Male Adaptations that Facilitate Success in War
KeywordsSocial Dominance Orientation Collective Action Problem Cognitive Adaptation Intergroup Conflict Adaptive Challenge
The male warrior hypothesis claims that the sex-specific adaptive pressures exerted by recurrent lethal intergroup conflicts in our ancestral past favored the evolution of a cognitive machinery enabling men to form and maintain aggressive coalitions with other males to the end of dominating and exploiting out-groups.
War is a gruesome phenomenon. Throughout (pre)history, humans engaging in this brutish activity have brought death, suffering, and destruction to an incalculable amount of their conspecifics. And although some scholars’ estimates indicate that the relative impact of wars on human survival and living conditions has been on the decline during the last couple of centuries (Pinker 2011), wars still continue to exert their devastating force today.
From an evolutionary perspective, wars pose a nontrivial puzzle. Why is it that humans are obviously ready and willing to put their lives at risk in collective lethal aggression against groups of conspecifics? Why is it that Homo sapiens is among the few species that engage in such murderous campaigns? Why is it, finally, that “[w]hile war may be everyone’s business, it has usually been men’s work,” as Keeley (1996, p. 35) trenchantly put it?
In this entry, we review selected previous literature on all three questions. Our focus, however, lies on sex differences in cognitive mechanisms potentially resulting from the differential adaptive challenges posed to the sexes by the frequent recurrence of violent intergroup conflicts throughout humanization. First, we outline theoretical work on the general cost/benefit structure faced by fitness relevant behavioral strategies in war. Second, we discuss how the main components of the resulting models of human belligerent behavior differentiate humans from many other animals. Third, we ask if human prehistory may have actually met the conditions required for war to entail sufficiently intense selection pressures in order to result in adaptive cognitive sex differences. Finally, we review contemporary evidence for the existence of these differences obtained in controlled psychological experiments, field studies, and surveys.
Theoretical Perspectives on Adaptive Problems Posed by Warfare
Modelling approaches to the evolution of human behavioral strategies commonly start from the assumption that most of our species’ evolutionarily relevant time on this planet was characterized by living conditions similar to those of contemporary hunter-gatherer bands and wild chimpanzees. For humans, this means living in relatively small, mobile bands consisting of up to a few dozens of individuals, co-residing with primary and more distant kin, spouses’ primary kin, other affines, and unrelated others (for recent demographical estimates, see, e.g., Hill et al. 2011). Multiple bands, then, together form clans, tribes, and ethnolinguistic cultures, which are often characterized by frequent exchanges of members between their subgroups. Most recent models of the evolution of bellicose behavior in humans locate violent conflicts at the between-band level (Choi and Bowles 2007; Lehmann and Feldman 2008; for a comprehensive review see Rusch 2014b).
Theorists have identified the main adaptive challenge that individuals living in such groups face when deciding whether or not to engage in intergroup conflicts as the trade-off that must be made between the potential benefits and prospective risks entailed by waging war. While some authors hold that a decision to participate in collective aggression against out-groups represents a genuinely altruistic act (Choi and Bowles 2007), other authors argue that the cost/benefit structure of war often favors the aggressor directly as long as certain conditions are met (Lehmann and Feldman 2008; Rusch 2014b; Tooby and Cosmides 2010). Relevant to this, one of the most important distinctions is the one between coalitionary defensive aggression, which can occur in the forms of preemptive first strikes as well as retaliatory counterstrikes, on the one hand, and unprovoked offensive aggression on the other. While the evolution of defensive aggression is relatively easy to explain because of its self- and kin-protective function, unprovoked offensive aggression poses a more complex problem (Rusch 2014a). Why should an individual be willing to participate in aggressing against an out-group, if (i) nonparticipation by this combatant would not substantially lower the chances of victory, if (ii) the spoils obtained in case of victory are shared by all members of the victorious group, and if (iii) the risks entailed by participating are high, potentially even as high as losing the own life.
