Germany (49.6%), Switzerland (49.1%), and Croatia (49.0) on the one hand, and less than 8% of all victims in, amongst others Honduras (6.9%), Paraguay (6.4%), Uganda (6.0%), Venezuela (5.0%), and Sri Lanka (3.7%). Historically, too, a similar range of values has been found across European societies between the Middle Ages and the 19th century (e.g. Eisner 2003, Hanawalt 1979, Sharpe 1981). The sex composition of homicide victims seems to differ systematically between high and low homicide societies. Using historical data the Finnish criminologist Veli Verkko (1967) found that in societies with low levels of homicide the proportion of female victims is usually large. In violent societies, in contrast, the majority of victims (often 80% and more) are usually men. This relationship seems to hold in many different societies over time and as levels of homicide change, and it appears to hold cross-sectionally (Roth 2009). It implies that in high violence societies most violence takes place between unrelated males. Relationship between Perpetrators and Victims: A common way to classify homicides is to distinguish between different groups of relationship between the victim and the perpetrator. The most basic distinction probably refers to whether the perpetrator and the victim were members of the same household or family on the one hand, or whether they were friends, acquaintances or strangers on the other (Wolfgang 1958). The number of domestic homicides as a proportion of all homicides committed in a society is highly variable. Amongst homicides recorded in judicial archives of the Middle Ages, for example, fewer than 10% of the cases related to the killing of a family member (Eisner 2003). In late 19th century England, in contrast, when overall homicide rates were much lower, more than half of all murders or manslaughters took place between family members. In England and Wales, members of the perpetrator’s family still accounted for about 50% of all cases in the 1970s, but the proportion dropped to about 25% by around 2002/3, when overall homicide rates in England and Wales were at their highest point during the 20th century (Smith, Coleman, Eder & Hall 2011). Several historians of homicide believe that the proportion of domestic homicides varies inversely with overall levels of homicide in the sense that high homicide societies generally have a low proportion of domestic homicides (Eisner 2003, Roth 2009, Spierenburg 2012). This would mean that overall variation in homicide rates is mainly driven by change in the level of lethal encounters between unrelated friends, acquaintances and strangers. Unfortunately, no comparative study of variation in perpetrator-victim relationship exists for contemporary societies or for primitive non-state societies, which would allow us to examine whether the hypothesized regularity universally exists across different types of cultures. Instrumental Homicide: By instrumental homicides I mean homicides that are committed to achieve some material advantage (e.g. robberies), that are

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part of organized crime or that can be seen as planned acts of private retaliation against an adversary. Unfortunately close to nothing is known about the extent to which instrumental homicide varies across societies. Qualitative studies suggest that motives related to protecting illegal markets, turf fights between opposing gangs, and retaliation in the context of vigilanti groups or revenge cultures play an over-proportionate role in high-homicide societies. The limited evidence that is currently available suggests that the typical profile of homicides in pacified societies is probably quite different from the profile of homicides in high homicide societies. In Table 1 I have tentatively put together some of the characteristics that probably distinguish the modal homicide in violent societies from that in a pacified society. They suggest that in low homicide societies the perpetration of assaults with lethal consequences is limited to a small group of highly pathological individuals, and that a considerable proportion of killings is committed within a domestic context. In societies with high levels of homicide, in contrast, homicides tend to be more likely to happen between men, to be linked to instrumental motives, to include violent entrepreneurs, and to happen in public space.

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Low Homicide Societies (< 2 per 100,000) Relatively large proportion of domestic homicides

High Homicide Societies (> 10 per 100,000) Predominance of homicides between acquaintances and strangers

Importance of Biological and Social Risk Factors

Biological risk factors dominate, many offenders with deficits in neuro-cognitive functioning

Social risk factors dominate

Social Class

Most perpetrators highly marginalised

High involvement of elite members, violence a strategic option for upward mobility

Sex of Victims

Relatively high proportion of female victims (> 30%)

Predominantly male victims, usually > 90%

Instrumental Violence

Low importance of strategic, instrumental homicide Weapons not usually carried in public, homicides committed without specialised murder instruments

A significant proportion of homicides has strategic purposes Weapons routinely carried by a significant proportion of men, a large proportion of homicides committed by means of specialised instruments

Characteristic V-P Relationship

Weapons

Table 1

Hypothesized pattern of characteristics of homicides in low and high homicide societies

V. Where does Variation come from? Across human societies the level of interpersonal within-group violence varies at least between 0.3 and 200 per 100,000 inhabitants per year for ‘conventional’ homicide, although the upper end is a lot higher if we include raids, civil wars, and short but extreme outbreaks of massacre and genocide in the picture. Comparative researchers have long been interested in the structural conditions that are associated with this kind of variation in levels of homicide. Emile Durkheim (1957), for example, wrote in his lectures held at the Sorbonne in 1902/3 about the relationship between homicide and individualism, observing that collectivist societies typically have higher homicide rates, because an indi-

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vidual’s life counts relatively little in comparison to the value of a collective (the family, the clan, the tribe), leading to a higher propensity to put one’s own life on the line in order to defend the integrity and honor of the group. Other researchers around the turn of the 19th and 20th century, like the Italian criminologist Augusto Bosco produced the first tables that compared homicide rates across a significant number of countries and interpreted them against other macro-level indicators such as illiteracy (Bosco 1889). More systematic research on structural correlates of macro-level variation in levels of homicide only started in the early 1970s (LaFree 1999). Since then considerable progress has been made in identifying conditions that are systematically associated with variation in homicide rates. However, there is still little agreement about which factors are most relevant and what the causal mechanisms are that link structural conditions to the likelihood of homicide. I therefore refer to these correlates as macro-level risk-factors. The notion of risk factors originates in individual-level research and denotes any variable that is consistently found to be associated with an increased likelihood of a negative outcome such as coronary heart diseases, even if the causal mechanism is not clear (Farrington). A macro-level risk-factor is therefore that is regularly found to be associated with variation in homicide rates between societies or countries.

VI. Correlates of Homicide in Modern Societies Over the past 50 years many studies have examined the correlates of crossnational differences in homicide rates in modern societies. There exist three major recent reviews of the literature. LaFree (1999) reviewed 34 studies published between 1965 and 1997. Trent and Pridemore (2012) summarized the field by providing a narrative systematic review of 70 peer-reviewed studies published up to 2010. And Nivette (2011a) conducted a meta-analysis of 55 empirical studies that have examined cross-national variation in homicide rates. The review by LaFree (1999) made six generalizations about risk-factors that predict homicide rates: LaFree found that economic inequality was the most consistent significant predictor of homicide rates; the study also reported much support for a negative relationship between homicide and economic development in the sense that economically advanced nations tend to have lower homicide rates than poor countries. In contrast, the study found no support for the hypotheses that urbanisation, unemployment rates or the demographic structure of a society were associated with variation in homicide rates, although LaFree suggests that high population was found to be an independent predictor of high homicide. In respect for effects of social and cultural heterogeneity the review by LaFree concluded that the evidence was contradictory. The review by Trent and Pridemore (2012) grouped the variables tested in the reviewed research into five bundles that are similar to those developed by

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LaFree (1999) namely development and industrialization, deprivation, urbanism, population structure, and social and cultural heterogeneity. Given that the studies examine the same universe of observation units and the same limited list of potential predictors it is sobering to learn that, in the authors view, “no definite generalizations” on the strength of theoretical perspectives or variables can be drawn (2012: 133). Like the earlier study by LaFree (1999) the authors conclude that evidence is contradictory or non-existing for the effects of urbanism and for the effects of the population structure on homicide. In contrast, evidence is found to be mostly supportive of the hypothesized links in two domains: Thus, the majority of studies are reported to find a negative association between modernization or development and homicide. Also, the authors conclude that the majority of studies find a positive association between measures of either absolute (child poverty) or relative (income inequality) deprivation and homicide. As to heterogeneity or fractionalization the authors conclude that several studies did find the expected positive effect, but that the overall picture is inconsistent. The study by Nivette (2011a) is particularly useful because the results of her meta-analysis provide, for the first time, a statistical summary of which predictors have been used in extant research, and what average effects (as measured by the standardized mean effect size correlation coefficients Mr) were found across the studies. In addition to macro level geographic variables such as ‘Latin America’ or ‘South East Asia’, which are best considered as substantially meaningless markers of something that is not understood, the variables found to be positively associated with homicide rates included income inequality - ratio measure (Mr = .416, Nstudies = 13), divorce rate (Mr = .277, Nstudies = 10) , population growth (Mr = .251, Nstudies = 9), income inequality – Gini Index (Mr = .224, Nstudies = 31), female labour force participation (Mr = .223, Nstudies = 13), infant mortality (Mr = .196, N = 8), and ethnic heterogeneity (Mr = .163, N = 12),. Factors found to be negatively associated with homicide rates were social welfare protection of the population (Mr = -.279, N = 4), ethnic homogeneity (Mr = -.247, N = 5), and modernization as measured by the human development index (Mr = -.163, N = 14). Some of these findings are in line with the two narrative reviews. In particular, the meta-analysis confirms that high social inequality is a robust predictor of high homicide rates and it provides further support to the assumption that countries with high ethnic and cultural heterogeneity tend to experience higher homicide rates. It also suggests that high socio-economic development as measured by the human development index may be linked to lower homicide rates, while high levels of poverty as measured by the infant mortality rate tend to be associated with higher homicide rates. The latter finding is consistent with the result that societies where citizens are better protected by welfare-state arrangements tend to have lower homicide rates. Finally, the study identifies two variable domains, namely the divorce rate and the female labour force participation rate, that were not addressed in the other two reviews. Both

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variables may be interpreted as indicating instability of family and socialization patterns, but they could also be indicators for other aspects of social structure that are less easily measured. It is of some concern that three recent reviews of the cross-national literature on homicide arrive at diverging conclusions about which bundles of variables of theoretical constructs are predictive of variation in homicide rates. Partly, this reflects the nature of the field, which has long been plagued by far from satisfactory and biased samples, a sometimes eclectic operationalization of theoretical constructs, very different theoretical interpretations of the same predictor variables, lack of attention to issuers of temporal order and spatial dependency, and the multicollinearity of conceptually different variables (Stamatel 2006). Nonetheless, the analyses lead to the conclusion that in modern nation states the homicide rate tends to be high in less-developed societies, in societies with high levels of social inequality and poverty, and in societies with a low integration of ethnic and cultural minorities. It is worth mentioning, however, that cross-national homicide research has tended to repeatedly use similar constructs over the past decades rather than exploring new theoretical and empirical avenues. One such new promising perspective are analyses of the link between variation in homicide rates and indicators of state functioning such as governance indicators, measures of corruption, or variables that capture state legitimacy (Neumayer 2003, Nivette & Eisner 2012). Overall, the few studies suggest that state functioning could prove to be a relevant but previously ignored factor associated with variation in intra-societal violence. For example, Nivette and Eisner (2012) examined the extent to which political legitimacy predicts levels of homicide rates. They found that political legitimacy is a strong and consistent predictor of homicide rates. As Nivette and Eisner state “The legitimate state, as measured in this paper, is one that evokes confidence in its ability to provide fair and equal rights and protection, conducts its government according to citizens' values, and does not prey upon its citizens (i.e. through political violence) (Gilley, 2006). Tested against a number of controls, the robust results show that where polities are considered legitimate, homicide is low.”

VII. Correlates of Homicide in Non-State Societies Anthropological research has generated an impressive volume of studies on violence in many societies around the globe (e.g. Bohannan 1960, Chagnon 1988, Heald 1990, Knauft 1987). However, few studies have tried to assess macro-level risk-factors for violence by taking an explicitly comparative perspective. Also, I am not aware of any systematic reviews of the cross-culturally comparative literature on homicide and violence in non-state societies. I therefore discuss three studies that have relied on relatively large samples of non-

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state societies with a view to drawing inferences about contextual factors that are associated with variation in homicide rates. One of the first such studies was conducted by political scientist Marc H. Ross (1985 #10824, 1986 #10825). It included a sample of 90 small-scale traditional societies and examined risk-factors for internal and external violence separately. In a multivariate model, four of the eleven included risk-factors for internal violence turned out to be significantly predictive: Two were structural characteristics. Internal violence tended to be high in societies with a low score in cross-cutting ties, i.e. weak political links between communities and the lacking sense of overarching solidarity ( = -.29) and in societies with strong fraternal interest groups, i.e. strong solidarity ties amongst kinsmen ( = .22). Although the two variables measure different constructs, they seem to tap on a common underlying dimension, namely the strength of bonds and allegiances within a primary group (the family, the clan) relative to the strength of ‘weak ties’ (Gravetter) between groups. The findings by Ross hence suggest that in non-state societies homicide was low where the sense of cohesion beyond the primary solidarity networks was high and where the mutual obligation within the kinship group was low. This finding echoes Durkheim’s hypothesis that the decline of homicide is correlated with the transition from mechanic solidarity (based on kinship) to organic solidarity (based on a generalized respect). The two other risk-factors found by Ross were related to socialization practices. Violence was found to be high in societies with harsh socialisation practices ( = .22) and lacking affectionate socialisation practices ( = -.31). This would suggest that levels of violence are transmitted over generations through a socialization pattern that emphasizes the warrior abilities of young men and that promotes notions of masculine honour and toughness. Interestingly, though, the effects of both socialization variables became non-significant once the variable measuring external conflict (i.e. war) was added to the equation. This probably suggests, as Ember and Ember (1994 #10199) have argued, that external conflict promotes more martial and aggressive socialisation practices, which in turn lead to higher levels of internal violence. In a different study, Rosenfeld and Messner (1991) analysed data for 32 societies derived from the Human Relations Area Files, a systematic collection of information about various aspects of cultures across the globe (Ember 1988). The study explicitly aimed at examining whether modern covariates of homicide can be replicated for pre-state societies, and included a range of potential covariates related to social inequality, social disorganization, and social development. Messner and Rosenfeld concluded that none of available measures of social inequality were related to levels of socially disapproved homicide. However, the authors found five macro-level risk factors that co-varied with levels of homicide. In particular, homicide rates were found to be lower in societies with high levels of political oppression (Kendall’s ba predominance of larger settlements (b = -.63), a large total population size (b = -.28), some degree of separate centralized political authority (b = -.28), and in societies with a differenti-

