Annals of the New York Academy of Sciences

This unedited manuscript has been submitted for publication in the Annals of the NYAS.This paper has not been copyedited.

SEXunedited DIFFERENCES IN INTERGROUPmanuscript AND : THE MALE WARRIOR HYPOTHESIS

Journal: Annals of the New York Academy of Sciences

Manuscript ID: annals-1474-000.R1

Values and Empathy across Social Barriers: A Neurocognitive Volume Title: Approach to Fairness

Date Submitted by the n/a Author:

Complete List of Authors: van vugt, mark; University of Kent, Psychology

intergroup relations, , sex differences, Keywords: aggression, cooperation, competition, violence, gender

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Van Vugt: Male Warrior Hypothesis 1 1 2 3 4 5 6 7 8 9 10 11 12 13 SEX DIFFERENCES IN INTERGROUP AGGRESSION AND VIOLENCE: 14 15 THE MALE WARRIOR HYPOTHESIS 16 unedited manuscript 17 18 19 20 21 22 23 24 1 25 26 27 University of Kent 28 29 30 31 32 33 1 34 Centre for the Study of Group Processes, Department of Psychology, University of 35 36 Kent United Kingdom 37 38 39 40 41 Mark Van Vugt, Centre for the Study of Group Processes 42 43 Department of Psychology University of Kent 44 45 CT2 7NP Canterbury, United Kingdom 46 47 48 +44 (0)1227 827468 or 3961 or 7030 (fax) 49 50 mobile: 07764765473 51 52 [email protected] 53 54 55 http://www.kent.ac.uk/psychology/department/people/van-vugtm/personal 56 57 Key words: SEX DIFFERENCES, INTERGROUP BEHAVIOR, AGGRESSION, 58 59 GENDER, VIOLENCE, COOPERATION, COMPETITION, EVOLUTIONARY 60 PSYCHOLOGY

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Van Vugt: Male Warrior Hypothesis 2 1 2 3 4 5 6 Abstract 7 8 The social science literature abounds with examples of human , the tendency 9 10 to categorize individuals on the basis of their group membership and treat ingroup 11 12 13 members benevolently and outgroup members malevolently. I argue that this tribal 14 15 inclination is an evolved response to the threat of intergroup violence and warfare 16 unedited manuscript 17 that were endemic in ancestral human environments (and are still common today). 18 19 20 Here I hypothesize that intergroup conflict has profoundly affected the social 21 22 psychology of human males in particular – the male warrior hypothesis -- and 23 24 present evidence consistent with this claim. I also discuss implications of this 25 26 27 hypothesis for managing intergroup relations in our society. 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Van Vugt: Male Warrior Hypothesis 3 1 2 3 SEX DIFFERENCES IN INTERGROUP AGGRESSION AND VIOLENCE: 4 5 6 THE MALE WARRIOR HYPOTHESIS 7 8 Alien biologists collecting data about different life forms on Planet Earth 9 10 would no doubt come up with contradictory claims about human nature. They would 11 12 13 witness our capacity to help complete strangers in sometimes large groups yet they 14 15 would also observe many incidents of extreme violence, especially between groups. 16 unedited manuscript 17 To make sense of the data, the alien researchers would probably conclude that 18 19 20 humans are a fiercely tribal social species. Some time ago, Charles Darwin speculated 21 22 about the origins of our tribal nature: “A tribe including many members who, from 23 24 possessing in a high degree the spirit of patriotism, fidelity, obedience, courage, and 25 26 27 sympathy, were always ready to aid one another, and to sacrifice themselves for the 28 29 common good, would be victorious over most other tribes; and this would be natural 30 31 selection.” (1871, p. 132).1 Unfortunately Darwin’s brilliant insight was ignored for 32 33 34 more than a century by fellow scientists, yet it is now gaining impact. Here I offer an 35 36 evolutionary perspective on the social psychology of intergroup relations, presenting 37 38 new insights and evidence about human intergroup psychology. 39 40 41 Social scientists are increasingly adopting an evolutionary approach to develop 42 43 novel hypotheses and integrate data on various aspects of human social psychology.2- 44 45 3 The evolutionary approach is based on the premise that the human mind is a 46 47 48 product of evolution by natural selection in the same way as human physiology. My 49 50 field, evolutionary social psychology, proposes that the human mind is essentially 51 52 social, comprising many functionalized mechanisms to cope with the challenges of 53 54 55 group living. One such specialized mechanism is coalition formation. Forming 56 57 alliances with other individuals confers considerable advantages in procuring and 58 59 protecting reproductively relevant resources – such as food, territories, mates, or off- 60 spring.4 Coalitional pressures may have led over evolutionary time to the emergence