Theorists have identified multiple factors that can provide solutions to this collective action problem (Gavrilets 2015; Rusch 2014b; Tooby and Cosmides 2010). Apart from the commonplace mechanisms – direct and indirect reciprocity as well as peer-punishment – that can of course also promote group-level cooperation in this context, coalitionary offensive aggression likely differs from other collective action problems in a number of crucial respects. For one, in groups as small as the ones our ancestors lived in, nonparticipation by a single individual quite likely had a significant effect on the outcome of an attack (Johnson and MacKay 2015). This makes every single combatant pivotal for the outcome of a threshold collective action problem, a factor which is known to foster cooperative individual decision-making (Barclay and van Vugt 2015). Second, small deviations from the, arguably unrealistic, assumptions that (i) the spoils of war are equally shared and that (ii) all combatants are equal in their contribution capabilities can already substantially alter the incentive structure of coalitionary aggression. This then leads to positive selection for belligerence in the more battle-ready and/or more privileged individuals (Gavrilets 2015; Gavrilets and Fortunato 2014). Third and finally, an interesting structural asymmetry exists between the risks entailed by offensive versus defensive warfare, caused by the greater strategic flexibility that offenders usually have. Normally, attackers can eventually decide if they want to follow through with a strike or not. If, for instance, a lack of numerical superiority or other situational factors discourage an attack, offenders can retreat safely as long as they are not spotted or ambushed by defenders. In addition, the strategic advantage of surprise is usually on the attackers’ side. In effect, these structural advantages often minimize the risks faced by offenders.
In summary, various models and simulations suggest that the benefits associated with coalitionary offensive aggression can sometimes outweigh the costs. It must be noted, though, that this likely only pertains to single strikes in hunter-gatherer warfare. The long-term outcome of continuous raiding and feuding between such groups can be quite substantial in terms of mortality risks, as we argue later. Assuming that costs are potentially low in the short term, the question is what the potential benefits of hunter-gatherer aggression are. In order for belligerence to be positively selected for in the long run, these must be large enough to offset not only the relatively small risks of single strikes but also the long-term increases in mortality risks, caused by continuous feuding between bands.
A recent model suggests that there are at least two ways through which aggressors can obtain significant direct and indirect fitness benefits through warring. Building on the conventional assumptions about hunter-gatherer population structure, and explicitly focusing on male combatants, Lehmann and Feldman (2008) showed that both (i) direct fitness increases for males obtained through winning additional reproductive access to females of other groups and (ii) the more indirect effect of increased reproduction of in-group females caused by seizing territory or other valuable resources from out-groups can independently suffice for a positive selection of male belligerence and bravery. Unlike other models, furthermore, Lehmann and Feldman explicitly analyze the role of relatedness between combatants. They find that the potentially high costs of warring can be substantially dampened by high within-group relatedness of male warriors. They show that, although an individual combatant may eventually have to pay with his life for victory over out-groups, the inclusive fitness benefits obtained by him through increased reproduction of his lineage can still offset these costs, even in groups of considerable size.
To summarize, various scenarios favoring a positive selection of belligerence and bravery in intergroup conflict exist in the theoretical literature today. They show consistently that warring may have had adaptive benefits, especially in ancestral men. An important question raised by these theoretical models of human warfare is why only a handful of species show coalitional aggression (Wrangham 1999).
One potential reason for the relative sparseness of intraspecific lethal coalitional aggression in other animals lies in the complex social adaptations that are required for warfare. These include sociality, communality, and advanced capabilities for coordinated action. Furthermore, in order for warring to be potentially beneficial in terms of inclusive fitness, the relatedness patterns resulting from a species’ way of living must meet certain criteria. As constantly killing close relatives would obviously result in negative selection against this trait, warring can only evolve if its benefits are more likely to be received by close kin while its costs are borne by more distantly or unrelated individuals. Intense lethal intergroup conflicts occur, for example, in some species of colonial eusocial insects. In these, individuals within colonies are genetically very closely related, while relatedness between colonies is much lower. For most vertebrates, however, relatedness patterns are more complex. In group-living mammals, for example, within- and between-group relatedness crucially depends on residence rules, that is, on the question whether groups frequently exchange individuals and if so which individuals.
For understanding the evolutionary roots of human intergroup aggression, and particularly for the question why it is mostly men who engage in this behavior, residence rules are central. A recent meta-study comparing 135 primate species, for example, found that species in which the dominant sex is philopatric, i.e., species in which the physically stronger sex tends to live together with close relatives while members of the physically weaker sex change round between groups, are more successful at defending their territories (Willems et al. 2013; also see Willems and van Schaik 2015). It seems that close family ties are required to foster the cooperation needed to sustain coalitional aggression in animals. Although no commonly accepted theory explaining the occurrence of lethal animal intergroup conflict exists yet, residence rules of the warring sex show similar patterns in all mammals for which such conflicts have been documented. Among them are hyenas, lions, wolves, and, of course, chimpanzees, and humans (Wrangham 1999).