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ated military organization such as a standing army (b = -.57). Similar relationships were found when substituting homicide by measures of wife beating and drunken brawling, suggesting that these variables are macro-level risk-factors not only for homicide, but for violence more generally. Messner and Rosenfeld (1991) interpret their findings as suggesting that violence is suppressed more effectively in societies that have formalized conflict resolution mechanisms, greater centralized formal authority, and formal coercive mechanisms. They are therefore in line with the assumption that the evolution of state structures reduces interpersonal violence (Hobbes 1968 [1660]). Also, the significant negative effect of political oppression (i.e. societies oppressed by others are more peaceful) is in line with the argument by Keeley (1996) on peoples with low levels of violence in Northern America, namely that they tended to be either isolated defeated refugees or groups that were forcefully pacified by the victors. Furthermore, the negative effects of settlement size and large population size may indicate that higher levels of social differentiation and the rise of interaction chains with people outside the close kinship group are associated with lower levels of homicide. In the third study to be discussed here, Ember and Ember (1994) analysed up to 186 societies included in the Standard Cross-cultural Sample developed by Murdock and White (1969). That study concluded that cross-cultural variation in homicide and other kinds of violence was related to socialisation patterns, in particular the encouragement of aggression during late childhood amongst boys (Ember & Ember 1994). The most important bivariate correlates of homicide were encouragement of aggression during late childhood amongst boys (r = .40) parental hostility, (r = .31), and low overall parental warmth (r = -.50). Similar to the findings in the study by Ross (1985), a multivariate analysis suggested that the frequency of external wars and socialisation for aggression turned out as main predictors of homicide. It is difficult to determine whether any general conclusions can be reliably drawn from these studies on why nonstate societies vary in their levels of interpersonal violence. However, some support appears to exist for three general patterns: First, more intensive socialisation for aggression and pugnacious manliness seems to predict rates of interpersonal violence, and socialisation for aggression in turn is probably related to the likelihood that a society is involved in warfare with its neighbours. Second, the studies by Ross (1985) and Rosenfeld and Messner (1991) suggest that rates of interpersonal violence in pre-modern societies co-varied with the complexity of social organisation, in that societies with more intense networks of interaction across kinship boundaries, larger settlements, and a stronger overarching sense of cohesion tended to experience less violence. Third, Rosenfeld and Messner (1991) found evidence to support the notion that interpersonal violence tends to be lower as specialists of social control evolve, such as some kind of political and legal authority or a differentiated military organization.

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VIII. Correlates of Homicide in Historical Research When analysing macro-level variation in homicide, most comparative historians of crime are less interested in comparisons between societies at a given point in time than in explanations of the long alternations between ups and downs in homicide rates over time within one society. For example, their research might typically entail the search for explanations of why levels of homicide in England dropped massively from the early decades of the 17th century until the mid-18th century (Roth 2001), or why homicide tended to increase in most Western societies between the late 1950s and the early 1980s (Eisner 2008, Spierenburg 2008). Over the past 20 years many theoretical approaches have been developed to explain long-term variation in homicide rates, especially the significant declining trend of lethal violence in Europe since the Middle Ages. Most of these approaches are influenced by the seminal work of Norbert Elias, a German sociologist whose book The Civilizing Process was published in 1939 (Elias 1978). In that book Elias argued that the growing monopolization of the use of force by the state and the increasing interdependence of people in long chains of interlocking networks creates a pressure towards cultivating self-control, good manners, and long-sightedness, which in turn is associated with a decline in impulsive and disruptive behaviour such as individual violence. To date, there have been hardly any attempts to operationalize hypotheses about the causes of macro-level historical variation in homicide, especially over longer periods of time. Rather, existing work has tended to rely on qualitative evidence to develop arguments about potential explanatory mechanisms. This has resulted in a number of suggestions about the relevant covariates of historical variation in violence. I briefly review three studies in this field, namely Eisner (2003), Roth (2009) and Pinker (2011). Each of the three studies proposes a number of general macro level risk-factors assumed to correlate with historical variation in homicide. However, neither of these studies provides more rigid empirical tests of the hypothesized relationships in the sense of attempts to demonstrate patterns of association with quantitative risk-factors. In two papers in the early 2000s I suggested that by combining the results of a large number of primary studies it is possible to gain a differentiated picture of homicide trends in various European regions over the past 800 years (Eisner 2001, 2003). The main finding was that homicide rates declined significantly across Europe from the Middle Ages to the 19th century, but that there were significant differences in the timing and the speed of the decline. In many regions of Northern Europe the decline appears to have started in the late Middle Ages already and by the mid-18th century all of England, the Netherlands, Northern Germany, Sweden, and Norway typically had homicide rates of around 13 per 100,000. In contrast, many areas in Southern Europe, especially along a rim of areas around the Mediterranean and the Balkans retained high homicide rates until well into the 19th century.

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In an attempt to identify plausible macro-level correlates I distinguished several layers of explanation: First, I argued that at the most general level the long-term decline in homicide covaries with the gradual increase in centralized state power and bureaucratic control over the lives of people, supporting Elias’s argument, which in turn echoes the arguments developed by Thomas Hobbes in the Leviathan, about the pacifying effect of the state, and its consequences on the display of self-control. The second explanatory bundle I proposed were ‘disciplining revolutions’  large-scale coordinated efforts that involve the state but also non-state moral agencies such as the church or the family, which aim at fostering self-control, discipline, conscience, and innerdirectedness. Such revolutions in social control appear to have often been associated with shifts in societal levels of homicide. The third mechanism that I identified as a potentially relevant covariate was the relationship between the state and civil society, in particular legitimate state structures and political integration. The core argument here was that the turn towards declining homicide often occurred at times when there was not necessarily any real increase in sheer state coercive power, but a shift towards a more broadly accepted state that was seen as delivering justice and operating for the benefits of its citizens. The fourth major underlying force that I proposed was culture, more specifically the ideas of Protestantism and modern moral individualism (Eisner 2003: 132). While I introduced the notion of Protestantism and the ethic of selfdiscipline described by Max Weber in the specific context of European history, I believe the underlying idea can easily be generalized to a broader crosscultural context. It implies the notion that homicide should be expected to be low in societies that emphasize duty, sobriety and frugality, a methodic conduct of life, inner-directedness and conscientiousness as major principles of conducting one’s life. In the fascinating monograph American Homicide the historian Randolph Roth (2009) has produced a very large number of homicide estimates for many regions in the United States since the early 17th century. His analyses suggest very high rates of homicide in the early years of settlement and a massive decline until the early 19th century. Since then homicide rates have fluctuated in long swings, but remained at levels considerably above those of Europe. Roth has argued that a bundle of four interrelated factors have been correlated with variation in homicide rates both in the United States and in Europe over the past 400 years. Each of these factors reflects a specific aspect of legitimacy. The first of these factors is the belief that government is stable and that its legal and judicial institutions are unbiased and will redress wrongs and protect lives and property. This factor could also be interpreted as the rule of law  the idea that laws are effective and member of a society can rely on the institutional mechanisms for redressing wrongs. The second factor is described by Roth as “a feeling of trust in government and the officials who run it, and a belief in their legitimacy”. It alludes to a broader role of the state than that implied in the first factor, namely the idea that the fictional contract at the basis Hobbes’s Levia-

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than, which transfers previously held individual rights (such as the right to kill a thief) to the state, is based on reciprocity. The third component is “patriotism, empathy, and fellow feeling arising from racial, religious, or political solidarity”. This element incorporates the idea that cooperative behaviour amongst people who are not related to each through kinship ties is easier if it can be based on an imagined collective identity. The fourth and final component identified by Roth is “the belief that the social hierarchy is legitimate, that one’s position in society is or can be satisfactory and that one can command the respect of others without resorting to violence”. These hypotheses imply that historical variation in homicide rates is mostly driven by a cluster of factors that combine into the perceived legitimacy of the state and its institutions. Throughout the analyses, American Homicide presents a wealth of qualitative historical evidence in support of the four hypotheses. In a recent publication Roth (2012) makes a significant step towards more formal quantitative tests of his hypotheses. In particular, he proposes a number of quantitative measures of the proposed macro-level risk factors, using data from a variety of sources ranging from data on political rebellions and riots to results from content analyses of newspapers and books, such as counts of the occurrence of racist language in English between 1640 and 1720. The latest attempt to make sense of large-scale historical change in interpersonal violence is The Better Angels of our Future by Harvard psychologist Stephen Pinker. The book examines the whole range of manifestations of violence from war and genocides to human sacrifice, the death penalty and homicide over the course of human history and prehistory. Essentially, Pinker argues that violence has declined over the past hundreds and thousands of years, and that five macro dynamics have contributed to the long-term decline in violence: He calls them the Leviathan, gentle commerce, feminization, the expanding circle, and the escalator of reason. With the exception of feminization, the mechanisms identified by Pinker reformulate, in an innovative way, ideas and observations made by social philosophers and sociologists such as Hobbes, Montesquieu, Bentham, Kant, and Elias over the past 400 years. The post powerful mechanism, in the view of Pinker, is the coercive power of the state, especially if it manifests itself under its more benevolent appearance of a democratic state based on the rule of law (Pinker, 2011: 680). The second mechanism that Pinker proposes, following a long string of enlightenment thinkers, relates to the pacifying effect of capitalism and commerce in that people and states who exchange goods and services can be expected to have a disincentive to fight each other, at least as long both believe that the exchange is to their own advantage (Pinker 2011: 682f). Pinker’s third argument is that large scale variation in violence is related to feminization, i.e. violence goes down where women ‘get a better deal’ (Pinker 2011: 684f). The concept not very well defined, but it seems to imply something like ‘female-friendly’ values (Pinker 2011: 685), respectively a society that moves away from manly honor, approval of violent retaliation, and veneration or martial glory (Pinker 2011). Mechanisms four and five elabo-

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rate the notion that the ideas that underpin the ‘project of modernity’ have a pacifying force because the universal power of reason will eventually overcome prejudice, hatred and the right of the stronger that lie at the heart of violence. Thus, Pinker expects violence to decline as sympathy and cosmopolitanism take hold in the modern world (the expanding circle) and to the extent that humans apply the principles of reason to their affairs (the escalator of reason).

IX. Conclusion In this paper I examined three research traditions that have examined macrolevel covariates of variation in human violence, namely cross-national criminological research on homicide across contemporary societies, comparative anthropological research on violence in non-state societies, and historical research on macro-level variation in homicide over time. My main goal was to understand how much the rates of homicide vary across human societies, what factors have been found to be predictive of such variation, and whether any covariates of homicide may exist across very different types of societies. The review suggests that human societies show an enormous variability in the likelihood that members of the same group kill each other. The societies with the lowest levels of interpersonal killing ever empirically observed had homicide rates below 0.5 per 100,000 per year. Taking the effects of modern medical technology into account I argued that rates of around 0.2 killings per 100,000 inhabitants may be lower boundary of what one might consider possible. It proved more difficult to determine an upper limit to lethal within-group violence. If we limit the perspective to acts that resemble criminal homicides in that they violate shared norms and are committed by individuals or groups of individuals, then the upper limit may be in the range of 100-400 per 100,000 per year. However, the transition from criminal homicide to organized use of force, civil war, and large scale massacres, lynchings or genocides is gradual rather than categorical, and there seems to be no upper limit for the lethality of such acts of organized violence. Societies with high and with low levels of homicide do not only differ in the overall amount of lethal violence, but the gradient from peaceful to violent societies appears to be systematically associated with change in the quality of violence. Evidence suggests that in low homicide societies the majority of killings are committed by highly marginalized people who usually experience a number of psychological, genetic, neuro-cognitive and family risk factors. In contrast, in high homicide societies violence is much more goal-driven, embedded in economies of violence and protection, and coordinated or carried out by powerful individuals. The examination of macro-level factors associated with homicide suggested that within each of the three traditions of research there remains considerable disagreement about the factors that account for high or low levels of homicide

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in a given society. While researchers working in each of the three fields appear to share some underlying assumptions, there is little sign of an emerging consensus yet. To some extent, this may be because each tradition of research has its own theoretical anchors which lead to diverging interests in examining specific potential correlates of variation in interpersonal violence. Also, there are important limitations in how reliably differences in homicide levels can be measured for any society other than modern affluent nation-states, and the extent to which relevant potential predictors can be collected. However, it is also likely that at least some factors associated with variation in violence are specific to some types of societies. For example, variation in levels of violence amongst simple hunter-gatherer societies may be driven by contextual forces (e.g. war-prone neighbours leading to socialization for aggression, beliefs in witchcraft as a source of harm-doing, etc.) that are much less relevant in a comparison between modern nation states, where factors such as the functioning of the police force or the importance of illicit markets may play an important role. Nonetheless, it seems that some shared general topics emerge in all three research traditions: In all three traditions researchers found evidence suggesting that the existence of an accepted authority which organizes power and delivers justice is associated with lower levels of interpersonal violence. In all three research traditions, too, at least some researchers concluded that higher social complexity and interdependence are associated with a lower likelihood of violence. Both of these findings align well with classical enlightenment thinking about proper state functioning and functional interdependence between its members as two major prerequisites for internal peace. Some other shared topics emerge in at least two of the reviewed research traditions, but not in the third. For example, the finding in anthropological research about the association between socialization practices based on affection and parental warmth on the one hand and low interpersonal violence on the other seems related to the idea, developed by historians of violence, that the historical decline in violence was linked to a greater emphasis on parental warmth, conscience, and domesticity in many societies. Also, the rather strong link between social inequality and homicide found in contemporary research is possibly echoed in Roth’s argument that variation in historical homicide rates is linked to the sense of living in a society with a fair hierarchy, where people are respected without having to resort to violence, but resource inequality has not been found to be covariate of homicide in anthropological research.

References Becker, M. (1976). Changing Patterns of Violence and Justice in Fourteenth Century Florence. Comparative Studies in Society and History, 18, 281–296.