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Van Vugt: Male Warrior Hypothesis 4 1 2 3 of some rather unique human traits such as language, theory of mind, culture, and 4 5 6 warfare. Some theorists argue that intergroup challenges created pressures on 7 8 humans to live in larger groups, producing an increase in brain size to make the most 9 10 of group living. 4-7 According to this hypothesis, our social brain is essentially a 11 12 13 coalitional or tribal brain. 14 15 In search of the origins of our tribal brain it is useful to make a distinction 16 unedited manuscript 17 between proximate and ultimate causes. An act of intergroup violence (such as a fight 18 19 20 between rival gangs or war between two nations) could be explained at two different 21 22 levels. First, why did this group attack the other? This proximate question interests 23 24 most sociologists, political scientists, historians, and social psychologists. Second, 25 26 27 one could ask why humans have evolved this apparent tendency to engage in 28 29 intergroup aggression – this ultimate question interests many evolutionary-minded 30 31 biologists and social scientists. Addressing both questions produces a more complete 32 33 2,8 34 picture, but they are different and should not be confused. 35 36 In terms of ultimate (evolutionary) causes, there are two classes of 37 38 explanations generally invoked. The first treat intergroup aggression as a by-product 39 40 41 of a highly developed in-group psychology. Being a highly social and cooperative 42 43 species, humans likely possess tendencies to help and favour members of in-groups.9 44 45 As a by-product of this, people will show indifference, or worse, a dislike for members 46 47 48 of out-groups. The second class of ultimate explanations focuses explicitly on people’s 49 50 psychological dispositions with respect to outgroups. The argument is that humans 51 52 likely evolved specific adaptations for dealing with intergroup threats because such 53 54 55 situations provided important reproductive challenges for ancestral humans. I believe 56 57 that this hypothesis is ultimately more persuasive because it accounts for the highly 58 59 textured psychological and behavioural reactions to outgroups. People do not have 60 some hazy negative feelings toward outgroups. In some instances, outgroups

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Van Vugt: Male Warrior Hypothesis 5 1 2 3 motivate desires to dominate, exploit or kill; in other instances, they inspire desires to 4 5 6 exclude and avoid. Recent work on prejudice, stereotyping, and intergroup processes 7 8 that recognizes this textured nature of intergroup psychology has generated many 9 10 new insights and empirical findings.6,10-12 Given the complexity of intergroup 11 12 13 relations, there are probably many different psychological tendencies pertaining to 14 15 interactions with outgroups depending upon the specific intergroup threat. For the 16 unedited manuscript 17 purpose of this article, I focus on the threat of intergroup aggression. 18 19 20 The Psychology of Warfare 21 22 Intergroup conflict is ancient. Not only was intergroup conflict common in 23 24 human ancestral environments, there is evidence that chimpanzees, our closest 25 26 27 genetic relative which are also a group-living species, are highly territorial and that 28 29 their intergroup encounters are often hostile1.5,7,13 Fossil evidence of human warfare 30 31 dates back at least 200,000 years, and it is estimated that as many as 20–30% of 32 33 14 34 ancestral men died from intergroup violence. Many present-day hunter–gatherer 35 36 groups are just as territorial and violent.15 Alexander (1987) argued that the biggest 37 38 threat for early humans came from other groups, which instigated an evolutionary 39 40 5 41 arms race to form ever larger coalitions. As Kurzban and Leary (2001) noted, 42 43 “membership in a potentially cooperative group should activate a psychology of 44 45 conflict and exploitation of out-group members—a feature that distinguishes 46 47 48 adaptations for coalitional psychology from other cognitive systems” (p. 195). 11 49 50 From the perspective of coalitional psychology, it becomes clear that not all 51 52 intergroup situations are equal; indeed, not all outgroups consist of coalitions of 53 54 55 individuals who engage in coordinated action—think of the homeless, the elderly, or 56 57 people with blue eyes. Humans are likely to have evolved coalition-detection 58 59 mechanisms that are responsive to various indicators of tribal alliances, for example, 60 “patterns of coordinated action, cooperation, and competition.”11 In modern

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Van Vugt: Male Warrior Hypothesis 6 1 2 3 environments, heuristic cues such as skin colour, speech patterns, and linguistic 4 5 6 labels—regardless of whether they actually signal tribal alliances—may engage these 7 8 mechanisms. 11,12 Perhaps equally important, many other salient cues—gender, age, 9 10 eye colour—may be far less likely to engage the coalitional psychology. We should 11 12 13 note that although this coalitional psychology evolved in the evolutionary context of 14 15 competition for resources (such as territories, food, and mates), this does not imply 16 unedited manuscript 17 that it is contemporarily activated only within contexts involving actual intergroup 18 19 16 20 conflict as proposed, for instance, by realistic conflict theory. 21 22 Furthermore, the specific psychological reactions of individuals in intergroup 23 24 contexts should depend on whether one’s group is in the position of exploiter or 25 26 27 exploited. For the would-be exploiters, desires to dominate—and the associated 28 29 psychological tendencies—would be functional. For the exploited, desires to avoid or 30 31 yield—and the associated psychological tendencies—would be functional. Of course, 32 33 34 in many situations, a group’s position as exploiter or exploited is transient or 35 36 ambiguous so it is likely that the two psychological tendencies are activated in similar 37 38 situations by similar cues and moderated by similar variables (e.g., social dominance 39 40 17 41 theory). 42 43 The Male Warrior Hypothesis 44 45 An important implication of the warfare hypothesis is that intergroup conflict 46 47 48 may have affected the evolved psychologies of men and women differently. 49 50 Intergroup aggression has historically involved rival coalitions of males fighting over 51 52 scarce reproductive resources, and this is true for early humans as well as 53 54 55 chimpanzees.13,15 As a consequence, this aspect of human coalitional psychology may 56 57 be more pronounced among men, an idea we refer to as the male warrior hypothesis 58 59 (MWH). 6 60

