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The Evolutionary Anthropology of War

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Abstract

Evolutionary anthropologists seek to understand the evolution of warfare across multiple timescales, from the roots of warfare in the intergroup aggression of our primate ancestors, to the causes of warfare among contemporary societies today. While warfare remains a contentious subject, considerable evidence supports the view that warfare is a strategy by which coalitions of males cooperate to acquire and defend resources necessary for reproduction. This strategy is not the result of a single "instinct" for war, but is instead an emergent property resulting from evolved psychological mechanisms (such as xenophobia and parochial altruism). These mechanisms are sensitive to ecological and social conditions, such that the prevalence and patterns of warfare vary according to subsistence strategies, military technology, cultural institutions, and political and economic relations. When economic conditions enable intergroup relations to change from zero-sum to positive-sum games, peaceful intergroup relations can emerge.

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... Warfare is considered by many anthropologists to have played a central role in human history, from early human evolution, emergence of complex societies, to our recent history (e.g., Bowles, 2009;Carneiro, 1970Carneiro, , 1990Choi and Bowles, 2007;Glowacki et al., 2017;Gómez et al., 2016;Keeley, 1996;Turchin, 2007;Turchin et al., 2013;Zefferman and Mathew, 2015). Consequently, identifying warfare in past societies and its importance for societal change is a significant challenge for archaeologists (Kintigh et al., 2014). ...
... In some cases, these new findings are leading to wholesale reinterpretations of warfare narratives (e.g., Fry and Söderberg, 2013;Jiménez, 2018;Smith-Guzmán and Cooke, 2018). While violence and warfare were demonstrably prevalent in the past, warfare is not an inevitable outcome of human social interaction (e.g., Glowacki et al., 2017). Thus, we must critically evaluate the evidence for war from the available archaeological and ethnohistorical evidence. ...
... Warfare is thus group-to-group violence at the scale of alliances, political communities, or other groups that share identity (Tefft and Reinhardt, 1974, p. 154;Younger, 2008). While this class of interaction includes lethal violence between individuals, the mechanisms leading to between-group conflict are distinct from those where a single individual might attack another -interpersonal violence might lead to warfare, but the latter occurs as a function of coordinated and cooperative effort between sets of individuals (Glowacki et al., 2017;Younger, 2011;Zefferman and Mathew, 2015). Here, we focus our treatment of warfare on scholarly assumptions about the 'Huri Moai' narrative for Rapa Nui: cases where the outcome was large-scale loss of life and monument destruction as a result of group-scale violence. ...
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Warfare is widely accepted as a transformative factor in human history. However, as warfare is not inevitable in human groups, archaeologists must critically assess the empirical evidence for war and its importance in the past. Here, we reevaluate the culture history of Rapa Nui (Easter Island), often interpreted as a case of warfare resulting in social upheaval. Common accounts hold that, prior to European contact, clan groups eventually ceased making moai statues and statue platforms (ahu), battled with obsidian spears, sought refuge in fortified caves, and toppled rivals' moai in a prolonged period of internecine warfare termed the 'Huri Moai' phase. Examining this culture historical framework and evidence for warfare and monument destruction, we find a lack of support in archaeological or historical records for a pre-contact Huri Moai phase. Overall, these findings highlight how archaeologists must carefully evaluate assumptions about the prevalence of violence and war in the past given the evidence for each case. In the case of Rapa Nui, our prior understanding of the island's culture history is in need of fundamental revision.
... However, relative to the chimpanzee model, human intergroup relations simultaneously entail greater risk taking in violence and also broad cooperation, ranging from deadly battles and revenge killing to relationships of intergroup trade and alliance (Wrangham and Glowacki, 2012). Taken together, this in part suggests that an evolutionary biological explanation for human warfare should be complimented with cultural evolutionary dynamics that may help to explain the incentive structures that facilitate risky violence and intergroup alliance in humans (Glowacki and Wrangham, 2013a;Glowacki, Wilson and Wrangham, 2017). ...
... In this sense, Kelly implicitly argues that warfare cannot be merely opportunistic. In contrast, long chronology scholars allow warfare to target individuals, to occur within kin groups, and do not require that warfare must demonstrate a particular motive or set of motives (Ember and Ember, 1992;LeBlanc and Register, 2003;Gat, 2006;Allen and Jones, 2014;Lopez, 2016b;Glowacki, Wilson and Wrangham, 2017;Hames, 2019). ...