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Hanawalt, B. A. (1979). Crime and Conflict in English Communities, 1300–1348. Cambridge: Cambridge University Press. Heald, S. (1990). Controlling Anger: The Sociology of Gisu Violence. Manchester: Manchester University Press. Hobbes, T. (1968 [1660]). The Leviathan. Harmondsworth: Penguin. Karonen, P. (2001). A Life for a Life versus Christian Reconciliation: Violence and the Process of Civilisation in the Kingdom of Sweden, 1540–1700. In H. Ylikangas, P. Karonen & M. Lehti (Eds.), Five Centuries of Violence in Finland and the Baltic Area (pp. 1-83). Columbus: Ohio State University Press. Keeley, L. H. (1996). War before Civilization: The Myth of the Peaceful Savage. Oxford: Oxford University Press. Knauft, B. M. (1987). Reconsidering Violence in Simple Human Societies: Homicide among the Gebusi of New Guinea. Current Anthropology, 28 (4), 457. doi: 10.1086/203549. Knauft, B. M., Abler, T. S., Betzig, L., Boehm, C., Dentan, R. K., Kiefer, T. M. & Rodseth, L. (1991). Violence and Sociality in Human Evolution [and Comments and Replies]. Current Anthropology, 32 (4), 391-428. LaFree, G. D. (1999). A Summary and Review of Comparative Cross-National Studies of Homicide. In M. D. Smith & M. A. Zahn (Eds.), Homicide: A Sourcebook of Social Research (pp. 125-145). Beverly Hills: Sage. Mead, M. (1928). Coming of Age in Samoa: A Psychological Study of Primitive Youth for Western Civilization. New York: William Morrow. Monkkonen, E. (2001). New Standards for Historical Homicide Research. Crime, Histoire et Société - Crime, History and Society, 5 (2), 7-26. Murdock, G. P. & White, D. R. (1969). Standard Cross-Cultural Sample. Ethnology, 8 (4), 329-369. Neumayer, E. (2003). Good Policy can lower Violent Crime: Evidence from a CrossNational Panel of Homicide Rates, 1980-97. Journal of Peace Research, 40 (6), 619-640. doi: 10.1177/00223433030406001. Nivette, A. E. (2011a). Cross-National Predictors of Homicide: A Meta-Analysis. Homicide Studies, 15 (2), 103-131. Nivette, A. E. (2011b). Violence in Non-State Societies. British Journal of Criminology, 51 (3), 578-598. doi: 10.1093/bjc/azr008. Nivette, A. E. & Eisner, M. (2012). Do Legitimate Polities have Fewer Homicides? A Cross-National Analysis. Homicide Studies. doi: 10.1177/1088767912452131. Pinker, S. (2011). The Better Angels of Our Nature: Why Violence has declined. London: Viking. Robarchek, C. A. & Robarchek, C. J. (1998). Reciprocities and Realities: World Views, Peacefulness, and Violence among Semai and Waorani. Aggressive Behavior, 24 (1), 123-133. Rosenfeld, R. & Messner, S. F. (1991). The Social Sources of Homicide in Different Types of Societies. Sociological Forum, 6 (1), 51-70. doi: 10.1007/bf01112727. Ross, M. H. (1985). Internal and External Conflict and Violence: Cross-Cultural Evidence and a New Analysis. The Journal of Conflict Resolution, 29 (4), 547-579. Roth, R. (2001). Homicide in Early Modern England, 1549-1800: The Need for a Quantitative Synthesis. Crime, Histoire et Société - Crime, History and Society, 5 (2), 3368. Roth, R. (2009). American Homicide. Cambridge, MA: Belknap Press.

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Roth, R. (2011). Biology and the Deep History of Homicide. British Journal of Criminology, 51 (3), 535-555. doi: 10.1093/bjc/azr029. Roth, R. (2012). Measuring Feelings and Beliefs that may facilitate (or deter) Homicide. Homicide Studies, 16 (2), 197-216. doi: 10.1177/1088767912442501. Schiefenhoevel, W. (1988). Indoctrination among the Eipo of the Highlands of WestNew Guinea. In I. Eibl-Eibesfeld (Ed.), Indoctrinability, Ideology and Warfare: Evolutionary Perspectives (pp. 109-132). Oxford: Berghahn. Sharpe, J. A. (1981). Domestic Homicide in Early Modern England. The Historical Journal, 24 (1), 29-48. Smith, K., Coleman, K., Eder, S. & Hall, P. (2011). Homicides, Firearm Offences and Intimate Violence 2009/10. Supplementary Volume 2 to Crime in England and Wales 2009/10 (2nd Edition). London: Home Office. Spierenburg, P. (2001). Violence and the Civilizing Process: Does it work? Crime, Histoire et Société - Crime, History and Society, 5 (2), 87-105. Spierenburg, P. (2008). A History of Murder: Personal Violence in Europe from the Middle Ages to the Present. Cambridge: Polity Press. Spierenburg, P. (2012). Long-Term Historical Trends of Homicide in Europe. In M. C. A. Liem & W. A. Pridemore (Eds.), Handbock of European Homicide Research (pp. 25-38). New York: Springer. Stamatel, J. P. (2006). Incorporating Socio‐Historical Context into Quantitative Cross‐ National Criminology. International Journal of Comparative and Applied Criminal Justice, 30 (2), 177-207. doi: 10.1080/01924036.2006.9678752. Trent, C. L. S. & Pridemore, W. A. (2012). A Review of the Cross-National Empirical Literature on Social Structure and Homicide. In M. C. A. Liem & W. A. Pridemore (Eds.), Handbook of European Homicide Research (pp. 111-135). New York: Springer. United Nations Office on Drugs and Crime. (2011). Global Study on Homicide. Vienna: UNODC. Verkko, V. (1967). Static and Dynamic "Laws" of Sex and Homicide. In M. Wolfgang (Ed.), Studies in Homicide (pp. 36-44). New York: Harper and Row. Wolfgang, M. E. (1958). Patterns in Criminal Homicide. Philadelphia: John Wiley.

One flew over the Cuckoo’s Nest: Violence, Uncertainty, and Safety MANFRED J. HOLLER AND BARBARA KLOSE-ULLMANN

Introduction Can we start a street fight to learn about the properties and effects of violence? Can we take away a family´s property to find out how its individual members and the family as a social entity behave under the threat of starvation? Can we throw people into a dungeon to let them feel the pain of uncertainty and hunger? Why not look at theatre plays when we want to find out more about violence and uncertainty? Plays can be understood as a substitute for social experiments. As such they can be of help, especially when it is impossible or even immoral to conduct experiments dealing with human suffering or violations of basic rights, or may end in uncontrollable catastrophes. In his review of Daniel Kahneman’s new book “Thinking, Fast and Slow”, Freeman Dyson, Professor of Physics Emeritus at the Institute of Advanced Studies in Princeton, points out that “strong emotions and obsessions cannot be experimentally controlled” (Dyson 2011: 43). He argues that the methods that earned the psychologist Kahneman a Nobel Prize in economics do not allow him to study them. “The part of the human personality that Kahneman’s method can handle is the nonviolent part, concerned with everyday decisions, artificial parlor games, and gambling for small stakes. The violent and passionate manifestations of human nature, concerned with matters of life and death and love and hate and pain and sex, cannot be experimentally controlled and are beyond Kahneman’s reach” (Dyson 2011: 43). He further argues that the “artistic” approach of Sigmund Freud (and William James) is the territory in which to study violence and passion. “Freud can penetrate deeper than Kahneman because literature digs deeper than science into human nature and human destiny” (Dyson 2011: 43). To make use of theatre plays does not necessarily imply that we have chosen a post-Freudian approach. But we have to admit that we started from the premise: What happens on stage can be taken as a blueprint and extract of real life – also when it comes to violence, uncertainty, and safety. Of course, there are numerous historical studies of violence, uncertainty, and safety, based on episodic evidence, on the one hand, or quantitative data, on the other. However, the plays that we consider in the following are distilled

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by the experience and understanding of their authors and filtered by an audience of millions. If they do not reflect fundamental human dimensions in an adequate way, then at least we can assume that they shaped the images of them. So let us take this material and check whether it helps us to further our understanding of violence, uncertainty, and safety. However, let us first ask about the relationship of violence and uncertainty. Why to combine these two concepts? ‒ They both seem adverse to safety and security: violence in a more direct, often physical way, while uncertainty threatens the mental and psychological balance. But uncertainty is also used to resist aggressive violence, especially if aggression is not induced by emotions but executed with some degree of rationality. A rational aggressor wants to be sure that the aggression is successful. Camouflage is a means to circumvent aggression as by its very nature it destroys or undermines information. Lack of information (or knowledge) implies uncertainty. Ketman1 is such a form of dissimulation, or political or religious camouflage. It helps to survive when open dissent would result in persecution and Gulag or KZ. Ketman has always been widely practiced by people in any forms of ideocracies as their only possibility to survive in a decent way. The Polish author and Nobel Laureat Czesław Miłosz finds strong similarities between Ketman and the customs cultivated in the totalitarian regimes of the Comecon countries. He observes that it “is hard to define the type of relationship that prevails between people in the East otherwise than as acting, with the exception that one does not perform on a theatre stage but in the street, office, factory, meeting hall, or even the room one lives in. Such acting is a highly developed craft that places a premium upon mental alertness. Before it leaves the lips, every word must be evaluated as to its consequences. A smile that appears at the wrong moment, a glance that is not all it should be can occasion dangerous suspicions and accusations. Even one´s gestures, tone of voice, or preference for certain kinds of neckties are interpreted as signs of one´s political tendencies” (Miłosz 1990: 54). In his book Réligions et philosophie dans l´Asie Centrale (Religions and Philosophy in Central Asia) of 1865, Joseph Arthur de Gobineau2 notes that the Muslims believe that “He who is in possession of truth must not expose his person, his relatives or his reputation to the blindness, the folly, the perversity of those whom it has pleased God to place and maintain in error”. In other words, he must hide his true beliefs. A

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The term comes from the Arabic word kitmān which means secrecy or concealment. During his six years as a diplomat in Persia, the French diplomat, writer and ethnologist Joseph Arthur de Gobineau became aware of Ketman as a widespread phenomenon in Persia. Gobineau attained a notorious reputation as author of the Essai sur inégalité des races humains – 4 vol. 1853-55 (Essay on the Inequality of Human Races) which influenced Wagner, Nietzsche and especially Adolf Hitler. Therein he tried to demonstrate the superiority of the Arian race over the two other “primary” races: the black and the yellow race.

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Persian once told Gobineau, “ there is not a single true Muslim in Persia.” “Nevertheless”, says Gobineau, “there are occasions when silence no longer suffices, when it may pass as an avowal. Then one must not hesitate. Not only must one deny one´s true opinion, but one is commanded to resort to all ruses in order to deceive one’s adversary…”

Thus one acquires the multiple satisfactions and merits of having placed oneself and one´s relatives under cover, of not having exposed a venerable faith to the horrible contact of the infidel, and finally of having, in cheating the latter and confirming him in his error, imposed on him the shame and spiritual misery the he deserves” (quoted in Milosz 1990: 57-58). Miłosz categorizes the different forms of Ketman, he observes in the totalitarian Comecon countries to include National Ketman, The Ketman of Revolutionary Purity and The Metaphysical Ketman, the latter occurring generally in countries with a Catholic past like Poland (cf. Miłosz 1990: 54-81, and Donskis 2008: 140-150). Miłosz’s Ketman is role acting in the real world to protect himself. However, history demonstrates that role acting in the real world can be most hazardous especially if a man in power confuses the real world and the theatre stage, and poetry and history. It is said that Alexander the Great “saw himself as the new Achilles, and along with his friend Hephaestion as the new Patroclus, to have been replaying the Trojan War (on one occasion cruelly reworking the scene in the Iliad in which Achilles drags the body of the dead Hector from his chariot around the walls of Troy  though in Alexander’s case the victim was, for a little while at least, still alive)” (Beard 2011: 27). In this paper, however, we will focus on theatre plays where blood is red marmalade and dead people go to the backstage bar and have a drink. But we will analyse well-known plays that show Ketman-type acting on stage, following this strategy to outbalance aggressive violence and to assure safety. Section 2 presents feigned madness as a form of dissimulation chosen as a survival strategy. In Shakespeare’s “King Lear” and “Hamlet” this strategy is straightforward – a means of the weaker party to avoid extermination. In Ken Kesey´s novel “One Flew over the Cuckoo´s Nest,” successfully adapted as theatre play and movie, this relationship is much more subtle as the discussion will show. Section 3 illustrates a relationship of security (and uncertainty) and the law when law is applied as instrument by the Great Elector to enforce his will. The submission to the law is declared as a safe haven in Heinrich von Kleist’s play “The Prince of Homburg” in which the Great Elector (Elector Frederick of Brandenburg) declares his and his country´s strict submission to the law. But when the application of legal rules is in conflict with his political intentions, he is prepared to sacrifice the certainty they are to guarantee his ambition. Section 4 refers to Friedrich von Schiller’s Wallenstein trilogy. Schiller presents Wallenstein as a military leader who wants to keep his strategic options in order to maintain his power to act. However, this policy of hesitation results in his violent downfall and the downfall of those who believed in him. Readers who are not curious to find

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our arguments distilled from theatre plays may start the reading of the paper with the concluding Section 5. This section briefly discusses the method chosen for this paper and generalizes some of its results.

II Madness and Safety In many cultures mad people are outside of society, they are discriminated against, and suffer from this discrimination, but also enjoy the freedom to live without obligations. There are cultures in which mad people are ranked to be the wise ones or even declared sacred. Who would deny food to a saint or even violate his bodily integrity by beating or killing him? Madness can give shelter against aggression and assure safety. To what degree can people play madness to protect themselves? Is this a viable strategy? Here we will try to find a preliminary answer to these questions by looking into three well-known plays: King Lear, Hamlet and One Flew Over the Cuckoo´s Nest.