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Van Vugt: Male Warrior Hypothesis 7 1 2 3 There is already considerable evidence for sex differences in morphology, 4 5 6 psychology, and behavior that are functionally related to different selection pressures 7 8 operating on men and women throughout human, primate, and mammal evolution.18- 9 10 21 Due to a combination of differences in and parental certainty 11 12 13 men and women pursue somewhat different mating strategies yielding implications 14 15 for social behavior.22 In humans, as in most other mammals, mothers must invest 16 unedited manuscript 17 more heavily and for a longer period in their off-spring and, as a consequence, 18 19 20 engaging in openly aggressive acts to acquire resources, either individually or as part 21 22 of a group, will be physiologically and genetically costlier for women.18,21 Conversely, 23 24 given the right conditions, it can pay for males to join forces in attacking others to 25 26 27 acquire valuable reproductive resources despite the substantial risks involved. 28 29 Tooby and Cosmides’ (1988) risk contract hypothesis specifies four conditions 30 31 for the evolution of inter-group aggression in men.23 First, the average long-term 32 33 34 gains in reproductive success (i.e., mating opportunities) must be sufficiently large to 35 36 outweigh the average costs (i.e., injury or death). Second, members of warfare 37 38 coalitions must believe that their group is likely to emerge victorious in battle. Third, 39 40 41 the risk that each member takes and the importance of each member’s contribution 42 43 to victory must translate into a corresponding share of benefits (cf. the free-rider 44 45 problem). Fourth, when individuals go into battle they must be cloaked in a “veil of 46 47 48 ignorance” about who will live or die. Thus, if an inter-group victory produces, on 49 50 average, a 20% increase in reproductive success then as long as the risk of death for 51 52 any individual coalition member is less than 20% (say 1 in 10 men) such warrior traits 53 54 55 can be selected for potentially. This analysis presents not only a checklist for 56 57 theoretical development, but also a set of specific hypotheses that can be tested with 58 59 psychological and behavioral data. 60

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Van Vugt: Male Warrior Hypothesis 8 1 2 3 Even without compensating individual benefits, a male tribal psychology could 4 5 24,25 6 have evolved via . Multilevel selection theory holds that if there is 7 8 substantial variance in the reproductive success among groups, then group selection 9 10 becomes a genuine possibility. As Darwin himself had noted (see his earlier quote), 11 12 13 groups in which self-sacrifice is more common will fare better, especially if there is 14 15 competition between groups. Although participating in intergroup competition may 16 unedited manuscript 17 be personally risky because of the risk of death or injury, genes underlying propensity 18 19 20 to serve the group can be propagated if group-serving acts contribute to group 21 22 survival. 1 23 24 One condition conducive to group-level selection occurs when the genetic 25 26 27 interests of group members are aligned, such as in kin groups. In kin-bonded groups, 28 29 individuals benefit not just from their own reproductive success, but also from the 30 31 success of their family members (concept of inclusive fitness). Interestingly, ancestral 32 33 34 human groups appear to have been based around male kin, with females moving 35 36 between groups to avoid inbreeding (so-called patrilocal groups). This could offer 37 38 another explanation for why men rather than women would have been more 39 40 41 concerned about intergroup conflict (i.e., intergroup conflict would have 42 43 consequences for their inclusive fitness). The same patrilocal structure is incidentally 44 45 found in chimpanzees. The males of these groups also engage in coalitional 46 47 48 aggression. 7,13 49 50 These evolutionary models do not preclude the possibility that cultural 51 52 processes may be at work that could exacerbate or undermine these stronger male 53 54 55 tribal instincts. In fact, many of the evolved propensities are likely to be translated 56 57 into actual psychological and behavioral tendencies by socialization practices and 58 59 cultural norms. Thus, it is entirely possible that, in certain environments, it could be 60 advantageous for societies to turn females into warriors. A modern day example is

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Van Vugt: Male Warrior Hypothesis 9 1 2 3 Israel, a country at war with surrounding nations. To increase the size of their 4 5 6 military, Israel has actively recruited female soldiers, and it currently has the most 7 8 liberal rules regarding the participation of females in wars. 26 We would expect the 9 10 socialization practices among Israeli girls to match those of boys, potentially 11 12 13 overriding any evolved psychological sex differences. 14 15 Evidence from the Behavioral Sciences 16 unedited manuscript 17 Evidence for various aspects of this male warrior phenomenon can be found 18 19 20 throughout the behavioural science literature, for instance, in anthropology, history, 21 22 sociology, political science, biology and psychology. Across all cultures, almost any 23 24 act of inter-group aggression and violence, for instance, warfare, genocide, rebellion, 25 26 26,27 27 terrorism, street-gang and hooligan violence, is perpetrated by coalitions of men. 28 29 Evidence of male-to-male inter-group violence goes back as far as 200,000 years ago 30 31 (e.g., mass graves containing mostly male skeletons with evidence of force). 14 On 32 33 34 average death rates due to warfare among hunter-gatherers are 13% (according to 35 36 archaeological data) and 15% (according to ethnographic data), suggesting strong 37 38 selection pressures operating on ancestral males. 24 The figure is sometimes even 39 40 41 higher. Among the Yanomamö in the Amazon Basin an estimated 20-30% of adult 42 43 males die through tribal violence. 15 This compares to less than 1% of the US and 44 45 European populations in the 20 th century. Furthermore, male warriors in traditional 46 47 48 societies are held in higher esteem, and they have more sexual partners and children, 49 50 suggesting a direct reproductive benefit -- the so-called “Duke of Marlborough” effect. 51 52 This relationship might still be operative in modern society. A US-study revealed that 53 54 55 male youth street gang members have more sexual partners than ordinary young 56 57 males.28 In some societies, military men also seem to have greater sex appeal.29 Thus, 58 59 there may be reputational benefits associated with “warrior” behavior, which could 60 make it a profitable strategy for men in particular.30