Chimpanzee “wars,” in particular, are the best studied cases of nonhuman violent intergroup conflicts. Since Jane Goodall made her famous observations in Gombe National Park, documenting the systematic extermination of all out-group males by a coalition of male chimps over a period of 5 years, many careful field studies have been conducted to understand when – and why – male chimpanzees engage in lethal coalitional aggression and how they might benefit from doing so. Michael Wilson and colleagues recently evaluated the available evidence (Wilson et al. 2014). Analyzing 152 killings in 15 communities, they found that (i) males were the most frequent attackers (92%) and victims (73%) in lethal violence in chimpanzees, (ii) a majority of these killings (66%) involved members of different communities, and (iii) attackers used risk-minimizing tactics (median attacker-to-victim ratio 8:1). In addition, Wilson et al. were able to extract two robust predictors of higher levels of intergroup conflict from their data. Violence increased with population density as well as with the number of males living in the respective communities. They conclude that “patterns of lethal aggression in Pan [… are …] better explained by the adaptive hypothesis that killing is a means to eliminate rivals when the costs of killing are low” (Wilson et al. 2014, p. 416).
One might be tempted to extrapolate this “chimpanzee model” directly to humans. Like Pan troglodytes, humans are a territorial, group-living species. And, like chimpanzees, Homo sapiens often lives patrilocally, that is, groups are often formed around “bands of brothers,” whereas females switch groups through exogamous marriage (e.g., 15 of the 32 contemporary hunter-gatherer societies surveyed recently by Kim Hill et al. 2011, follow this residence rule; matrilocal: 5, ambiguous/unknown – 12).
Support for the notion that residence patterns, and the kinship structures they induce, are key to understanding human warfare comes from early systematic anthropological work. Adams (1983) analyzed evidence from 67 prestate human societies. He found that in nine of these societies, some forms of active female participation in lethal intergroup violence were documented. Strikingly, all of the nine societies followed residence rules that preclude that women may have to fight relatives in enemy groups. Furthermore, Adams found that patrilocally living societies were more likely to exhibit higher levels of intergroup conflict, as compared to societies following other rules for marital residency. More supportive evidence for the relevance of the “chimpanzee model” for understanding human intergroup conflict was recently compiled by Wrangham and Glowacki (2012). Carefully reviewing the available anthropological literature, they identified many additional similarities between chimpanzee and hunter-gatherer warring. Also in humans, victors often seize territory from defeated out-groups, females are frequently abducted in raids, and attackers mostly use risk-minimizing surprise tactics.
Thus, let us assume for a moment that human prestate intergroup violence resembles chimpanzee warring closely enough to follow a similar fitness logic – an assumption which is certainly debatable (see, e.g., Wrangham and Glowacki 2012 for a comprehensive discussion). The remaining question then is if war in ancestral human populations was frequent and intense enough in order to actually create differential selection pressures on human males and females across evolutionary time.
In terms of absolute numbers, the large-scale wars of the twentieth century have certainly been the deadliest campaigns ever carried out. As Pinker has argued, though, their relative impact on human survival may have been comparably small, taking the size of the world population into account (Pinker 2011). Interestingly, Pinker’s calculations are paralleled in the anthropological literature. Where authors have tried to estimate the mortality rates caused by lethal intergroup conflict in human prestate societies, they have often arrived at similar conclusions. Keeley (1996), for example, in his reconstruction of the prehistory of war, estimates that in some traditional societies, between 8 and 59% of male deaths may have been caused by war, while this figure lies below 1% for the twentieth century Europe and USA. Also, where data is available, Keeley’s estimates indicate that men in traditional societies roughly had a 6.5 times higher chance of dying in war than females (Keeley 1996, p. 196). Similar numbers are reported by Walker and Bailey (2013) who recently analyzed ethnographic reports of 1,145 killings in 44 lowland South American societies. They confirm that attackers in individual raids suffered hardly any casualties and that a major reason for raiding was the capture of women (with ca. 0.6 women being displaced per raid). In addition, they found that a total of 1,045 of these events, representing an average of roughly 28% of all deaths, male and female, were caused by war in this sample. Again, though, men had a substantially higher risk of dying in war than women.