II.1 King Lear The Earl of Gloucester has two sons: Edgar, the first-born, and an illegitimately born second son named Edmund. In the beginning of Shakespeare’s “King Lear” it seems to be made obvious that both sons are appreciated and loved by their father: “Our father´s love is to the bastard Edmund/ As to th´legitimate” (I, 2, 17f.). Thus, Edmund does not have to be afraid to be left out in his father´s last will on the basis of being the second-born and not in wedlock. The old AngloSaxon law provided for the inheritance being equally shared among all children, irrespective whether in wedlock or not. However, this principle became questionable by (a) the principle of primogenitur, based on Roman law, whereby the first-born would be the sole heir and (b) arbitrary changes of the testament by the testator. Note that between 1590 and 1610,3 the social attitude towards illegitimate children changed substantially: The latter were more and more considered belonging to lower classes and their chances as equal heirs were diminishing (see Schülting 2007: 368-369). As a consequence, Edmund is appalled by King Lear´s decision to divide the country among his two daughters Regan and Goneril and disinherit his youngest daughter Cordelia, instead of dividing it among the three as he had intended before the “love test.” Edmund becomes afraid, being the secondborn and “bastard” son, to get his equal share of Gloucester´s heritance. Edmund: “Wherefore should I Stand in the plague of custom, and permit

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Shakespeare wrote “King Lear” between 1603 and 1606.

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The curiosity of nations to deprive me, For that I am some twelve or fourteen moonshines, Lag of a brother? Why bastard? Wherefore base?” (I, 2, 2-6)

Therefore he forges a letter in Edgar´s name which reads: “This policy and reverence of age makes the world bitter to the best of our times; keeps our fortunes from us till our oldness cannot relish them. I begin to find an idle and fond bondage in the oppression of aged tyranny, who sways, not as it hath power, but as it suffer´d. Come to me, that of this I may speak more. If our father would sleep till I wak´d him, you should enjoy half his revenue for ever, and live the beloved of your brother, EDGAR.” (I, 2 46-54)

Edmund shows the letter to his father and Gloucester believes that it was written by Edgar. He is furious about his son´s “conspiracy” and wants to punish him. Gloucester: “This villain of mine comes under the prediction; there´s son against father: the King falls from bias of nature; there´s father against child.” (I,2, 109ff)

Edgar is forced to flee. He disguises as “mad Tom O´ Bedlam” or Poor Tom. In Britain, since the time of Shakespeare, the term ‘Tom O´Bedlam’ or ‘poor Tom’, or ‘Abraham-men’ was used for beggars and vagrants who were or pretended to be lunatics. They claimed to have been former patients at the Abraham ward at Bethlehem Royal Hospital (Bedlam) which was a mental institution. The term was adopted as a technique of begging, or was used for playing the role of a madman. Says Edgar “…poor Tom!/ That´s something yet: Edgar I nothing am.” (II, 3, 20f.). Edgar survives all the scams and intrigues. When Lear and the Fool seek shelter from a storm out in the heath, they enter a hovel in which Edgar is sitting disguised as a madman. Calling himself poor Tom he sings “Pillicock sat on Pillycock hill:/Alow, alow, loo, loo!” This is a children´s song, but also obscene pun: Pillicock has the meanings 1. honey, darling and 2. penis; Pillicock Hill is, in this context, a circumscription for vagina. Without being recognized, poor Tom, i.e. Edgar, takes care of his father who was in the meantime blinded by Regan and her husband Cornwall. He challenges Edmund (V, 3, 110ff) to become Earl of Gloucester and wounds him fatally in a duel. Before Edmund´s death (V, 3, 168ff), Edgar reveals himself to the dying brother. At the very end of the play, the Duke of Albany, the husband of dead Goneril, asks Kent and Edgar ”… to rule in this realm…” (V, 3, 320). King Lear and his three daughters are dead. It remains open whether Edgar becomes King and will lead England to a better, and less violent future.

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Edgar survived because he feigned madness and madness gave him shelter. He used madness to hide from his brother Edmund who hated him and preferred to see him dead, and from his father who is furious at him. It is a rather efficient means to be safe, as his dissimulating madness allows him to disguise as a poor beggar while still being able to help his father in disguise. Apparently, in Shakespeare’s days, pretending on stage to be somebody else, and play a theatre in the theatre, was quite common. In King Lear, the Earl of Kent takes on the role of a servant called Caius after he was banished by Lear when he implored the king to be less rigid towards Cordelia and not disinherit and abdicate her. In the mask of Caius he can continue serving and protecting his king. Faking madness provides Edgar with safety without making use of violence. Taking on the role of Poor Tom proves to be a successful survival strategy. He switches back to his “real” life when he considers the time right, i.e., at the very end of the tragedy. In the end, the time of role playing and uncertainty is over.

II.2 One flew over the Cuckoo´s Nest McMurphy thought that pretending to be mad and being transferred to a mental clinic rather than serve the rest of his penalty term in prison would give him an easier life. However, his transfer to hospital turns out to be fatal for him. Ken Kesey´s novel was published in 1962. A year later, it was adapted into a Broadway play by Dale Wasserman, followed by a film version in 1975 which won five Academy Awards. The author got the inspiration for his book while working on a night shift at a veteran´s hospital, talking to patients. He did not believe that they were insane but that society had pushed them out. At this time it was rather common in the US, but not only there, to lock people who deviated from standard behaviour, into a mental institution. The story is set in an Oregon mental clinic and tells much about the institutional process in such a place. Its focus is on the character Randle Patrick McMurphy who successfully faked to be insane while he was in prison. So he was transferred to a mental hospital. He constantly antagonizes the ward´s head nurse Mildred Rached who rules the place most rigidly. McMurphy is convinced that he will stay only for a few months in the institution, i.e. as long as his remaining prison term. Only later does he learn that he is not one of the regular ‘acutes’ who are there on a voluntary basis and have chances to leave the place. He is there for an undefined time. That is his first misjudgement. His second misjudgement is the following. He misses the opportunity to flee from the institution as he had become friends with other patients at the ward: Chief Bromden, Billy Bibbit, Dale Harding, etc. He wants to help them against Big Nurse Rached’s extremely oppressive ruling that they did not understand. Her methods are too subtle for them to grasp.

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McMurphy motivates them to fight these methods, more than once he gets electro shock treatments when he behaves too obnoxiously for the nurse´s taste. When Billy Bibbit committed suicide after the Big Nurse had threatened him to tell his mother of his “sexual experience”, McMurphy is so furious that he almost strangles the nurse. That is the point where he is forced to undergo a lobotomy.4 After being lobotomised McMurphy is in a vegetative state, silent and motionless. The Chief is appalled and deeply sad when McMurphy is wheeled back lying on a Gurney: “There´s nothin´ in the face. Just like one of those store dummies…” (Kesey 1974: 308) Before Chief Bromden jumps through the broken window and escapes from the hospital, he smothers McMurphy with a pillow, commenting his action: “I lifted the pillow, and in the moonlight I saw the expression hadn´t changed from the blank, dead-end look the least bit, even under suffocation” (Kesey 1974: 309). In general, a hospital symbolizes security and personal safety for the patients. It is run by people who are meant to help their clients with their physical or psychological problems. But for McMurphy being in hospital did not mean safety, on the contrary. As it turned out McMurphy exchanged possible open brutality in prison with subtly exercised oppressiveness, in other words, an outright prison with a ward in a hospital and this hospital was run by the Big Nurse as restrictively as a prison. The punishment for McMurphy’s non-compliance to her rules was death: first his soul was killed by lobotomy and then he was physically killed through an act of grace committed by Chief Bromden. The rules celebrated by Big Nurse Ratched were meant to reduce uncertainty in an environment which is rather unstable by its nature due to the mental state of its clients. However, the suicide of Billy Bibbit signals that the rules do neither guarantee security for the institution nor safety for its patients. But this was not a threat to the functioning of the institution as long as McMurphy’s non-compliance did not make this defect obvious. The response was lobotomy. This was an act of violence, and not a medication, as McMurphy, quite different from Edgar in the preceding section, was poorly faking insanity.

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Lobotomy was rather common for mentally disturbed “overactive” or violent people to make them become again a “normal, satisfied member of society.” It is a type of neurosurgery. It consists of cutting the connections between the frontal lobes (or prefrontal cortex) and the rest of the brain. It was believed that severing such connections would calm patients´ emotions and stabilize their personality without affecting their intelligence and motor functions. In the USA, it was a common treatment in the 1950s with gradual decline of the procedure in the 60s. By the early 1970s, the application of lobotomy had nearly ceased. Instead, by then, a large range of psycho-pharmaceuticals was available to treat mentally troubled people.

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II.3 The Hamlet Plot Shakespeare wrote Hamlet around 1600 and published it in 1603. Already several centuries before, in the turn of the 12th to the 13th century, the Danish historian Saxo Grammaticus had narrated a similar story about crimes and feigned madness called Amletus. Here, Amletus is the legitimate successor of his father Horwendillus, governor of Jutland. He is afraid that his uncle Fengo, who had killed his father and married his mother Gerutha, will also kill him. Therefore he pretends to be insane by doing all sorts of very odd things. His mother becomes suspicious but he sticks firmly to his disguise as a madman. Yet, Fengo remains doubtful and sends him to England. His two nephews who accompany him are supposed to transmit to the king of England Fengo´s order to kill Amletus. The latter manages to alter this order such that the English king is asked to kill his nephews which he does. Amletus marries the king´s daughter and returns to Jutland. Fengo tries again to kill Amletus but Amletus survives killing Fengo instead. Finally Amletus becomes the governor of Jutland and succeeds his grandfather, i.e. his mother´s father, RØrik, as Danish king. But, in the end, he was slain in a battle against Wiglek, RØrik´s successor. In 1589, a play called Ur-Hamlet was performed in London and alluded to the author of “The Spanish Tragedy,” Thomas Kyd. It got lost, but might have been Shakespeare´s inspiration. In Kyd´s play, it is known among the Danes that Fengo killed his brother Horwendillus, allegedly in order to liberate Gerutha from her brutal husband. In Shakespeare´s Hamlet, Claudius murders his brother, King Hamlet, in all secrecy by pouring poison into the sleeper´s ear. Thereafter, Claudius marries King Hamlet´s wife Gertrude. Both are now reigning over Denmark. Hamlet gets to know about his uncle´s horrible deed only a few months later when he visits his father´s grave. There, his father´s ghost appears telling him about the murder and makes Hamlet and his friends Horatio and Marcellus swear by Hamlet´s sword “never to speak of this…” and to revenge his father. Hamlets asks his friends (I, 5, 177ff) “How strange or odd some´er I bear myself – As I perchance hereafter shall think meet To put an antic disposition on – That you, at such time seeing me, never shall, With arms encumber´d thus, or this head-shake, Or by pronouncing of some doubtful phrase,…. That you know aught of me  this do swear….”

As Hamlet intends to proceed with his revenge in all secrecy, he decides to use the disguise of madness exemplified by extremely foolish behaviour (“to put an antic disposition on); his behaviour changes between madness and the clownish performance. When his former school friends Rosencrantz and Guildenstern tell him of a group of play-actors that they met on their way to the castle and who are com-

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ing to offer their services he asks them whether they can perform “The Murder of Gonzago”: Hamlet muses: “…I have heard That guilty creatures sitting at a play Have, by the very cunning of the scene, Been struck so to the soul that presently They have proclaim´d their malefactions…” …I´ll have these players Play something like the murder of my father Before mine uncle. I´ll observe his looks… …The play´s the thing Wherein I´ll catch the conscience of the king” (II,2,583ff)

This scene shows two quite different but related aspects: (a) Shakespeare considered the effect of a theatre play on the audience so direct and impressive, strong enough to reveal a spectator´s malefaction. Indeed, Claudius reveals his secret when he sees on stage how Gonzaga is murdered by having poison poured in his ear.  This demonstrates how well the stage is suited to be used for re-enacting real life and social experiments. (b) The travelling theatre players’performance was only possible because Claudius and Gertrude considered Hamlet to be insane or at least quite disturbed although they were not sure of the reason for his state of mind. In any case, they did not refuse to watch the performance that Hamlet had arranged with the travelling players. Whenever necessary, Hamlet puts on the disguise of a madman. Hiding behind his alleged madness or foolishness he wants to conceal that he knows of Claudius´ crime. His plan to prove Claudius guilty works out, but Hamlet hesitates to kill his uncle. In the last scene (V, 2, 314ff) the Queen dies of poisoned wine which according to Claudius was meant to be offered to Hamlet. Instead, Claudius is deadly wounded by Hamlet with Laertes’ poisoned rapier and thereafter forced by Hamlet to drink the potion that he prepared for Hamlet. “Here, thou incestuous, murd´rous, damned Dane, Drink off this potion. Is thy union here? Follow my mother” (The King dies.) (V,2, 328ff). Hamlet´s disguise strategy is successful to the effect that he reveals the murderous crime committed jointly by his uncle Claudius and his mother. This turns the uncertainty into certainty, but this certainty does not provide safety to Hamlet. On the contrary, because this certainty makes him drop the veil of madness and now his uncle knows that Hamlet knows, violence is the winning pattern. Instigated by Claudius, Hamlet is challenged by Laertes to have a duel with him. Deadly wounded, Laertes tells Hamlet that his rapier is poisoned and Hamlet is bound to die, too, although he is not seriously wounded.

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III. The Safe Haven of “Law and Order” As alluded in Kleist’s play “The Prince of Homburg,” a rigorous legal setting provides a firm framework of security which the ruler and the people can rely on. Quintessentially the ruling Elector is convinced that a well-functioning state could not exist without laws and the understanding that they have to be abided by (“pacta sunt servanda”). Non-compliance with the law leads to chaos. However, the play demonstrates that there might be circumstances where a deviation from the tight path of the law is necessary to stabilize the state and to assure safety to its Prince and its people. Heinrich von Kleist wrote the play in 1809/1810. He wanted to motivate the Prussian people and other German states to fight against the Napoleonic occupation. The play takes place in 1675, when – so the historical facts – the Brandenburgers under their Elector Friedrich Wilhelm (“The Great Elector”) fought against the Swedes. In the play, Friedrich Wilhelm’s explicit intention is to annihilate the Swedes in this final battle, the Battle of Fehrbellin (I, 5). The Prince of Homburg is Colonel of the Cavalry. He returns from the battle as hero since he achieved a splendid victory against the Swedes who fled from the battle field leaving behind their flags and many other trophies. However, as it turns out, he acted against the Elector’s deliberate order “to not move from the post allocated him…until he will receive a personal order.” (I, 5, p. 35f.) During the briefing, knowing his impetuous behaviour, the Elector even admonished Homburg to stay calm and wait for the command telling him “…you´ve already cost me two Battles in this campaign…” (I, 5, p. 37). Nevertheless, disregarding the order, Homburg charged too early. After the battle, in Berlin, they all gather in front of the cathedral laying down their Swedish trophies. The Elector renders a little speech: “…Today was a famous victory ….However, its being ten times grander Would not excuse its being won by chance: This was by no means my final battle, And I must, must have my orders obeyed. Whoever it was, who led that attack, He´s committed a capital offence, And I repeat; I want him court-martialled.” (II, 9, p. 56)

In what follows, Homburg is imprisoned to be court-martialled immediately and sentenced to death, as the law says. When Natalie, the Elector’s niece and Homburg’s fiancée, asks her uncle to pardon Homburg, the Elector answers (IV, 1, p. 74): My dear girl, Look; a single word from you Would melt the hardest tyrant´s heart; were I one, I feel absolutely sure it would mine. But tell me; can I really overturn

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A sentence that a court of law has passed? Think of what the consequences would be.