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Van Vugt: Male Warrior Hypothesis 10 1 2 3 Further, a classic social psychological study, the Stanford prison experiment, 4 5 6 which highlighted some disturbing aspects of human inter-group psychology, was 7 8 conducted with males only. 31 Similarly, in economic game experiments involving 9 10 competing teams researchers frequently only use males groups (in a personal 11 12 13 communication, one of the authors, Gary Bornstein suggested that female groups 14 15 were less competitive). Finally, the primate literature reveals that among 16 unedited manuscript 17 chimpanzees adult males form coalitions to engage in violence against members of 18 19 7 20 neighbouring troops. This suggests that there is phylogenetic consistency between 21 22 intergroup violence in humans and one of our most closely related species. 23 24 Psychological Mechanisms Underlying Male Warrior Phenomenon 25 26 27 The MWH offers an integrative, conceptual framework in which findings from 28 29 diverse literatures including anthropology, biology, political science, and sociology 30 31 can be woven into a coherent story. However this approach runs the risk of being a 32 33 34 “just so” story about the role of warfare in shaping human psychology. It would be 35 36 more persuasive if we could make specific predictions about the psychological 37 38 mechanisms underlying the male warrior hypothesis and test these predictions in 39 40 41 carefully controlled studies. In the remainder I review the psychological literature on 42 43 sex differences in intergroup cognition and behaviour to test various aspects of the 44 45 male warrior hypothesis. I will be focusing in particular on areas such as inter-group 46 47 48 relations, intra-group dynamics, political support, self-esteem, and social 49 50 development. Naturally, not all out-groups are alike and we should expect sex 51 52 differences to emerge only when an outgroup constitutes a coalitional threat.31 53 54 55 Intergroup Aggression 56 57 A first generic prediction from the MWH is that there will be sex differences in 58 59 the willingness to initiate, plan, and participate in acts of intergroup aggression. On 60 the whole men are expected to be more belligerent than women. We can test this

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Van Vugt: Male Warrior Hypothesis 11 1 2 3 prediction in various ways. While a variety of studies across human and non-human 4 5 6 primates suggest that this is the case, more recent studies have tested these 7 8 predictions in controlled experimental conditions. 32 9 10 First, we can look at how men and women make decisions in war games 11 12 13 simulated in the laboratory. Upon being told that they are the leader of a fictitious 14 15 country interacting with leaders of other countries, a study by Johnson et al. (2006) 16 unedited manuscript 17 found that men are significantly more likely to attack another country without 18 19 33 20 provocation. Moreover, warfare is most intense when men are playing against other 21 22 men despite not knowing the sex of their rivals. As the authors noted: “Even though 23 24 players were unaware of the identity or gender of their opponent, wars and 25 26 27 unprovoked attacks were highest among male–male dyads, next most common 28 29 among mixed dyads, and least common in female–female dyads (p. 2516).”33 Men 30 31 also expressed higher positive illusions about winning these simulated intergroup 32 33 34 conflicts, a belief that increased the probability that they would attack their 35 36 opponent.33 Finally, another study analyzing the same dataset found that more male- 37 38 typical 2D:4D ratios, which are thought to index pre-natal testosterone exposure, 39 40 41 predicted aggression in the wargame experiment, over and above sex (i.e., 2D:4D 42 43 explained variance in aggression within as well as between sexes.34 These sex 44 45 differences also emerge when individuals play economic games between groups: All 46 47 48 male groups tend to be more competitive than all female groups or mixed-sex 49 50 groups.35 51 52 Second, there is some evidence that men and women differ in their 53 54 55 involvement in competitive inter-group encounters in the real-world.36 When asked 56 57 to indicate the frequency of various categories of social interactions over the past 58 59 month men reported more group-to-group interactions (M = 18.47, SD = 73.48) than 60 women (M = 12.77, SD = 59.68). Furthermore, men rated these interactions as more

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Van Vugt: Male Warrior Hypothesis 12 1 2 3 competitive ( Mmale vs. female = 3.17 vs. 2.31, SD ’s = 2.50 and 2.22; scale is 1 = very 4 5 36 6 cooperative, 5 = very competitive). 7 8 Intergroup Prejudice and Stereotyping 9 10 The MWH further predicts that men are more likely to derogate outgroup 11 12 13 members, especially when they constitute a coalitional threat. One specific form of 14 15 outgroup derogation is infrahumanization -- considering individuals less than human 16 unedited manuscript 17 – which might play a role in inducing intergroup violence.37. The rationale is that by 18 19 20 considering an out-group member as sub-human or animal-like, it will be 21 22 psychologically easier to treat them badly. In a recent study we did, men and women - 23 24 - all Christians -- were asked to describe a Christian or Muslim target using either 25 26 36 27 human (e.g., civil) or animal-typical (e.g., feral) words. Christian men were more 28 29 likely to describe the Muslim target in animal-typical ways, thus showing some 30 31 evidence of infra-humanization (Figure 1). It remains to be seen whether 32 33 34 infrahumanization strategies are particularly likely when outgroup targets are male, 35 36 as the MWH would predict. 37 38 Intergroup biases such as racism and also appear to be more 39 40 41 strongly held among men than women. Several experiments yield a greater sensitivity 42 43 of out-group stereotypes for in-group men, especially under conditions of inter-group 44 45 threat. Mark Schaller and colleagues have shown, for example, that men are more 46 47 48 strongly affected by cues of ambient darkness when using danger-relevant 49 50 stereotypes. 12 The notorious out-group homogeneity effect disappears when in-group 51 52 members are shown faces of angry out-group males rather than females.38 Again we 53 54 55 would expect this tendency to be particularly pronounced among in-group males. 56 57 Intra-Group Dynamics 58 59 The MWH also predicts potential sex differences in intra-group dynamics as a 60 result of inter-group threat. Being successful in inter-group competition requires