Before we move on to review evidence from recent psychological and social science studies supporting the hypothesis that the potentially high mortality rates caused by war under prehistoric living conditions could have resulted in cognitive adaptations for intergroup conflict, one important point needs to be addressed. As, for example, Gat (2015) elaborately discusses, one should not view human prehistory as a history of perpetual extreme intergroup violence. As Fry and Söderberg (2013), among others, point out, many human societies of all times and regions have most likely also been able to sustain long-term peaceful coexistence with neighboring groups (also see Rusch 2014b; Wrangham and Glowacki 2012). When we speak of psychological adaptations for coalitional aggression and intergroup conflict in the following section, thus, these should be understood as domain-specialized, cognitive mechanisms that are being activated only in adaptively relevant contexts and not (so much) as psychological instincts constantly seeking war as the only solution to conflict.
The Male Warrior Hypothesis
Let us assume, for now, though, that warfare and intergroup conflict have been important selective forces for humans, contributing to a relatively unique human coalitional psychology, with some parallels in other species. Then, it is likely that warfare has shaped the coalitional psychologies of men and women differently. This is the basic thrust behind the male warrior hypothesis which claims that specific cognitive machinery has evolved enabling ancestral men to form and maintain aggressive coalitions with other males with the aim of dominating and exploiting members of out-groups (McDonald et al. 2012; van Vugt and Janssen 2007). What cognitive adaptions constitute this specific warrior psychology, what cues activate it, and how do the sexes differ in the activation of these cognitive mechanisms?
We can distinguish between at least four different coalition mechanisms – or psychological systems – that need to be in place to plan, execute, and be victorious in intergroup conflict. First, coalition detection mechanisms need to be in place to be vigilant about potentially, hostile coalitions, remember who is in-group and who is out-group, and assess the strength and formidability of these coalitions. Second, an adaptive warrior psychology requires mechanisms to quickly recruit allies to form strong and sometimes sizeable coalitions, pick the right members to form a combat force, and form emotional attachments to these coalitional groups quickly. Furthermore, a cognitive adaptation to assume the identity of a male warrior is helpful (cf. the combat identity hypothesis by Tooby and Cosmides 2010) and so is an above-average interest in the aesthetics of warfare. Coalitional maintenance adaptations are needed in order to ensure that coalitions remain together and unified, and they include adaptations for reward cooperative members (e.g., war heroes) and punish defectors (deserters and cowards). Finally, coalition exploitation systems allow individuals to time a collective aggressive act and reap the benefits of intergroup aggression by converting the spoils of war into reproductive benefits. Cognitive adaptations that enable individuals to make the right trade-offs between the risks and benefits of coalitional aggression attack and kill members of out-groups only if potential costs are low and potential benefits are high, dominate and exploit members of out-groups, and morally justify these actions are some examples of exploitation mechanisms that together may constitute this warrior psychology.
Such coalitional systems are likely coupled to specific emotions that allow individuals to make adaptive decisions in the domain of intergroup conflict such as feelings of in-group loyalty, out-group hate, anger toward in-group defectors, admiration of war heroes, etc. Furthermore, these mechanisms are activated by adaptively relevant external cues. Seeing a collection of out-group males together, for example, should activate the coalition detection system more so than seeing a group of out-group females and perhaps especially if the men are (i) physically strong, (ii) in possession of weapons, and (iii) in or near in-group territory. Similarly, individual differences in physical formidability, aggression, social class, age, and social networks likely influence how people trade off the risks of engaging in collective violence. Finally, cognitive adaptations for coalitional aggression, although they may partly overlap with interpersonal aggression, are likely to be activated by distinct, domain-specific cues (such as the formidability and power of the leader of a group; Holbrook and Fessler 2013).
This brings us to consider sex differences. Due to differences in parental investment and, as discussed earlier, differences in physical formidability and co-residence patterns, there is an asymmetry in the costs and benefits of engaging in warfare between men and women. This was especially true in ancestral environments where most forms of intergroup conflict involve physical combat. By virtue of a lower paternal investment, coupled with a stronger physique, it may be adaptive for men under certain conditions to be warriors as this can increase their access to reproductively relevant resources (mates, territories). These conditions are specified in Tooby and Cosmides (1988) risk contract theory of warfare that views warfare as a collective action problem in which free-riding dominates unless (i) there are compensating benefits for cooperators, (ii) defectors are punished, and (iii) there is a veil of ignorance making it impossible to know who will die and who will survive an attack a priori.