Not adhering to the law means acting like a tyrant to the Elector. But such behaviour would neither suit the Elector nor his soldiers, nor his people. They all love their country as a law-abiding entity. When Natalie asks her uncle about the consequences of pardoning Homburg, thus overturning a sentence that a court of law had passed, the Elector indicates that the country might fall into chaos. Yet, he is very empathetic to the plight of his niece who is rather upset about how Homburg  faced with death  is behaving (IV, 1, p. 75). She observes: “His mind´s a blank, but for one thought: save me. Staring down a set of rifle barrels Has got him so scared, stunned stupid with fear, Nothing, except staying alive, is left”.

Homburg’s reaction shows signs of madness – madness of fear. The conversation between Natalie and her uncle ends with the Elector writing a reprieve and sending Natalie with the letter to the prison. The letter says that Homburg will be pardoned if he feels that the Elector has done him wrong. However, when Homburg reads the letter, he becomes convinced that he has committed insubordination and deserves death. Obviously, the letter hands the responsibility for judgement to the delinquent himself. This trading of position changes Homburg’s view on life and death substantially and defeats both his madness of fear and fear of death. On the other hand, during the conversation with Natalie, the Elector started to face his dilemma. It seems that he can no longer adhere to the strict rules of his own code of conduct generalized by the law without reference to himself. In fact, there are two incidents where he burdened himself with guilt, at least to a certain extent. In section V, 5, p. 97 Homburg´s friend Hohenzollern reminds the Elector with what happened in the night before the battle when Homburg was nightwalking in the gardens of the Palace gardens. “…He seemed to be dreaming of the next day´s fight, And had a wreath of laurel in his hand You, as if to probe the workings of his mind, Took his laurels from him… Then handed the wreath and chain, intertwined, To her Highness, the Princess Natalie. – and he blushes… But you, pulling the Princess back with you … And he – is holding a glove in his hand…”

Hohenzollern reports that his friend Homburg was dumbfounded when he became aware that it was the Elector´s niece Natalie´s glove that he found in his hand after a “dream of love” co-arranged by the Elector. He could not concentrate during the briefing being held before the battle and consequently did

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not take in the instructions given for the battle as he was totally absent-minded. Hohenzollern considers the Elector, by playing with Homburg´s dreams and longings, to be at least partially responsible for Homburg´s absent-mindedness. As a consequence, Homburg cannot be held fully responsible for his disregarding the order. Another incident that shows the Elector´s deviation from law-abiding behaviour is evident in the very beginning of V,5 when he asks Kottwitz as to who ordered his regiment´s march to town. It turns out that it was Natalie, who claimed that this was on behalf of her uncle, the Elector. Without hesitating, the Elector pretends to know of this order, allegedly given on his behalf which, however, was not the case. To sum up, the Elector is not only partly responsible for Homburg´s charging too early in the battle but also covers the insubordination of his niece Natalie, the regiment´s formal commander. Natalie´s behaviour is at least as grave a misdemeanour as Homburg´s but the Elector does not react at all. Possibly he considers this offence not really serious as it was committed by a woman. However, there is danger that the Army revolts if he would not pardon Homburg. They love Homburg and consider him a great hero. On top of it, after reading the Elector’s letter, Homburg feels deep remorse, being convinced that death is his just and fair penalty – an attitude that the Elector hoped to see. The Elector is ready to pardon Homburg. He wants Homburg to firmly believe in his war strategy, namely to annihilate the Swedish army completely, with the Elector being the “mastermind” and every officer in the Prussian army executing his instructions only. This extremely focused command structure reflected Napoleon´s approach in the Battle of Austerlitz in 1805 where his troops defeated the Russian and Austrian armies. It has been said that in Austerlitz for the first time this structure was executed in a major battle. Given Homburg´s popularity among the officers and soldiers, the Elector is sure that they will all follow Homburg and agree with above strategy as well. Therefore, by deviating from his original attitude of “pacta sunt servanda” and pardoning Homburg, the Elector could reinstall security for his governance. But using the law he played with the safety of Homburg. However, from the play it is not clear that the Elector has chosen this strategy right in the beginning in order to implement the strict command structure and to install the unrestricted will within the army to exterminate the enemy rather than just to defeat him – getting away from ideals of knighthood and bravery and become a mere instrument in the hand of the commander. This somehow reflects the substitution of medieval battleship ideology (and emotions) by modern warship and managed violence.5 The play notoriously ends with “everybody” shouting: “In den Staub mit allen Feinden Brandenburgs!” (a sentence which is in general omitted in post World War II performances) or in its somewhat less drastic translation, “Dust. Down into the dust with our enemies.” _____________ 5

For a game theoretical analysis, see Holler & Klose-Ullmann (2012).

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IV. The Wallenstein Paradox6 Friedrich Schiller completed his trilogy “Wallenstein´s Camp”, “The Piccolomini”, and “Wallenstein´s Death” in 1799. In this dramatic poem there is hardly any action, aside from the murderous ending, but much talking, guessing, thinking and even dreaming. Wallenstein wants to leave all options open in order to keep the possibility of acting, i.e., the power to move. As long as the choice between the strategies ‘siding with the Swedes’ or ‘staying loyal to the Emperor’ remain a thought experiment, he feels powerful and power seemed to guarantee safety to him. Therefore, instead of moving and acting, he hesitates. But it is his undecided indifferent behaviour that leads to failure. He is assassinated by order of Emperor Ferdinand II. His daughter, wife and sisterin-law die and his closest friends are murdered as well. In his play, Schiller experimented with regard to Wallenstein´s behaviour. He was familiar with the historic facts but had not the intention to be limited by them in what he wanted to see on stage. There is no evidence that the historic Wallenstein wanted to cooperate with the Swedes. Yet the historic Wallenstein was also sentenced to death by Ferdinand II. As professor of history at the University of Jena, Schiller had studied the Thirty-Years War (1618–1648) for around five years before lecturing about it to students. Thereafter, he started to write the play. The play takes place in winter 1633/1634, in the middle of the Thirty-Years War between the catholic Austria and Southern Germany, the League, and the protestant Sweden and some principalities of Northern Germany. Wallenstein is the Commander-in-Chief of the League, having an immense scope of power. As the Sergeant-Major states in “Wallenstein´s Camp”, Scene 11, “He has the power absolute To make war or peace, as it may suit; He can confiscate money or property, Can execute, or show clemency, He can promote, or grant a commission, All the matters of honour are at his disposition: The Emperor himself gave him this as his right.”

Some lines later he adds, “Is he not a prince, as good as the next? Hasn’t he his own coinage like Ferdinand? His own people and his own land? Men call him Your Highness, and bow to him deep. His own soldiers he must be able to keep.”

The key political issue in Schiller´s play is the question whether or not Wallenstein will cooperate with the Swedes. In “Wallenstein’s Death” I, 5, the Swedish _____________ 6

Parts of this section derive from Holler & Klose-Ullmann (2008).

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Colonel Wrangel comes to Wallenstein on a secret mission. The long-lasting bargaining between Wallenstein and the Swedes concerning joint action is finally to be concluded. By allying with the Swedes, Wallenstein hopes to gain the Bohemian crown and to reinstall Protestantism in Bohemia as well as to finish the war between the Catholic League and the protestant North. The Swedes, on the other hand, want to expand the protestant territory, under their rule or dominance, to the largest extent possible. They started their talks with a review of the siege of Stralsund in 1628. Wrangel defended the city for the Swedes. Wallenstein’s then besieging troops were forced to withdraw without success. Wallenstein: You wrested from my head the admiral’s hat. Wrangel: Today I come to set crown upon it. Wallenstein: Your papers. Have you full authority? Wrangel: So many doubts remain to be resolved. Wallenstein (after reading): A letter to the point. It is a shrewd/ And wily master, Wrangel, that you serve./ Your chancellor declares that it is but/ The late king’s own intent he carries out/ In helping me to the Bohemian crown.

Wrangel hints at the Swedes’ doubting that he, Wallenstein, the Duke of Friedland, would indeed become unfaithful to the Emperor and ally with the Swedes. To them it is even less imaginable that he would succeed in persuading a force of 18,000 men “… To break their oath of loyalty.” Wallenstein points out that the Emperor’s army, which he recruited, consists of soldiers from many different countries. Actually, some of them came from Bohemia. Indeed, their fathers and grandfathers had already vehemently been opposed to the Hapsburgs who had forced them to become Catholic. Regarding the nobility and the officers under his command, he goes so far as to say “In any circumstances, they are mine.” He shows Wrangel a declaration signed by all generals and commanders (except Max Piccolomini) which confirms their loyalty. This convinces Colonel Wrangel. “I will drop my mask – indeed!/ I have authority to act in this.” Nevertheless, for safety reasons, as “Everything might yet be trickery,” Wrangel demands from Wallenstein to disarm the Spanish regiments loyal to the Emperor, and to hand the border fortress Eger as well as Prague over to the Swedes. Wallenstein strongly rejects the occupation of Prague: “Give up my capital to you! Why, rather/ Rejoin my Emperor!” Wrangel: “If there is time.” Wallenstein: “That I am free to do, today and always.” However, Wrangel points out to him that he lost such freedom to act when his secret emissary Sesina was captured by soldiers loyal to the Emperor. Wallenstein, taken aback, offers no reply. Yet, the degree of his impotency is neither obvious to him nor to Wrangel. Wallenstein still believes that his troops are absolutely loyal to him. At the end of the meeting Wallenstein says that he would consider Wrangel´s proposal. Although Wallenstein, after a discussion with his sister-in-law, Countess Terzky, decides to agree to the Swedish proposal, the situation remains doubtful. Is he really willing to cooperate with the Swedes or is it just a tactical

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movement? Does he rather pretend to accept their proposal and then defeat them. It seems that Wallenstein wants to take advantage of the Swedes. But what are Wallenstein’s true intentions? Wallenstein to Terzky: “. . .Might I not mean/ To make you all look foolish? Do you know me?/ I do not think I ever let you see/ The secrets of my heart . . . ” (“The Piccolomini”, II, 5). Does he play with uncertainty and hope that it will give him protection against the other players in this power game? He seems to waver between two options: either to obtain the Bohemian crown through Swedish support or alternately, bring peace to the German lands. Yet, the second alternative would also imply a certain amount of cooperation with the Swedes - which would displease the Emperor as well. However, Wallenstein does not want to make a choice. He does not want to let go his power and become the agent of his own decisions. He confesses to his brother-in-law Terzky: “It is my pleasure to know the power I have.” (“The Piccolomini”, II, 5.) In his article, published in the program of the Stein production in Berlin in 2007, Rüdiger Safranski (2007: 83) writes, “Of course, for Wallenstein ‘power’ means nothing else than the strength to have his will rule politically and in society. Power means the ability to act. As Wallenstein says: “If I can no more act, then I am nothing” (Wallenstein’s Death, I, 7). However, his hesitation in deciding implies yet another meaning of power. As a man of power Wallenstein is, like Hamlet, also a man of possibilities. He wants to remain master over his possible actions. Reality is but a constriction, it reduces the possibilities. Reality is what remains when the multitude of possibilities are squeezed through the eye of the needle of decision. The reality which you have opted for is captivating and entangles you in the independent logic of facts, although it is you who created them. This is why Wallenstein hesitates. He wants to keep his options. As a man of power he wants to act, and yet he shuns the irreversibility of action. He wants to be both, a man of power and a man of possibilities.” Wallenstein overrates himself and misjudges his generals, colonels and commanders. This certainly contributes to his sharp descent and to his eventual assassination. But he has already been declared an outlaw before coming on stage, i.e. everybody can kill him and receive a reward from the Emperor. Safety exists in Wallenstein´s imagination only and the uncertainty that he creates by his inactivity, does not protect him. But the notion of safety is not in the forefront of his thinking. His maneuvering and manipulating apparently caused the Emperor to outlaw him at a time when Wallenstein considered himself as most powerful, having all the options to act which meant for him overall security and firmness. But this sense of firmness is a grave misjudgement of the situation and of the people surrounding him that annihilates him, his family and confidents.

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It has never been said that Wallenstein suffered from madness, but from a misinterpretation and misjudgement of reality. McMurphy played with madness while Wallenstein played with reason and rationality. They both failed.

V. On Method The method we applied in this paper is not a strictly scientific one. We did not collect history data and run multi-stage regressions and we did not bribe students to visit laboratories and play Battle of Sexes or War of Attrition experiments. One reason that we did not collect historical data is that there are too many when it comes to violence and the selection problem seems hardly solvable in a convincing way. (In fact a similar problem we faced when selecting theatre plays.) Another reason is that there is such a variety of violence. Moreover, doing statistics with violence would hardly shed light on its relationship to uncertainty and safety, two concepts that are much less represented in any statistical data, unless we dig in the archives of insurance companies. The latter approach, however, implies that we limit ourselves to a rather restricted view of both concepts. Obviously, there were serious “limits of representation”7 to experimenting on violence and on uncertainty and safety related to it. This brings us back to the argument of Freeman Dyson that we referred to in the introduction to this paper which says that “violent and passionate manifestations of human nature, concerned with matters of life and death and love and hate and pain and sex, cannot be experimentally controlled” (Dyson 2011: 43). We subscribe to this. So we have looked for material that reflects violence in a most explicit way and found theatre plays to serve our needs. Of course, movies and artwork such as paintings and sculptures also offer a host of suitable material. Following an approach suggested, e.g. by Baxandall8 (1985) we (re-) constructed a “pattern of intention” of the main characters of the plays that captured violence and its relationship to uncertainty and safety. This reconstruction and the selection of plays revealed to us some of the importance of madness to achieve safety – but also madness in view of the threat of death as experienced by the Prince of Homburg. There might be even a more fundamental argument why the analysis of theatre plays or, more general, objects of art are substantial for understanding of violence, uncertainty and safety as we experience them today. V.S. Ramachandran, director of the Center for Brain and Cognition at the University of _____________ 7

8

For a discussion of “limits of representation” see various contributions in Farinelli et al. (1994). The volume discusses geographical (“the map”), political, linguistic, and historical representation and the moral and cognitive power of it. Michael Baxandall (1985) applies his approach that focuses on a reconstruction of (possible) patterns of intentions of the agents to such diverse objects as Benjamin Baker’s Forth Bridge, finished in 1889, and Piero della Francesca’s “Baptism of Christ” (about 1440-50).