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Van Vugt: Male Warrior Hypothesis 13 1 2 3 membership of a strong, cohesive and coordinated in-group.6 Intergroup efforts 4 5 6 might be thwarted by free-riders, individuals who do not contribute to any intergroup 7 8 activity but nevertheless profit from the spoils of an intergroup victory. Based on the 9 10 MWH we argued that men would be more likely to respond to intergroup threats 11 12 13 through cooperating with their in-group, thus reducing the free-rider problem. 14 15 Consistent with this prediction, in public good games we found that men raised their 16 unedited manuscript 17 group contributions but only when we activated a symbolic competition between 18 19 6 20 groups. In Exp. 1 Van Vugt et al. found that during intergroup competition 92% of 21 22 men contributed to the public good but only 53% of women (Figure 2). As an 23 24 extension of this idea, in another study we found that in inter-group competition men 25 26 27 showed greater in-group loyalty by sticking with the group when it was more 28 29 (financially) attractive to leave. 36 30 31 A further prediction concerns differences in leadership emergence and 32 33 34 effectiveness in times of intergroup conflict. In a recent study we found that when two 35 36 equally suited candidates vied for the position of group leader, groups preferred the 37 38 male leader (78%). 39 A male leader was also more likely to raise group contributions 39 40 41 during intergroup competition. Interestingly, when the problem shifted towards 42 43 competition between in-group members virtually all groups chose a female leader 44 45 (Figure 3). 46 47 48 Consistent with the MWH, there is some evidence that male and female groups 49 50 have different group dynamics. Whereas female groups are more egalitarian, groups 51 52 of males form more hierarchical groups and these hierarchies tend to be more stable 53 54 55 over time. The difference in group structure corresponds with sex differences in 56 57 leadership style. Hierarchy formation is an effective response in dealing with 58 59 intergroup conflict requiring an urgent, coordinated response.40 60

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Van Vugt: Male Warrior Hypothesis 14 1 2 3 These claims are backed up by research on developmental differences in social 4 5 6 play. Boys play in larger groups than girls and more often play complex competitive 7 8 team games, which sometimes involve the use of weapons such as toy guns and 9 10 swords. 20 Boys also put greater social pressure on team members to conform to group 11 12 13 norms during play activities and they have more transient friendships with a larger 14 15 number of peers than girls. 20,41 16 unedited manuscript 17 Tribal Politics 18 19 20 The MWH predicts sex differences in political attitudes towards inter-group 21 22 conflict. We hypothesized that men would show relatively stronger political support 23 24 for warfare as a solution to conflict between countries. We tested this prediction 25 26 27 using data from a random selection of 10 recent national and international opinion 28 29 polls that we were able to find on the Internet and found consistent sex differences 30 31 (sometimes large, other times small but always in the same direction). 36 For instance, 32 33 34 a Washington Post-poll in 2003 (N = 1,030) asked the question “Do you support the 35 36 US having gone to war in Iraq?” to which 82% of men agreed versus 72% of women. 37 38 As another example, a recent poll by Gallup News (N = 7,074) found that 46% of men 39 40 41 (vs. 37% of women) disagreed with the question “Do you think the Iraq war was a 42 43 mistake?” 44 45 We also expected men to have a stronger preference for between-group 46 47 48 dominance hierarchies, which is the inevitable outcome of intergroup competitions. 49 50 To test this prediction, we asked an international survey of people to complete the 51 52 short 10-item social dominance orientation scale.17 This 7-point scale contains items 53 54 55 such as “Some groups of people are simply inferior to others;” “We should do what we 56 57 can to equalise conditions for different groups” “To get ahead in life, it is sometimes 58 59 necessary to step on other groups.” Consistent with other data, we found that men 60

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Van Vugt: Male Warrior Hypothesis 15 1 2 3 were significantly more socially dominant ( M = 2.56, SD = 1.13) than women ( M = 4 5 36 6 2.28, SD = 1.0). 7 8 Tribal Self-concept 9 10 A final prediction is that men and women should differ in self-concept. 11 12 13 According to the MWH men’s self-concept is primarily derived from the associations 14 15 with larger tribal groups, whereas women’s self-concept is derived primarily from 16 unedited manuscript 17 connections with other individuals. Consistent with this prediction men indeed have 18 19 20 a more collective sense of self that is more strongly derived from their group 21 22 memberships and affiliations. Gabriel and Gardner (1999) asked students to describe 23 24 themselves by completing the statement “I am…” They found that male students were 25 26 27 twice as likely to make statements referring to a tribal association (e.g., “I am a 28 29 member of a fraternity”). 42 In a recent study, we asked 100 people around the 30 31 University of Kent campus to indicate their favourite colour and to explain why they 32 33 34 picked this particular colour. Among men, almost 30% mentioned a tribal association 35 36 (e.g., their favourite football team, the colours of the flag of their country of origin); 37 38 none of the women did so. 36 39 40 41 Implications for Intergroup Relations 42 43 I presented a framework for studying the social psychology of intergroup 44 45 relations from an evolutionary perspective. This analysis suggests that not all 46 47 48 intergroup relations are alike because not all out-groups are alike. How people 49 50 interact with members of outgroups is determined in part by the specific challenges 51 52 (and opportunities) these groups present to the ingroup. When such challenges 53 54 55 correspond to evolutionarily relevant threats—threats that were significant enough in 56 57 ancestral social environments that humans have evolved to deal with them—they 58 59 elicit a specific intergroup psychology. Here I discussed the threat of intergroup 60 aggression and argued that it has produced a distinct evolved ingroup–outgroup