Coalitional aggression can be directly fitness enhancing for men as one gains access to women (see above). Other benefits of coalitional aggression are indirect fitness enhancers. That is, by killing or injuring the males of rival groups, these out-groups weaken in strength, and their females may eventually migrate to the in-group (analogous to observed behavior in chimpanzees; Wilson and Wrangham 2003).
Research Support for the Male Warrior Hypothesis
The MWH asserts that men will respond more quickly and strongly to cues associated with intergroup conflict and convert coalitional aggression into reproductive opportunities. This hypothesis does not suggest that all men will always prefer war as an option to manage intergroup relations. Whether or not this warrior psychology is activated depends upon cues that are ancestrally linked to be on the winning side in an intergroup conflict. Second, the MWH allows for individual variation between men in their sensitivity to cues associated with warfare. For instance, warrior behavior may be easier activated in men who are physical stronger, have higher degree of dispositional aggression, possess a higher baseline testosterone, and have larger male kin or friendship networks. Nevertheless, we believe that there should be sex differences in the ease with which different coalitional systems that support intergroup aggression and warfare are being activated. The research literature supports this by and large.
Earlier we reviewed the evidence from primatology and anthropology suggesting selection for male warrior traits. Now we turn to the social sciences. There is an abundance of studies in psychology and other fields (political science, sociology, behavioral economics) that show that men have a lower threshold for engaging in coalitional aggression. First, surveys reveal that men report having more experiences with competitive intergroup interactions (e.g., team sports) than women (Pemberton et al.1996). In simulated war games between countries, men as the leader of their nation are also more likely than women leaders to make a preemptive strike against another nation (Johnson et al. 2006). Exclusively male groups make more defective choices in prisoner’s dilemmas between groups than all-female groups (Wildschut et al. 2003). Finally, in nationwide opinion polls, conducted in various countries, men are always more supportive of military action as a solution to intergroup conflict (van Vugt 2009). This is ironic, of course, considering the fact that men are also the most likely victims of warfare, but it makes sense if we follow the logic of the MWH.
An indirect test of the MWH involves looking at sex differences in preferences for group-based dominance hierarchies, which is the inevitable outcome of warfare. On an individual measure of this preference, called social dominance orientation (SDO), men score systematically higher than women, even in countries where gender roles are relatively egalitarian such as Sweden (Lee et al. 2011). Importantly, scores on SDO are positively associated with a wide variety of social attitudes and ideologies that tend to legitimize intergroup dominance hierarchies, including racism, patriotism, and the explicit endorsement and support for aggressive wars (Sidanius and Pratto 1999). Men, on average, also display more xenophobic and ethnocentric attitudes than women (Sidanius and Ekehammar 1980), and men are more likely to dehumanize out-group members (van Vugt 2009), which may help ease the psychological discomfort associated with killing or injuring men from rival groups.
The way men and women form and maintain their coalitions also differs. In line with the MWH, developmental studies show that boys form larger, more flexible coalitions than girls, which suggests an ability to form flexible war-size parties (Hawley 2014). Men also identify themselves more strongly with large, tribal groups than women. In one study, researchers showed that men had a more collective self-concept, whereas women had an interpersonal self-concept (Gabriel and Gardner 1999). They asked men and women to complete 20 statements beginning with “I am….” They found that men reported a higher proportion of collective self-descriptors (e.g., “I am a member of a fraternity.”), whereas women provided a higher proportion of relational descriptions (“I am best friends with Michelle”). In another study (a summary is reported in van Vugt 2009), we asked men and women what their favorite color was and why. More than twice as many men as women mentioned a tribal association with their favorite color (e.g., “I like the color red because it is in our national flag”).
Men are also more motivated to defend their in-group and cooperate with fellow in-group members, particularly when there is an intergroup conflict. In a series of laboratory experiments, we found that men contributed more to a public good for their group when their group was in competition with other groups, rather when they were competing individually. This difference was absent for women. Furthermore, men identified more strongly with their group when it was facing intergroup competition, and again this effect was absent for women (van Vugt et al. 2007). Other studies show that men are also more aesthetically attracted to stimuli depicting intergroup conflict, be it war movies or gangster movies, war documentaries, and books depicting intergroup aggression such as biographies of war heroes (Goldstein 2002; van Vugt 2009).