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California, San Diego, suggests that mirror neurons or neuron systems enable us to absorb the culture of previous generations. In his recent book “The TellTale Brain: A Neuroscientist’s Quest for What Makes Us Human” he observes that “culture consists of massive collections of complex skills and knowledge which are transferred from person to person through two core mediums, language and imitation.”9 The transformation is executed by mirror neurons that “act like sympathetic movements that can occur when watching someone else perform a different task – as when your arm swings slightly when you watch someone hit a ball with a bat” (McGinn 2011: 34). Anatomy rules, not psychology, or liberal arts. In fact, V. S. Ramachandran extends the mirror-imagine approach into a science of art: “Enunciating nine ‘artistic universals,’ he propounds what he allows is a ‘reductionist’ view of art, attempting to provide brain-based laws of aesthetic response. Peacocks, bees, and bowerbirds possess rudimentary aesthetic responses…, and we are not so different” (McGinn 2011: 34). Perhaps we are not so different when we watch someone hit a ball, but how do our mirror neurons work when we see a murder? How do they react when Hamlet puts his rapier into Polonius who is hiding behind a curtain? Are theatre plays a means to train our mirror neurons to understand violence and respond to it in a world where violence is not openly experienced every day and, luckily, most major violence we only know from TV news? Still, all kinds of violence are part of our culture as are the theatre plays that bring violence on stage.

References Baxandall, M. (1985). Patterns of Intention: On the Historical Explanation of Pictures. New Haven and London: Yale University Press. Beard, M. (2011). Alexander: How great? New York Review of Books, 58 (October 27), 3537. Donskis, L. (2008). Power and Imagination: Studies in Politics and Literature. New Studies in Aesthetics Vol. 39. New York: Peter Lang Publishing. Dyson, F. (2011). How to dispel Your Illusion. New York Review of Books, 58 (December 22), 40-43. Farinelli, F., Olsson, G. & Reichert, D. (Eds.) (1994). Limits of Representation. München: Accedo-Verlag. Holler, M. J. & Klose-Ullmann, B. (2008). Wallenstein´s Power Problem and Its Consequences. AUCO Czech Economic Review, 2, 197-218. Holler, M. J. & Klose-Ullmann, B. (2012). Friedrichs Spiel mit dem Prinzen von Homburg, manuscript. Kesey, K. (1974). One flew over the Cuckoo´s Nest. New York: The Viking Press.

_____________ 9

Quoted in McGinn (2011: 34) in his review of V. D. Ramachandran’s book.

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Kleist, H. v. (2002). The Prince of Homburg. A New Version by Neil Bartlett with David Bryer. London: Oberon Books. McGinn, C. (2011). Can the Brain explain your Mind? New York Review Books, 58 (March 24), 32-35. Miłosz, C. (1990). The Captive Mind. New York: Vintage International. Safranski, R. (2007). Friedrich Schiller oder die Erfindung des deutschen Idealismus. Programmbücher des Berliner Ensemble, 89, 80-84. Schülting, S. (2007). Good Girls – Bad Girls: King Lear und seine Töchter. In Shakespeare King Lear, zweisprachige Ausgabe, Deutsch von Frank Günther (pp. 354-375). München: Deutscher Taschenbuch Verlag. Schiller, F. (1979). Wallenstein, translated with an Introduction by F. J. Lamport, The Robbers and Wallenstein. London: Penguin Books. Shakespeare, W. (2007). König Lear, zweisprachige Ausgabe, Deutsch von Frank Günther. Munich: Deutscher Taschenbuch Verlag (English Text: Arden 1972). Shakespeare, W. (2010). Hamlet, zweisprachige Ausgabe, Deutsch von Frank Günther. Munich: Deutscher Taschenbuch Verlag (English Text: Arden 1972).

 

Index Abortion 110 Achilles 165 Adaptation 6, 72, 18, 19, 50, 84, 88, 97, 98, 99, 112, 115 Adaptive function 44 Adultery 139, 144 Adversary 15, 17, 147, 165 Affectionate socialisation practices 152 Africa 46, 141 African Homicide and Suicide 142 Agent 131, 177 Aggression v, vii, 1, 2, 3, 4, 5, 9, 12, 13, 14, 15, 16, 17, 18, 23, 25, 26, 28, 29, 38, 39, 41, 42, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 61, 63, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 77, 78, 79, 80, 81, 82, 83, 87, 89,90, 91, 92, 93, 95, 96, 97, 98, 101, 102, 103, 104, 105, 106, 107, 113, 114, 115, 116, 117, 118, 119, 121, 153, 158, 159, 164, 166 Aggressive interactions 29, 45, 46 Aggressive species 1 Coalitional aggression 107 Coalitionary aggression 44 Counter-aggression 62 Direct aggression 104 Human aggression 13, 15, 24, 42, 69, 91, 104 Human aggressiveness 3 Indirect aggression 104, 105, 106 Intraspecific aggression 56 Non-human aggression 13 Optimality model of aggression 52 Physical aggression 4, 59, 105, 113, 114 Aggressor 26, 30, 36, 38, 164 Aka pygmies 110 Albany, Duke of 73, 81, 167 Alexander the Great 165

Allies 15, 16 Allied high command 133 Altruism v, 6, 65, 69, 71, 81, 83, 99, 121, 122, 123, 124, 127, 129, 134 Altruistic individuals 124 Altruistic motives 124 Reciprocal altruism 123, 124, 127, 129, 132, 134 Amazonas 28 America (see also United States) 13, 18, 46, 98, 117, 153 Americans 12 Latin America 150 Native Americans 144 American Homicide 139, 155, 156 Amok 1, 4 Anger 24, 37, 38, 39, 94, 95, 105, 118, 121, 160 Animal 1, 2, 3, 6, 14, 23, 24, 28, 29, 31, 32, 33, 38, 39, 40, 41, 42, 44, 45, 46, 47, 52, 53, 59, 70, 71, 72, 73, 74, 75, 77, 78, 80, 81, 82, 83, 84, 89, 95, 97, 98, 115, 117, 122 Anthropological research 158 Anthropologist 13, 40 Antlers 1, 43, 45, 53, 60 Ants 2, 3, 6, 43, 75 Ant workers 3 Cardiocondyla ants 2 Honeypot ants 3 Ape (see also Monkey, Chimpanzee, Baboon) 73 Great Apes 46, 74, 79, 107 Armaments 1, 13 Asia 46, 78, 116, 164 Eastern Asia 141 South East Asia 150 Assault 105, 106, 137, 138, 145 Asymmetric contest model 34 Asymmetry 32, 33

182 Attack 16, 17, 26, 29, 37, 43, 44, 47, 67, 69, 71, 78, 81, 87, 92, 172 Aumann, Robert J. 9, 15, 19, 39 Austerlitz, Battle of 174 Austria 141, 175 Authority 4, 6, 12, 126, 144, 152, 153, 158, 176 Axelrod, Robert 16, 19, 126, 133, 134 Baboon 43, 46, 47, 48, 51, 64, 66, 73 Hamadryas Baboon 3, 49, 58, 61, 63, 65, 77, 82, 85 Bahrain 141 Battle 2, 81, 102, 126, 128, 129, 130, 131, 170, 172, 173, 174, 178 Battle of the Sexes see Sex Beating 137, 142, 153, 166 Bees 3, 91, 179 Behaviour 6, 12, 13, 14, 15, 17, 18, 38, 39, 40, 41, 42, 43, 44, 45, 47, 48, 49, 50, 51, 53, 56, 57, 65, 66, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 80, 81, 82, 83, 84, 89, 90, 92, 93, 96, 97, 98, 102, 104, 106, 114, 115, 117, 121, 123, 126, 127, 128, 130, 131, 132, 134, 154, 156, 168, 170, 172, 173, 174, 175 Animal behaviour 41, 42, 44 Behavioural Ecology see Ecology Behavioural mechanism 66 Cooperative behaviour see Cooperation Human behaviour 18, 69 Natural behavioural repertoire 43 Ritualized behaviour 17, 91 Spiteful behaviour see Spite Violent behaviour 87, 90, 137 Youth Risk Behaviour Survey (YRBS) 105 Behavioural category 145 Belief 10, 127, 128, 131, 133, 155 First order belief 17, 131 Second order belief 17, 128, 131 Benefit 26, 51, 52, 55, 59, 62, 90, 92, 93, 123 Cost-benefit approach 45 Fitness benefits see Fitness Reproductive benefits see Reproduction

Index

Bentham, Jeremy 156 Berlin ix, 16, 20, 72, 73, 172, 177 Biologist vii, 23, 24, 87 Bird 19, 33, 45, 60, 79, 83, 96, 101, 102, 103, 112, 117 Bowerbirds 179 Passerine bird 112 Birth rates 18, 19 Body counts 11 Bohemia 176 Borders 67 Bosco, Augusto 149, 159 Bourgeois strategy 33 Bowerbirds see Bird Brain 5, 12, 74, 89, 114, 115, 116, 119, 169, 178, 179, 180 Human brain 4 Bravery 174 Britain 167 Brutality 1, 169 Bundeswehr 13 Burkina Faso 142 California 70, 74, 94, 118, 144, 179 Camouflage 164 Canadian musk ox 29 Candide 10 Canines 1, 43, 48, 49, 58, 60, 107, 108, 119 Capital punishment see Punishment Cardiocondyla ants see Ants Captivity 25 Cattle-raiding 144 Causal mechanisms 149 Centralized state power 155 Characteristics of homicides 148 Chicago 71, 72, 77, 78, 79, 80, 81, 82, 97, 98, 106, 119 Chimpanzee 41, 44, 53, 68, 71, 73, 74, 84 Chinese 18, 109 Choice 24, 27, 37, 60, 63, 78, 80, 81, 94, 99, 102, 121, 123, 126, 127, 128, 130, 132, 175, 177 Citizens 126, 150, 151, 155 Civil war see War Civilians 12, 15 Civilization 3, 4, 9, 10, 20, 23, 28, 117, 144, 160 Civilizing process 10 Clan 149, 152

Index

Coercion 20, 58, 63, 64, 72, 77, 79, 81, 82, 110, 123 Coercive power 122 Coexistence 32 Cognitive biases 93 Collectivist societies 148 Colonialisation 144 Colony 3, 45, 70 Columbus 28 Combat 32, 74 Command structure 174 Comparative anthropological research 137 Competition 6, 25, 27, 34, 39, 45, 49, 50, 55, 56, 57, 58, 59, 60, 61, 66, 69, 71, 73, 74, 75, 76, 77, 79, 80, 81, 82, 83, 84, 90, 91, 97, 98, 99, 101, 102, 103, 107, 108, 109, 110, 111, 112, 113, 114, 115, 118, 119, 134, 135 Intra-sexual competition see Sex Male-male competition 60 Sexual competition see Sex Comprehensive Meta-Analysis 105 Conflict v, 1, 3, 6, 14, 16, 17, 19, 21, 32, 33, 35, 36, 39, 49, 57, 59, 62, 63, 64, 66, 69, 70, 71, 72, 73, 77, 78, 79, 80, 81, 82, 83, 90, 91, 93, 95, 96, 98, 121, 122, 129, 135, 138, 139, 141, 152, 153, 159, 160, 165 Conflict of interest 122 Conflicting interests 124 Darwinian theory of conflict see Darwin Evolutionary conflicts 90 Global conflicts 10 Human conflicts 14 Inter-group war see War Conflict resolution 32, 33, 35, 91, 153 Conspecifics 1, 2, 13, 17, 41, 42, 43, 44, 46, 51, 57, 68, 84 Contextual forces 158 Contract 125, 155 Convention see also Social convention 32, 33, 121, 122, 135, 130 Conventional homicide 148 Cooperation vii, 2, 3, 16, 17, 18, 19, 21, 29, 56, 65, 69, 74, 75, 76, 79, 80, 81, 82,

183 93, 94, 97, 98, 99, 119, 124, 126, 127, 129, 132, 133, 134, 135, 177 Cooperative behavior 16, 89, 126, 156 Cooperative equilibrium see Equilibrium Co-operator 132 Coordination 65, 73, 122, 125, 129, 132 Coordination game see Games Social coordination 129 Cornwall 167 Corruption 151 Corsica 144 Cosmopolitanism 157 Cost 3, 17, 30, 31, 38, 45, 51, 52, 53, 55, 57, 75, 92, 93, 94, 123, 130, 172 Cost-benefit analysis see Benefit Cost-benefit approach see Benefit Cost of fighting 17 Côte d’Ivoire 143 Crime 1, 11, 20, 24, 38, 107, 116, 118, 137, 138, 147, 154, 159, 160, 161, 171 Criminal interpersonal violence 143 Criminological research 137 Croatia 146 Cross-cutting ties 152 Cruelty 17, 20 Culture 1, 4, 5, 18, 24, 69, 95, 96, 97, 99, 113, 115, 116, 155, 159, 179 Cultural evolution see Evolution Dark Ages 10, 15 Darwin, Charles 1, 3, 6, 18, 45, 49, 50, 60, 63, 72, 88, 97, 101, 102, 108, 113, 115 Death rates 11 Decision 29, 37, 39, 92, 93, 96, 97, 122, 123, 128, 132, 166, 177 Rational decision 37 Defection 126, 127, 129, 132 Defector 132 Demographic structure 149 Denmark 141, 170 Deprivation 150 Determinism 4 Deterrence 16, 32, 37, 38 Deterrent 17 Dilemma 123, 126, 127, 128, 129, 131, 132, 173 Prisoner’s Dilemma 126, 131