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Van Vugt: Male Warrior Hypothesis 16 1 2 3 psychology, consisting of an interrelated set of functional cognitive and behavioural 4 5 6 responses, to neutralize the intergroup threat. 7 8 Based on evolutionary reasoning, I argued that over evolutionary time men 9 10 must have been particularly affected by this intergroup threat, and dubbed this the 11 12 13 male warrior hypothesis. I reviewed the literature on sex differences in intergroup 14 15 psychology in light of predictions from the male warrior hypothesis and found them 16 unedited manuscript 17 to be generally supportive. Further tests are needed. For instance, some outgroups 18 19 20 are more likely to be infrahumanized (they are considered animal-like) whereas 21 22 others dehumanized (they are considered robot-like). It would be interesting to know 23 24 which of these strategies is more likely against which outgroups (depending upon 25 26 27 their size, strength, competence, etcetera) and whether such tendencies are stronger 28 29 in male-male intergroup interactions. Furthermore, in addition to warfare there 30 31 might be a host of other significant ancestral group challenges that have created their 32 33 34 own unique intergroup psychology which I did not discuss here. Disease avoidance is 35 36 one example and we would expect a different set of functional responses to a 37 38 contagion threat from an outgroup (e.g., avoidance rather than aggression).31,43 39 40 41 Moreover we would not necessarily expect differential reactions from men and 42 43 women when a disease threat is salient. There is some evidence that women show 44 45 greater intergroup prejudice when in their most fertile phase, thus when the risk of 46 47 48 contagion is highest. 44 49 50 The evolutionary framework makes various suggestions for interventions to 51 52 improve intergroup relations. When outgroups pose a coalitional threat interventions 53 54 55 might be targeted specifically at male-to-male interactions because they are the most 56 57 likely perpetrators and victims of intergroup aggression. In terms of their objectives, 58 59 interventions will be particularly successful when they eliminate the sense of threat 60 associated with particular outgroups altogether. Attempts must be made to

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Van Vugt: Male Warrior Hypothesis 17 1 2 3 individuate members of such outgroups, for instance, by accentuating their personal 4 5 6 achievements rather than the achievements of their group (e.g., British Asian 7 8 doctors). A second aim of interventions is to alter the perceptual cues that elicit threat 9 10 responses towards particular outgroups such as new immigrant groups. For instance, 11 12 13 language, dress code, and particular rituals or customs serve as tribal markers, and 14 15 the less noticeable they are the more these outgroups will receive positive treatment. 16 unedited manuscript 17 Thus, for the sake of attenuating the effects of coalitional psychology, it is important 18 19 20 for societies to make it easier for new immigrant groups to adopt the language and 21 22 customs of the ingroup. Third, interventions might be focused on changing the 23 24 specific cognitive and affective responses towards outgroups. However, if it is true 25 26 27 that these responses are evolved, then the link between threat and response might be 28 29 difficult to inhibit or extinguish (cf. fear of snakes and spiders). 45 Nevertheless, we 30 31 suspect that frequent positive interactions with members of outgroups will over time 32 33 34 reduce initial aversion or hostility. For instance, the Jigsaw class room experiments 35 36 demonstrate that cooperative relations between members of different ethnic groups 37 38 are a good means of reducing prejudice. 46 39 40 41 Coda 42 43 The social psychological literature on intergroup relations is rich and diverse. 44 45 It is relatively mute about the origins of tribal tendencies in humans and therefore 46 47 48 lacks a coherent framework for understanding why different outgroups elicit vastly 49 50 different responses. I presented a preliminary framework, inspired by insights from 51 52 evolutionary psychology and biology, that links particular intergroup challenges, 53 54 55 notably warfare, to particular functional responses. Although such responses may 56 57 have emerged because they were adaptive in ancestral times, they might not 58 59 necessarily be functional in modern times. Nonetheless, understanding why 60 particular out-groups elicit particular emotions, cognitions, and behaviours and in

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Van Vugt: Male Warrior Hypothesis 18 1 2 3 whom is the first step towards a sensible policy to improve intergroup relations in our 4 5 6 society. 7 8 9 10 11 12 13 14 15 16 17 unedited manuscript 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Van Vugt: Male Warrior Hypothesis 19 1 2 3 References 4 5 6 1. Darwin, C. (1871). The descent of man, and selection in relation to sex. 7 8 London: Murray. 9 10 2. Buss, D. M. (2005). Handbook of Evolutionary Psychology. Hoboken, NJ: 11 12 13 Wiley. 14 15 3. Van Vugt, M., & Schaller, M. (2008). Evolutionary perspectives on group 16 unedited manuscript 17 dynamics: An introduction. Group Dynamics, 12, 1-6. 18 19 20 4. Kenrick, D. T., Li, N. P., & Butner, J. (2003). Dynamical evolutionary 21 22 psychology: Individual decision rules and emergent social norms. 23 24 Psychological Review, 110, 3-28. 25 26 27 5. Alexander, R. D (1987). The biology of moral systems. London: Aldine 28 29 6. Van Vugt, M., De Cremer, D., & Janssen, D. P. (2007). Gender differences in 30 31 cooperation and competition: The male-warrior hypothesis. Psychological 32 33 34 Science, 18, 19-23. 35 36 7. Wrangham, R. W., & Peterson, D. (1996). Demonic males: Apes and the 37 38 origins of human violence. Boston, MA: Houghton Mifflin Co. 39 40 41 8. Van Vugt, M., & Van Lange, P. A. M. (2006). Psychological adaptations for 42 43 prosocial behavior: The altruism puzzle. In M. Schaller, J. A. Simpson, & D. T. 44 45 Kenrick (Eds.), Evolution and social psychology (pp. 237-261). New York: 46 47 48 Psychology Press. 49 50 9. Brewer, M. B. (1979). In-group bias in the minimal intergroup situation: A 51 52 cognitive–motivational analysis. Psychological Bulletin, 86, 307-324. 53 54 55 10. Cottrell, C. A., & Neuberg, S. L. (2005). Different emotional reactions to 56 57 different groups: A sociofunctional threat-based approach to “prejudice.” 58 59 Journal of Personality and Social Psychology, 88, 770-789. 60