... We have an increasingly fine-grained perspective on what ancestral coalitional aggression might have looked like. In addition to the clear reproductive benefits of raiding (notwithstanding an ambiguous case for battles), it is likely that ancestral collective violence often contained at least some of the following behavioural and motivational attributes: territorial stakes; occurring by ambush of vulnerable individuals; risky and often involving weapons; involving the exploitation of defensive locations; causing high death rates between groups; likely to involve or implicate inter-group alliances; most commonly motivated by revenge or women (Keeley, 1996;Kelly, 2000;LeBlanc and Register, 2003;Otterbein, 2004;Gat, 2006;Allen and Jones, 2014;Glowacki, Wilson and Wrangham, 2017). ...
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Introduction: The evolution and history of warfare has been investigated by philosophers, historians, practitioners, social scientists and life scientists. Common questions in this endeavour are: How far back into human evolution and history do we find evidence of warfare? How frequent was warfare in any given historical period? How lethal was warfare? In short, scholarship on the evolution and history of warfare has focused on questions of origins, frequency, and intensity. Despite the fact that scientific interest in these questions is perhaps broader and more methodologically sophisticated than ever, consensus on these questions remains elusive for at least two reasons. First, the archaeological record of warfare is incomplete. Second, we do not agree on what warfare is or how to unambiguously distinguish it from other forms of violence. Beyond an agreement that warfare is something more than violence between two individuals, there is little consensus on the proper scope of our main unit of analysis. Given these hurdles, it would seem that an investigation into the evolutionary origins of human warfare is destined merely to perpetuate academic stalemates, in which old arguments are continuously repackaged with each new discovery of a mass grave or 'peaceful' society. Although this is a rather pessimistic view, I establish it at the forefront of this chapter since my argument will be that these hurdles (e.g. knowledge of ancestral phenomena and consensus on definitions) are not insurmountable. Entire disciplines thrive on their ability to successfully infer and model the unobserved past based on imperfect historical, geological and archaeological evidence. And the question of definitions must be placed in its proper scope-as a methodological, rather than ontological, consideration. One of the core dynamics of the evolutionary process is natural selection, which is the only force known to organise biological design-that is; natural selection builds adaptations. Given that biological adaptations are solutions to recurrent and reproductively significant problems in an organism's environment, these adaptations themselves convey some information about the environment in which they evolved. In other words, the form and function of adaptations contains information about the (socio)ecology in which they were built. Therefore, if there is an argument to be made about the ancestral frequency and intensity of warfare, we should expect that the form and function of our evolved psychology should reflect the ancestral existence of such challenges. This is a way of saying that if warfare was evolutionarily recurrent and reproductively significant for our ancestors, evidence of this fact lies in our very brains.
... Anthropologists and sociologists realized early on that leadership and followership were critical to understanding human psychology, social organization, and culture (e.g., Firth, 1927;Morgan, 1877;Mumford, 1906;Myres, 1917). Leaders are documented among every ethnographically observed society (Brown, 1991;Lewis, 1974), and in diverse contexts, such as kin groups (Dussart, 2000), ritual (Singh, 2017), economic groups (Macfarlan, Remiker, & Quinlan, 2012), conflicts between groups (Glowacki, Wilson, & Wrangham, 2017), and nonindustrial political groups (Cohen & Middleton, 1967). For review, see Garfield, von Rueden, and Hagen (2019). ...
... Leaders also play important roles in ritual and religious contexts (Singh, 2017;Winkelman, 2020), facilitating marriages (Walker, Flinn, Ellsworth, & Hill, 2011), and organizing feasts (Wiessner & Schiefenhövel, 1996). Leadership in between-group conflict and cooperation is also common across nonindustrial societies, including hunter-gatherers (Apicella, Marlowe, Fowler, & Christakis, 2012;Glowacki et al., 2017;Hames, 2019;Richerson et al., 2016). ...
... Pastoralists and coastal populations often face similar pressures concerning grazing lands and fishing access (e.g., Stevens, 1990;Widmer, 1988). Defensibility of resources creates increased opportunities for resource management and is often associated with increased territoriality and inter-group conflicts (Glowacki et al., 2017). Community leaders among populations more reliant on domesticated plant foods, livestock, and specific territories are often required to manage military operations (Lopez, 2017). ...