184 Social dilemma 123, 124, 126, 129 Dinka 144 Dioula 142 Disciplining revolutions 155 Disease 58, 110, 137 Divorce rate 150 DNA 7, 71, 74, 111 Dominance hierarchies 59 Domination 139 Doves 14, 53, 54, 59 Drowning 27 Drunken brawling 153 Dunnock 112, 115 Durkheim, Emile 148, 152, 159 Dutch 144 Dyson, Freeman 163, 178, 179 Early modern period 144 Eastern Asia see Asia Eclectic operationalization 151 Ecology 6, 20, 21, 39, 67, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 115, 116 Behavioural Ecology 71, 72, 77, 89, 97 Socio-ecological model of evolution 58 Socio-ecological theory 56 Economics ix, 15, 19, 39, 56, 90, 93, 94, 123, 134, 135, 163 Economic development 149, 150 Economic inequality 149 Edgar 166, 167, 168, 169 Edmund 166, 167, 168 Eggs see Ova Egoism 123 El Salvador 143 Elias, Norbert 10, 11, 20, 21, 154, 155, 156, 159 Emotion 24, 37, 96, 97, 121, 127, 163, 164, 169, 174 Enemy 14, 15, 16, 42, 133, 174 England 97, 98, 117, 146, 154, 160, 161, 167, 170 Enlightened cooperation 134 Enlightenment 158 Environment 4, 18, 19, 34, 48, 50, 55, 56, 57, 58, 84, 93, 95, 96, 101, 114, 169 Environmental conditions 23, 50, 56, 89

Index

Equilibrium / Equilibria 124, 126 Cooperative equilibrium 132 Psychological equilibrium 128, 129 Social equilibria 129 Escalator of reason 157 Ethnographic Atlas 111 Europe 9, 15, 16, 23, 37, 39, 138, 141, 142, 143, 154, 155, 159, 161 European Homicide Database see Homicide European colonialisation 144 Evolution 1, 2, 3, 6, 19, 21, 24, 26, 27, 28, 31, 38, 39, 42, 44, 48, 56, 57, 58, 60, 62, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 90, 92, 94, 95, 97, 98, 99, 102, 103, 107, 115, 116, 117, 118, 119, 134, 135, 153, 160 Cultural evolution 5, 29, 42, 95 Evolutionary approach 50, 96 Evolutionary biology 24, 26, 31, 45, 65, 69, 87, 88, 90, 96 Evolutionary conflicts see Conflict Evolutionary framework 65, 113 Evolutionary perspective 4, 49, 67, 69, 70, 90 Evolutionary psychology see Psychology Evolutionary theory 87, 92, 95, 96 Evolved behavioural repertoire see Behaviour Gene-centered view of evolution 2 Non-selective evolutionary forces 92 Selective evolutionary forces 92 Socio-ecological model of evolution see Ecology Evolutionary models 130 Ex-ante probability 122 Expanding circle 157 Expectations 113, 123, 125 Experience 37, 48, 95, 96, 130, 150, 153, 157, 164, 169, 178 Infantile experience 5 External factors 47, 48, 89, 110 Facial width 108 Family 93, 111, 146, 149, 151, 152, 155, 157, 163, 177 Family members 146 Fear 18, 24, 38, 173

Index

Female 2, 13, 25, 27, 34, 42, 43, 46, 47, 48, 56, 58, 60, 61, 62, 63, 64, 65, 66, 67, 71, 72, 73, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 91, 102, 103, 106, 108, 109, 110, 111, 112, 113, 115, 116, 119, 146, 148, 150, 156 Female victims see Victims Female labour force participation rate 150 Feminization 156 Ferdinand II, Emperor 175 Ferguson, Niall 133, 134 Fig wasp see Wasp Fight 2, 13, 14, 16, 17, 29, 32, 33, 34, 36, 39, 43, 45, 51, 52, 53, 54, 57, 59, 61, 101, 105, 145, 156, 163, 169, 172, 173 Lethal fighting 2, 13 Fitness 2, 14, 18, 25, 26, 27, 31, 34, 35, 36, 45, 48, 50, 51, 52, 53, 54, 55, 56, 57, 62, 63, 64, 65, 67, 69, 77, 88, 89, 90, 91, 92, 94, 95, 96, 97, 115 Darwinian fitness 88 Fitness benefits 65, 89, 94 Fitness consequences 25, 51, 67, 89, 92, 94, 96 Fitness costs 63, 91, 92, 94, 95 Individual fitness 52, 62, 88, 91 Florence 144, 158 Food 3, 32, 33, 34, 48, 49, 52, 55, 56, 57, 58, 59, 60, 61, 65, 67, 74, 75, 77, 81, 82, 89, 90, 95, 101, 102, 112, 115, 166 Fraternal interest groups 152 Freedom 126, 166, 176 Freud, Sigmund 163 Friedland, Duke of 176 Friedrich Wilhelm 172 Führer (Adolf Hitler) 15 Gains 35, 48, 55, 67, 123 Games v, 32, 39, 78, 94, 98, 121, 123, 124, 125, 128, 129, 130, 131, 132, 134, 135, 163 Bargaining game 124, 125, 127, 128, 129, 132 Battle of the Sexes game 129 Coordination game 125 Game theoretic formalism 124 Game theoretic literature 123, 130 Game-theoretical approach 51

185 Game theoretical models 14 Game Theory 2, 15, 16, 23, 29, 30, 31, 32, 37, 39, 53, 74, 98, 121, 124, 127, 130, 131, 133, 134, 135 One-shot-games 15 Psychological games 124, 129, 131 Repeated games 15 Gender roles 13 Gene 2, 5, 7, 50, 72, 88, 89, 92, 98, 149, 150 Gene-environment interactions 5 Genetic changes 23 Genetic factors 18 Genetic influence 5 Genocide 19, 142, 148 Genome 5 Germ cells (gametes) 101 Germany 16, 20, 37, 146, 154, 175 German soldier 133 Germans 16, 19 Gini Index 112, 150 Global Study on Homicide 141, 143 Gloucester, Earl of 166, 167 Gobineau, Joseph Arthur de 164, 165 Goneril 166, 167 Goodall, Jane 41, 53, 66, 67, 68, 74 Governance 151, 174 Great Apes see Apes Group ix, 1, 2, 3, 29, 42, 44, 46, 50, 52, 57, 58, 59, 60, 61, 62, 63, 65, 66, 67, 68, 71, 72, 73, 74, 75, 76, 78, 79, 83, 84, 90, 97, 107, 111, 125, 126, 139, 141, 142, 143, 144, 147, 148, 152, 157, 170 Group living animals 2, 65 Group living mammals 3 Group selectionist thinking 2 Guilt 123, 134, 173 Guilt aversion 123 Gulag 164 Habits 24 Hair 108, 113, 118 Hamlet 165, 166, 170, 171, 177, 179, 180 Harsh socialisation practice 152 Hate 53, 133, 163, 178 Hawks 14, 53, 54, 59 Hector 165 Hedge sparrow 112 Hereros 19

186 Heritage 114 Heterogeneity, social/cultural 149, 150 High Homicide Societies 147, 148, 157 High levels of political oppression 152 High socio-economic development 150 Historical homicide rates 158 Historical Research 154 Historical Research 137, 154 Hobbes, Thomas 3, 21, 126, 153, 155, 156, 160 Hobsbawm, Eric 10, 20 Hohenzollern 173 Homburg, Prince of 172, 173, 174, 179 Homicide v, 4, 11, 20, 23, 28, 38, 106, 112, 115, 116, 119, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161 European Homicide Database 138, 141 FBI homicide figures 106 Homicide levels 158 Homicide rates 11, 23, 28, 112, 137, 138, 139, 140, 141, 142, 143, 144, 146, 148, 149, 150, 151, 152, 154, 155, 156, 157, 158 Homicide trends 154 Instrumental Homicides 146 Homo sapiens 1, 25 Honduras 143, 146 Honesty 123 Hong Kong 141 Honor 149, 156 Humanities vii, 4, 5, 9, 12 Humanity 4, 73 Humans v, 1, 3, 4, 5, 9, 12, 17, 19, 23, 25, 28, 30, 39, 41, 42, 44, 46, 68, 69, 71, 72, 73, 76, 77, 78, 79, 82, 83, 84, 87, 93, 94, 95, 96, 97, 98, 99, 101, 103, 107, 109, 111, 112, 113, 114, 117, 118, 137, 142, 157 Human aggression see Aggression Human aggressiveness see Aggression Human altruism 69 Human brain see Brain Human conflicts see Conflict Human genome see Genome

Index

Human history 1, 10, 18, 19, 107, 156 Human mating system 114 Human Relations Area Files 152, 159 Human societies 140 Human violence 13, 18, 157 Huxley, Thomas Henry 3, 6 Iliad 165 Illiteracy 149 Imitation 4, 179 Indians 28, 29 Individualism 26, 27, 148, 155 Industrialization 150 Inequality 104, 110, 111, 112, 119, 123, 149, 150, 151, 152, 158, 164 Economic inequality 149 Inequality aversion 123 Infanticide 2, 6, 25, 46, 58, 61, 62, 71, 73, 75, 78, 79, 80, 83, 137 In-group 29, 93 Insects 6, 33, 39, 96, 101 Instrumental motives 147 Intelligence 5, 71, 169 Interaction 126, 134, 153 Interdependence 158 International classification of Diseases 138 Instrumental homicides see Homicide Interpersonal killing 142, 157 Intruders 3, 11, 34, 67 Invaders 23, 28 Iraq 143 Iraq Body Count project 143 Jamaica 143 James, William 163 Japan 76, 141, 146 Japanese 17, 47, 76, 77, 80, 133 Japanese soldiers 17, 133 Jews 12 Joint offspring see Offspring Józefów 12 Judicial category 144 Kahneman, Daniel 93, 97, 98, 99, 163 Kant, Immanuel 156 Kato tribe 144 Kent, Earl of 167, 168

Index

Kesey, Ken 165, 168, 169, 179 Ketman 164, 165 Killing 2, 12, 13, 17, 28, 39, 41, 42, 43, 44, 46, 61, 68, 74, 76, 84, 107, 115, 124, 133, 142, 146, 157, 166, 170 King Lear 165, 166, 167, 168, 180 Kinship group 152, 153 Kleist, Heinrich von 165, 172, 180 Knighthood 174 Korea 146 Kottwitz 174 KZ (concentration camp) 164 Language 5, 13, 89, 156, 179 Large total population size 152 Large-scale Variation 137 Law 39, 102, 106, 155, 156, 159, 165, 166, 172, 173, 174, 175, 176, 177 League 175, 176 Learning 4, 24, 39, 96, 129, 130, 131, 134, 135 Learning Fairness 129 Lethal combat 2, 25 Lethal fighting see Fight Leviathan 155, 156, 160 Lion[s] 2, 25, 42, 43, 45, 60, 62, 71, 74, 85, 91, 98 Logic of Reciprocity 127, 128 Lorenz, Konrad 1, 6, 25, 26, 28, 39, 42, 50, 77 Love 124, 163, 166, 173, 174, 178 Low Homicide Societies 148,157 Lynching 157 Macro-level historical variation 154 Macro-level variation 154 Madness 165, 166, 168, 170, 171, 173, 178 Malaysia 142 Male 1, 2, 5, 6, 12, 13, 25, 27, 28, 32, 36, 38, 41, 42, 46, 47, 48, 49, 50, 51, 53, 56, 58, 60, 61, 62, 63, 64, 65, 66, 67, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 89, 91, 95, 97, 98, 101, 102, 103, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 145, 148 Male Threat Displays 107

187 Mammals 3, 34, 42, 46, 56, 60, 62, 63, 64, 73, 76, 81, 82, 83, 101, 103, 107, 112, 118 Mandibles 1 Manifestations of violence 137 Manoeuvres 17 Manslaughter 141 MAOA-L 5 Martial glory 156 Maryland 144 Massacre 28, 40, 148 Mating 1, 2, 5, 13, 27, 29, 33, 36, 56, 57, 58, 62, 71, 72, 73, 75, 78, 84, 101, 102, 103, 104, 111, 112, 114, 115, 116, 117 Maturation Rates 109 Maynard Smith, John 2, 6, 14, 23, 29, 31, 33, 39, 51, 53, 54, 56, 78, 88, 98, 102, 117, 122, 135 Meta-analysis 150 Middle Ages 20, 144, 146, 154, 161 Middle East 141 Milgram, Stanley 4, 6, 12 Military reviews 17 Miłosz, Czesław 164, 165, 180 Mismatch Hypothesis 93 Mixed strategy 30, 32, 54 Modernity 10, 12, 157, 159 Mogadishu 143 Monetary considerations 127 Monkey (see also Ape) 68, 71, 73, 74, 82 Monogamy 77, 103, 103, 110, 111, 112, 114 Montesquieu, Charles de Secondat (Baron de) 156 Morgenthau, Henry 16 Mortality 39, 43, 62, 72, 82, 103, 110, 113, 117, 138, 139, 141, 150 Moral Emotions 127 Moral individualism 155 Moral rules 139 Moral sense 133 Moral sentiments 19 Morally motivated individual 127 Motivation 52, 65, 115, 132 Morally motivated individual see Morality Non-selfish motivation 124, 129, 132

188 Motive 123 Altruistic motives see Altruism Psychological motive 10 Multicollinearity 151 Multivariate 134 Multivariate model 152 Murder 1, 11, 42, 117, 137, 141, 143, 148, 161, 170, 171, 179 Muslims 164 Mutilation Mutilating 13 Mutual beliefs 122 Mutual cooperation 134 Mutual defection 129 Mutual help 3 Mutual punishment 129, 133 Mutual surveillance 3 Napoleon 174 Natalie 172, 173, 174 Nation-states 139, 158 Native Americans see America NATO 37 Natural environment 25 Natural selection 101 Nature 5, 6, 9, 13, 18, 21, 23, 26, 32, 38, 39, 42, 70, 72, 74, 77, 78, 79, 80, 88, 97, 98, 99, 115, 117, 118, 126, 128, 135, 151, 160, 163, 164, 167, 169, 178 Negotiation 125 Neighbours 15, 41, 67, 126, 127, 144, 153, 158 Netherlands 141, 154 Neuron 179 Noble savage 28 Non-selfish motivation see Motivation Norms 65, 95, 96, 121, 123, 124, 125, 126, 135, 157 Social norms 123 Norway 141, 154 Nuclear retaliation 37 Nurture 5, 77 Nyoro 142 Offspring 2, 3, 19, 25, 28, 45, 48, 50, 60, 61, 63, 64, 78, 81, 82, 90, 95, 101, 102, 103, 104, 110 Joint offspring 2 Oman 141