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Van Vugt: Male Warrior Hypothesis 20 1 2 3 11. Kurzban, R., & Leary, M. R. (2001). Evolutionary origins of stigmatization: The 4 5 6 functions of social exclusion. Psychological Bulletin, 127, 187-208. 7 8 12. Schaller, M., Park, J. H., & Faulkner, J. (2003). Prehistoric dangers and 9 10 contemporary prejudices. European Review of Social Psychology, 14, 105-137. 11 12 13 13. Goodall, J. (1986). The chimpanzees of Gombe: Patterns of behavior. 14 15 Cambridge, MA: Harvard University Press. 16 unedited manuscript 17 14. Keeley, L. (1996). War before civilization. New York: Oxford University Press. 18 19 20 15. Chagnon, N. A. (1988). Life histories, blood revenge, and warfare in a tribal 21 22 population. Science, 239, 985-992. 23 24 16. Campbell, D.T. (1965). Ethnocentric and other altruistic motives. In: D. 25 26 27 Levine, (Ed.), Nebraska symposium on motivation , University of Nebraska 28 29 Press, Lincoln. 30 31 17. Sidanius, J., & Pratto, F. (1999). Social dominance: An intergroup theory of 32 33 34 social hierarchy and oppression. New York: Cambridge University Press. 35 36 18. Campbell A. (1999). Staying alive: Evolution, culture and intra-female 37 38 aggression. Behavioural and Brain Sciences, 22 , 203–252. 39 40 41 19. Eagly, A. H., & Wood, W. (1999). The origins of sex differences in human 42 43 behavior: Evolved dispositions versus social roles. American Psychologist, 54, 44 45 408-423. 46 47 48 20. Geary, D. C. (1998). Male, female: The evolution of human sex differences . 49 50 Washington, DC: American Psychological Association. 51 52 21. Taylor, S. E., Klein, L. C., Lewis, B. P., & Gruenewald, R. A. R. (2000). 53 54 55 Biobehavioral responses to stress in females: Tend-and-befriend not fight-or- 56 57 flight . Psychological Review, 107, 413-429. 58 59 22. Trivers, R. (1972). Parental investment and . In B. Campbell 60 (Ed.), Sexual selection and the descent of man, pp. 136-179 . Chicago: Aldine.

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Van Vugt: Male Warrior Hypothesis 21 1 2 3 23. Tooby, J., & Cosmides, L. (1988). The evolution of war and its cognitive foundations 4 5 6 (Institute for Evolutionary Studies Tech. Rep. No. 88-1). Palo Alto, CA: Institute for 7 8 Evolutionary Studies. 9 10 24. Bowles, S. (2006). Group competition, reproductive levelling, and the 11 12 13 evolution of human altruism. Science, 314, 1569-1572. 14 15 25. Wilson, D. S., Van Vugt, M., & O’Gorman, R. (2008). Multilevel selection 16 unedited manuscript 17 theory and its implications for psychological science. Current Directions in 18 19 20 Psychological Science, 17, 6-9. 21 22 26. Goldstein, J. (2003). War and gender. Cambridge: Cambridge University 23 24 Press. 25 26 27 27. Atran, S. (2003). The genesis of suicide terrorism. Science, 299, 1534-1539. 28 29 28. Palmer, C. T., & Tilley, C. F. (1995). Sexual access to females as a motivation 30 31 for joining gangs: An evolutionary approach. Journal of Sex Research. 32, 213- 32 33 34 217. 35 36 29. Leunissen, J. & Van Vugt, M. (2008). I love the man in the uniform: Women’s 37 38 preferences for male warriors. Unpublished Manuscript: University of Kent. 39 40 41 30. Hardy, C. L., & Van Vugt, M. (2006). Nice guys finish first: The competitive 42 43 altruism hypothesis. Personality and Social Psychology Bulletin, 32, 1402- 44 45 46 1413 47 48 31. Van Vugt, M. & Park, J. (in press). Guns, germs, and tribal social identities: 49 50 Evolutionary perspectives on intergroup relations. Social and Personality 51 52 53 Psychology Compass. 54 55 32. Wrangham, R. W. & Wilson, M. L. 2004 Collective Violence: Comparisons 56 57 between Youths and Chimpanzees. Annals of the New York Academy of 58 59 60 Sciences 1036, 233-256.