What about the choice for leaders during episodes of intergroup aggression? Both men and women prefer to appoint men as leaders during intergroup conflicts, presumably because they believe that men fit the physical and psychological prototype of a war leader better. In a public goods experiment, participants who thought the study’s aim was to compare how teams of different universities were doing were much more likely to appoint men as their team captains than participants who believed that the study’s aim was to compare individual team players in terms of their public good contributions – the majority of them chose women as team captains (van Vugt and Spisak 2008). Various studies, using mock presidential election data, revealed that when people are asked to vote for a president when their country is facing war, a majority of voters prefer a candidate with a masculine-looking face over a candidate with a more feminine-looking face (Little et al. 2007; Spisak et al. 2012).
Finally, consistent with the MWH and in line with sexual selection theory, there are sex differences in the extent to which warfare is driven by sexual motives. That is, for men, warfare may be a platform on which they can show off their physical skills and psychological qualities as valuable coalition partners or sexual mates. Regarding the latter, recent studies found that women were more sexually attracted to men who had shown bravery in warfare than those who had not (Rusch et al. 2015). This effect was both domain and sex specific. Women were more interested in dating a war hero than a hero in a natural disaster. Furthermore, men were not particularly interested in dating female war heroes, showing evidence for a sex-specific war psychology. In addition, an archive study revealed that male war heroes (Medal of Honor recipients of World War II) sired more offspring than ordinary soldiers – suggesting evidence for reproductive benefits associated with war behavior.
Despite the accumulating evidence for the MWH and specific male adaptations for warfare, a number of contentious issues remain. First, are there specific female cognitive adaptations for warfare? We think there are. Ancestral women’s reproductive interests were not served by actively joining in physical combat for the reasons we have already outlined. Nevertheless ancestral women could profit from the extra resources that would be gained by warrior males as long as their group emerged victoriously from combat. Thus, under certain conditions such as a food shortage, women could have encouraged their men to go on a raid. Similarly women may encourage male warrior behavior indirectly through selectively preferring male warriors as their mates (Rusch et al. 2015). Nevertheless, on the whole, we hypothesize that women’s psychology is probably more geared toward peacekeeping and peace maintenance between groups as this serves their reproductive interests best. Not surprisingly, both sexes prefer female leaders to keep the peace in intergroup relations (van Vugt and Spisak 2008). This is not to say that women have not evolved specific cognitive adaptions to deal with warfare. First, out-group men pose specific challenges to the reproductive choices of in-group women. Thus, we expect women to have evolved specific mechanisms that are activated in particular after exposure to out-group male warriors. One such adaptation is rape avoidance, and there are findings suggesting that women are particularly fearful of formidable out-group men versus in-group men (McDonald et al. 2012). Second, it would be in a woman’s best interest to opportunistically switch sides once a particular coalition of men emerges victorious after battle. Thus we expect women to have evolved a tend-and-befriend strategy (cf. Taylor et al. 2000) toward victorious out-group men to protect themselves and possibly their offspring from danger. This hypothesis remains to be tested in humans.
Third, the MWH does not preclude the possibility of women to play an active role in combat. Although the notorious army of Amazon warriors is likely a myth, women do sometimes join a fight. Female warrior behavior seems to be driven by factors such as the size of the war coalition needed in battle. For instance, Israel is surrounded by a number of countries that are hostile to Israel, and not surprising, it is also the country that has the largest portion of active female soldiers in service (Goldstein 2002). Thus, although women may not be predisposed to be warriors, there may be critical conditions – such as the overwhelming coalition size of the out-group – that override these evolved tendencies. In addition, although offensive coalitional actions such as raids and ambush killings are largely the domain of men, this may be different when a group is defending itself against a hostile out-group.
A final challenge is what specific cues trigger this male warrior psychology and if there are individual differences. Cues of out-group males teaming up together should activate this warrior psychology and probably more so when this male coalition is sizeable in numbers and has a physically formidable leader. Men should be better at detecting coalitions than women, being able to remember better to which tribal groups a person belongs (Kurzban et al. 2001). Resource scarcities should probably activate male warrior tendencies and so does the presence of young, fertile out-group women. Finally, one’s own physique and personality might matter. That is, warrior tendencies are easier to activate in men who are physically formidable, have high base levels of testosterone, and have a fearless personality.
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