Index

Operational Sex Ratio see Sex Operationalization 141 Order see Social order Out-group 29, 93 Ova 101 Eggs 3, 34, 60, 91, 101, 102, 103 Pain 163, 178 Paley, William 88, 98 Pangloss 10, 21 Paraguay 146 Parental investment 60, 102 Parental Investment Theory 102 Passerine bird see Bird Pathological murder 143 Payoff 30, 33, 34, 35, 57, 124, 127, 128, 129, 130, 131, 132 Payoff matrix 30 Peacefulness 82, 141, 142, 159, 160 Peacocks 60, 179 Perceived transgressions 144 Persecution 164 Persia 164, 165 Persian 165 Phylogenetic perspective 48 Piccolomini, Max 175, 176, 177 Pinker, Steven 9, 10, 12, 15, 21, 70, 80, 106, 107, 111, 112, 118, 144, 154, 156, 160 Pleistocene 116, 137 Poland 12, 20, 165 Policing 3, 6, 90, 99 Polyandry 112, 115, 117 Polygynous marriage 111 Polygynous pattern 103 Polygyny 103, 109, 110, 111, 112, 113, 114, 117 Population structure 42, 93, 150 Poverty 150, 151 Prague 176 Predisposition 23 Preference 124, 133, 164 Prehistory 10, 156 Primates v, 2, 3, 5, 9, 19, 41, 44, 45, 46, 56, 57, 58, 59, 60, 61, 63, 64, 65, 66, 67, 68, 69, 70, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 94, 103, 107, 108, 113, 114, 118 Primate social systems 57, 58

Index

Prince of Homburg 165, 172, 178, 180 Prison 29, 39, 168, 169, 173 Prisoner 124, 126, 127, 128, 129, 131, 132 Prisoner’s Dilemma see Dilemma Prisoners of war see War Project of modernity 157 Protestantism 155, 176 Proximate causes 19 Proximate explanations 89, 90 Proximate mechanisms 68, 94 Psychohydraulic model 26 Psychologist 10, 13, 114, 117, 156, 163 Psychology v, vii, ix, 5, 6, 29, 41, 44, 71, 74, 84, 90, 93, 94, 97, 98, 102, 114, 115, 116, 117, 159, 179 Psychological balance 164 Psychological equilibrium see Equilibrium Psychological games see Games Psychological motive see Motive Punishment 4, 17, 24, 65, 72, 75, 90, 94, 97, 126, 129, 133, 139, 169 Capital punishment 139, 144 Puritans 144 Rabin, Matthew 123, 124, 127, 128, 129, 134, 135 Raiding 3, 44, 70, 107 Raids 13, 25, 29, 43, 67, 148 Ramachandran, V. S. 178, 179 Rape 97, 137 Rationality 14, 16, 30, 97, 164, 178 Rational decision see Decision Rational players 37, 130 Reason 10, 26, 29, 36, 51, 92, 133, 134, 145, 156, 171, 178 Reciprocity 17, 71, 74, 75, 127, 128, 129, 132, 134, 135, 156 Reciprocal altruism see Altruism Reconciliation 65, 66, 70, 72, 73, 76, 77, 81, 160 Red deer 36 Regan 166, 167 Relationship between perpetrators and victims 146 Reproduction 6, 18, 48, 50, 57, 62, 71, 72, 74, 91, 101, 102, 112, 114 Reproductive advantage 145 Reproductive benefits 61

189 Reproductive opportunities 45, 57, 58 Reproductive success 2, 23, 56, 88, 103, 107, 110, 111, 113 Resource 2, 14, 17, 33, 45, 49, 51, 52, 53, 57, 59, 60, 73, 75, 77, 79, 83, 91, 112, 119, 158 RHP (Resource Holding Power) 52, 53, 56 Return of investment 15 Reward 30, 31, 32, 177 Riot 1 Risk 2, 3, 11, 25, 27, 32, 33, 50, 51, 53, 58, 59, 70, 91, 92, 94, 95, 96, 105, 106, 110, 113, 115, 122, 126, 127, 141, 148, 149, 151, 152, 154, 156, 157, 159 Ritual 17 Ritualization 17 Ritualized aspects of human warfare see Warfare Ritualized contests 52 Ritualized fights 28 Robbery 23, 137 Rules of combat 17 Russian prisoner see Prisoner Rwanda 19, 142, 143 Sadism 1 Safety v, 39, 126, 163, 164, 165, 166, 168, 169, 171, 172, 174, 175, 176, 177, 178 Sahara 142 Sahel 142 Sanctions 33, 90, 122 Schelling, Thomas 14, 15, 16, 21, 37, 39, 135 Schiller, Friedrich von 165, 175, 180 Scientist ix, 4, 7, 12, 78, 152 Selection v, 2, 4, 6, 13, 18, 23, 26, 27, 29, 30, 31, 39, 44, 45, 50, 52, 55, 65, 69, 70, 72, 73, 74, 77, 78, 79, 80, 82, 83, 84, 88, 89, 90, 92, 93, 94, 95, 96, 97, 98, 99, 101, 102, 103, 109, 110, 111, 113, 114, 115, 116, 119, 124, 131, 134, 140, 145, 159, 178 Non-selective evolutionary forces see Evolution Selective evolutionary forces see Evolution Sexual selection see Sex

190 Self-domestication 25 Self-interest 122, 123, 132 Selfishness 123 Selfish individuals 3, 124 Semai 142, 159, 160 Separate centralized political authority 152 Sex v, 2, 6, 13, 32, 46, 61, 62, 72, 73, 74, 80, 81, 101, 102, 103, 104, 105, 106, 107, 109, 112, 113, 114, 115, 116, 117, 118, 145, 146, 148, 161, 163, 178 Battle of the sexes (see also Games) 128, 130, 178 Intra-sexual competition 62 Operational Sex Ratio 112 Sexual coercion 63, 64 Sexual competition 61 Sexual dimorphism 101, 102, 108 Sexual maturation 103 Sexual selection 13, 18, 45, 50, 60, 61, 101, 102, 103, 104, 107, 109, 110, 111, 113 Sex difference[s] 104, 109, 113 Sex of victims 145 Sex ratio 114 Sexually selected species 109, 113, 114 Shakespeare, William 165, 166, 167, 168, 170, 171, 180 Shame 11, 165 Singapore 20, 21, 141 Singer, Peter 70, 81, 105, 119 Slovenia 141, 146 Social and cultural heterogeneity 150 Social animals 2, 3, 96 Social communities 3 Social complexity 158 Social convention 121, 122, 124, 125, 130 Social dilemma see Dilemma Social Disadvantage of Perpetrators 145 Social emotions 121 Social evolution 2 Social experiments 163, 171 Social insects 2 Social learning 24 Social life 125 Social norms see Norms Social order vii, 17, 24, 26, 27, 33, 35, 36, 37, 46, 59, 61, 62, 65, 107, 122, 124,

Index

128, 129, 131, 132, 133, 139, 149, 151, 165, 170, 172, 174, 175 Social outcomes 122 Social preferences 123, 124 Social primates see Primates Social sciences 4, 70, 114 Social scientist 3, 13, 14, 141 Social system 58 Social welfare see Welfare Socialisation patterns 153 Society 1, 2, 4, 5, 6, 11, 18, 20, 39, 70, 71, 75, 76, 78, 80, 84, 87, 95, 97, 98, 112, 115, 117, 118, 119, 122, 123, 124, 125, 126, 137, 139, 141, 142, 146, 147, 149, 153, 154, 155, 156, 158, 159, 160, 161, 166, 168, 169, 177 Agropastoral societies 144 Animal societies 3, 33 Fission-fusion society 67 Modern societies 11, 19, 139, 142, 149, 153 Non-state societies 107, 137, 139, 140, 142, 146, 151, 152, 157 Societal influences 5 Socio-biology 42, 48 Soldier 133 Solidarity 152, 156 Somalia 19, 143 South East Asia see Asia South Europe see Europe Soviet Union 16 Spanish regiments 176 Sperm (Spermatozoa) 2, 63, 101, 102 Spiders 33, 34, 36, 38 Spite 65, 75, 77, 83, 97, 99, 121 Spiteful behaviour 65, 94, 124, 127 Spontaneous coordination 121 Sri Lanka 146 Stillbirth 110 Stockholm 144 Strangers 127, 146, 148 Strategic competence 23 Strategy 14, 16, 17, 21, 23, 24, 30, 31, 32, 33, 35, 39, 48, 50, 51, 52, 53, 54, 55, 56, 59, 60, 63, 77, 80, 83, 98, 128, 130, 131, 135, 139, 165, 166, 168, 171, 174 Dove strategy 14, 15, 54 Hawk strategy 14, 15

Index

War strategy 174 Strength 42, 53, 103, 108, 109, 113, 118, 150, 152, 177 Stress 10, 50, 77, 110, 113, 117 Struggle 6, 27, 36, 51, 72, 101 Success 10, 18, 25, 36, 54, 56, 72, 73, 77, 79, 83, 88, 98, 107, 110, 115, 117, 176 Sudan 144, 159 Surrender 2, 17, 19, 20, 133 Surveillance see Mutual surveillance Survival of the fittest see Fitness Sweden 141, 154, 160, 175 Swedes 172, 175, 176, 177 Swedish army 174 Switzerland 141, 146 Sympathy 37, 157 Teamwork 3 Tenant 125 Territory 3, 29, 33, 34, 35, 37, 41, 42, 43, 59, 67, 69, 90, 115, 144, 163, 176 Territory borders 3 Terzky, Countess 176, 177 Testosterone 47, 50, 68, 70, 71, 72, 79, 84, 89, 93, 108, 113, 119 Theft 139, 144 Third Reich 11, 15, 19 Tinbergen, Nikolaas 18, 21, 47, 82, 89, 99 Titus Maccius Plautus 3 Toad 27 Torture 1 Toy models 124 Trapp, W., Major 12 Trauma 139, 141, 144 Tribal communities 28, 29 Tribe 17, 144, 149 Trotha, Lothar von, General 19, 20 Troy 165 Trojan war see War Truthfulness 123 Tutsi 142 Uganda 67, 83, 142, 146 Ultimate causes 19 Ultimate explanation 18, 87, 89, 90, 91, 92, 93, 94, 95, 96 Ultimate question 48, 90, 94 Uncertainty v, 163, 164, 165, 168, 169, 171, 177, 178

191 Unemployment 149 United Nations Office on Drugs and Crime (UNODC) 138, 141, 143, 146, 161 UNODC database 141 United States (see also America) 9, 98, 110, 115, 116, 117, 138, 141, 144, 155 Urbanisation 149 Urbanism 150 Utrecht 144, 159 Variability 112, 114 Variation 138, 144, 148 Venezuela 146 Vertebrates 42, 72, 101, 108, 113 Victims 43, 44, 141, 143, 145, 146, 148 Female victims 146, 148 Vietnam 12, 17, 20 Vietnamese 12 Vikings 15 Violence v, vii, 1, 3, 4, 9, 10, 11, 12, 13, 14, 15, 16, 18, 19, 20, 21, 23, 24, 25, 26, 28, 29, 39, 40, 42, 68, 69, 71, 73, 76, 77, 80, 84, 87, 93, 95, 107, 110, 113, 115, 118, 119, 137, 138, 139, 142, 143, 144, 145, 146, 148, 151, 152, 153, 154, 156, 157, 158, 159, 160, 161, 163, 164, 165, 168, 169, 171, 174, 178, 179 Different species of violence 11 Human violence 13, 18, 157 Intergroup-violence 67 Interpersonal violence 137, 143, 145, 153, 156, 158 Lethal state violence 11 Lethal violence 12, 137, 144, 154, 157 Political violence 143 State’s monopoly of violence 11 Violent behaviour see Behaviour Violent action 23 Virginia 144 Virtues 123 Vocalizations 36, 113, 108 Voltaire 10 Wales 146, 161 Wallenstein 165, 175, 176, 177, 178, 179, 180 War 9, 11, 12, 14, 15, 16, 17, 19, 20, 21, 37, 41, 42, 67, 79, 83, 116, 117, 133, 134,

192 138, 139, 143, 144, 145, 152, 156, 158, 159, 160, 165, 174, 175, 176, 178 Chimp war 41, 67 Civil war 138, 143, 148, 157 Inter-group war 107 Prisoners of war 17, 124, 133 Race war 19 Thirty-Years War 175 Trojan war 165 War of Attrition experiment 178 War strategy see Strategy World War I 10, 16, 174 World War II 174 Warfare 1, 3, 9, 14, 16, 17, 20, 21, 29, 38, 44, 45, 68, 70, 71, 77, 78, 91, 107, 153, 159, 161 Inter-cultural warfare 17 Ritualized aspects of human warfare 17 Warrior 5, 7, 13, 119, 152 Warsaw Pact 37

Index

Wasp 3, 6, 32 Fig wasp 2, 32 Weapons 1, 13, 15, 25, 42, 43, 45, 53, 60, 74, 101, 103, 105, 107, 148 Nuclear weapons 3 Weber, Max 155 Welfare 118, 123, 124, 135, 150 Social welfare 123, 127, 150 William the Conqueror 15 Wilson, Edward O. 2, 3, 6, 40, 41, 42, 43, 45, 60, 69, 75, 84 Winner 3, 16, 17, 53, 54, 59 Witchcraft 144, 158 World Health Organisation 138 Wrangham, Richard 29, 39, 43, 44, 56, 57, 58, 61, 63, 67, 68, 69, 72, 74, 77, 78, 79, 80, 81, 82, 84 Yanomamö (or Yanomamo) 28, 29, 110 Y-Chromosome 111 Youth Risk Behaviour Survey (YRBS) see Behaviour