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Van Vugt: Male Warrior Hypothesis 22 1 2 3 33. Johnson, D. D. P., McDermott, R., Barrett, E. S., Crowden, J., Wrangham, R., 4 5 6 McIntyre, M. H., & Rosen, S. P. (2006). Overconfidence in war games: 7 8 Experimental evidence on expectations, aggression, gender and testosterone. 9 10 Proceedings of the Royal Society B, 273, 2513-2520. 11 12 13 34. McIntyre, M., Barrett, E., McDermott, R., Johnson, D., Cowden, J. & Rosen, S. 14 15 2007 Finger length ratio (2D:4D) and sex differences in aggression during a 16 unedited manuscript 17 simulated wargame. Personality and Individual Differences 42, 755-764. 18 19 20 35. Wildschut, T., Pinter, B., Vevea, J. L., Insko, C. A., & Schopler, J. (2003). 21 22 Beyond the group mind: A quantitative review of the interindividual– 23 24 intergroup discontinuity effect. Psychological Bulletin, 129, 698-722. 25 26 27 36. Van Vugt, M., Johnson, D. D. P., McDermott, R., O'Gorman, R., Spisak, B, & G. 28 29 Tendayi Viki. The Male warrior hypothesis: Sex differences in intergroup 30 31 aggression. Unpublished Manuscript: University of Kent. 32 33 34 37. Haslam, N. (2006). Dehumanization: An integrative review. Personality and 35 36 Social Psychology Review, 10 , 252–264 . 37 38 38. Ackerman J.M., Shapiro J.R., Neuberg S.L., Kenrick D.T., Becker D.V. and 39 40 41 Griskevicius V. (2006). They all look the same to me (unless they’re angry), 42 43 Psychological Science, 17, 836–840. 44 45 39. Van Vugt, M., & Spisak, B. R. (2008). Sex differences in leadership emergence 46 47 48 in conflicts within and between groups. Psychological Science, 19, 854-858. 49 50 40. Van Vugt, M. (2006). The evolutionary origins of leadership and followership. 51 52 Personality and Social Psychology Review, 10, 354-372. 53 54 55 41. Sherif, M., Harvey, O. J., White, B. J., Hood, W. R., & Sherif, C. W. (1961). 56 57 Intergroup conflict and cooperation: The Robbers Cave experiment. Norman, 58 59 Oklahoma: University Book Exchange. 60

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Van Vugt: Male Warrior Hypothesis 23 1 2 3 42. Gabriel, S., & Gardner, W. L. (1999). Are there his and hers types of 4 5 6 interdependence? The implications of gender differences in collective versus 7 8 relational interdependence for affect, behavior and cognition. Journal of 9 10 Personality and Social Psychology, 77, 642-655. 11 12 13 43. Schaller, M., & Neuberg, S. L. (in press). Intergroup prejudices and intergroup 14 15 conflicts. In C. Crawford & D. L. Krebs (Eds.), Foundations of evolutionary 16 unedited manuscript 17 psychology: Ideas, issues, and applications. Mahwah, NJ: Erlbaum. 18 19 20 44. Navarrete, C. D., Fessler, D. M. T., & Eng, S. J. (2007). Elevated ethnocentrism 21 22 in the first trimester of pregnancy. Evolution and Human Behavior, 28, 60- 23 24 65. 25 26 27 45. Öhman, A., & Mineka, S. (2001). Fear, phobia, and preparedness: Toward an 28 29 evolved module of fear and fear learning. Psychological Review , 108 , 483– 30 31 522. 32 33 34 46. Aronson, E., Blaney, N., Stephan, C., Sikes, J., & Snapp, M. (1978). The Jigsaw 35 36 classroom. Beverly Hills, CA: Sage. 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Van Vugt: Male Warrior Hypothesis 24 1 2 3 4 5 6 Table 1. Domains of evidence, hypothesized psychological mechanisms, and 7 8 predictions from Male Warrior Hypothesis. 9 10 Domain of Psychological Prediction Supported 11 12 Evidence Mechanism 13 1. Inter-group Propensity to Men are more likely Yes 14 aggression engage in inter- to make 15 group aggression unprovoked attacks 16 uneditedin manuscript war games 17 Men have more Yes 18 19 competitive inter- 20 group experiences 21 2. Inter- Infra/de- Men are more likely Yes 22 group humanization of to infra-humanize 23 prejudice antagonistic out- members of out- 24 25 groups groups 26 3. Intra- Greater Men contribute Yes 27 group cooperation with more to group 28 dynamics ingroup in during intergroup 29 response to threat competition 30 Displaying in- Men are more likely Yes 31 32 group loyalty to show in-group 33 during inter- loyalty during 34 group conflict inter-group conflict 35 Male leadership Groups show Yes 36 bias in intergroup stronger preference 37 38 settings for male leaders 39 during intergroup 40 competition 41 4. Tribal Support for Men show stronger Yes 42 Politics intergroup political support for 43 aggression warfare in opinion 44 45 polls 46 Preferences for Men score higher Yes 47 and justifcation of on social 48 hierarchies dominance 49 between groups orientation scale 50 51 5. Tribal self- Spontaneous Men are more likely Yes 52 concept activation of to make 53 inter-group spontaneous tribal 54 association associations when 55 defining themselves 56 57 58 59 60

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Van Vugt: Male Warrior Hypothesis 25 1 2 3 Figure Captions 4 5 6 Figure 1. Evidence for infra-humanization when Christian males ascribe fewer 7 8 human-like words and more animal-like words to Muslim targets. 36 9 10 Figure 2. Intergroup competition increases public good contributions among males in 11 12 6 13 particular. 14 15 Figure 3. Intergroup competition increases the preference for male leaders, whereas 16 unedited manuscript 17 intragroup competition increases the preference for female leaders. 39 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 unedited manuscript 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Figure 1. Evidence for infra-humanization when Christian males ascribe fewer human-like words and 33 more animal-like words to Muslim targets.36 34 254x190mm (96 x 96 DPI) 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 http://www.nyas.org/forthcoming Page 27 of 28 Annals of the New York Academy of Sciences

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 unedited manuscript 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Figure 3. Intergroup competition increases the preference for male leaders, whereas intragroup 34 competition increases the preference for female leaders.39 35 254x190mm (96 x 96 DPI) 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 http://www.nyas.org/forthcoming Annals of the New York Academy of Sciences Page 28 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 unedited manuscript 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Figure 2. Intergroup competition increases public good contributions among males in particular.6 34 254x190mm (96 x 96 DPI) 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 http://www.nyas.org/forthcoming