Article
Many researchers have turned to evolutionary theory to better understand diversity in leadership. Evolutionary theories of leadership, in turn, draw on ethnographic cases of societies thought to more closely resemble the smaller-scale, face-to-face communities in which humans evolved. Currently, though, there is limited systematic data on the nature of leadership in such societies. We coded 109 dimensions of leadership, including costs and benefits relevant to evolutionary models, in 1212 ethnographic texts from 59 mostly nonindustrial populations in Human Relations Area Files (HRAF). We discovered evidence for both cultural universals in leadership, as well as important variation by continental region, subsistence strategy, group context, and leader sex. Candidate universals included that leaders were intelligent and knowledgeable, resolved conflicts, and received material and social benefits. Evidence for other leader dimensions varied by group context (e.g., there was more evidence that leaders of kin groups were older and tended to provide counsel and direction), subsistence (e.g., hunter-gatherers tended to lack leaders with coercive authority), and sex (e.g., female leaders tended to be associated with family contexts). There was generally more evidence of benefits than costs for both leaders and followers, with material, social, and mating benefits being particularly important for leaders, and material and other benefits important for followers. Shamans emerged as an important category of leaders who did not clearly conform to influential models that emphasize two leader strategies: using knowledge and expertise to provide benefits to followers vs. using physical formidability to impose costs. Instead, shamans and other leaders with supernatural abilities used their knowledge to both provide benefits and impose costs on others. We therefore propose a modified scheme in which leaders deploy their cognitive, social, material, and somatic capital to provide benefits and/or impose costs on others.
... In addition to paleo-archaeological evidence, the main lines of evidence for such inference are comparisons with nonhuman primates, comparisons with modern hunter-gatherer groups believed to approximate ancestral conditions (Boehm, 2011;Glowacki, Wilson, & Wrangham, 2017;Kissel & Kim, 2018;Lopez, 2016), along with statistical and game theoretic estimations of ancestral regularities (Bowles, 2009;Gómez et al., 2016). Lastly, and crucially, to the extent that the character of biological design reveals information about the world in which it evolved, we can also examine evidence of psychological design itself as a useful source of information for drawing inferences about the character of ancestral environments (Barkow, Cosmides, & Tooby, 1992;Cosmides & Tooby, 1987;Mayr, 1983;Williams, 1966). ...
... 2015). However, somewhat problematically, the term "collective action" sometimes refers narrowly to public goods dilemmas, in which the benefits of n-person action are non-excludable and non-rival (Glowacki et al., 2017;Jordan, Jordan, & Rand, 2017), but at other times collective action is referred to more generally as any form of n-person exchange (Pietraszewski, 2016;Tooby, Cosmides, & Price, 2006). It is not my contention that public goods are necessarily always at stake wherever there is coalitional aggression; indeed, the analysis that follows is premised upon growing evidence that this is not the case. ...
Article
Warfare is a collective action problem, and groups often stand to benefit from the quick and coordinated action that leaders can provide. This basic principle is as true in modern political contexts as it has been across our evolutionary history, and there is growing evidence that leadership has evolved, in part, to solve such collective action problems. Despite the material and reproductive benefits of leadership for groups, leaders may also seek private gains at the expense of group interests. Drawing upon insights from social and evolutionary psychology, I explain how leaders solve collective action problems in warfare, but also how leaders manipulate audience preferences when their own interests do not align with group interests. Specifically, when leaders anticipate great private gain from foreign aggression while facing steep public resistance at home, leaders will misframe the conflict as defensive rather than offensive in nature. I provide an evolutionary analysis that explains why leaders exploit this framing specifically, and I identify the specific aspects of conflict framing that are most likely to be exploited toward this end.
... While much of the competition between groups is indirect (e.g. via scent marking behaviour or vocal exchanges), some species directly engage in physical aggression between groups. Although intergroup conflict was once thought to be rare outside of humans [1], violence between groups has now been described across a wide range of taxa including social insects [2][3][4], birds [5,6] and mammals [7][8][9][10][11]. These studies have provided an important comparative perspective on human warfare [12], which has been argued to have been frequent in our evolutionary history [1,13] and an important force in human social evolution [14,15] (although see [16]). ...
... Although intergroup conflict was once thought to be rare outside of humans [1], violence between groups has now been described across a wide range of taxa including social insects [2][3][4], birds [5,6] and mammals [7][8][9][10][11]. These studies have provided an important comparative perspective on human warfare [12], which has been argued to have been frequent in our evolutionary history [1,13] and an important force in human social evolution [14,15] (although see [16]). ...
Article
Violent conflicts between groups have been observed among many species of group living mammals and can have important fitness consequences, with individuals being injured or killed and with losing groups surrendering territory. Here, we explore between-group conflict among meerkats (Suricata suricatta), a highly social and cooperatively breeding mongoose. We show that interactions between meerkat groups are frequently aggressive and sometimes escalate to fighting and lethal violence and that these interactions have consequences for group territories, with losing groups moving to sleeping burrows closer to the centre of their territories following an intergroup interaction and with winning groups moving further away. We find that larger groups and groups with pups are significantly more likely to win contests, but that the location of the contest, adult sex ratio, and mean within-group genetic relatedness do not predict contest outcome. Our results suggest that intergroup competition may be a major selective force among meerkats, reinforcing the success of large groups and increasing the vulnerability of small groups to extinction. The presence of both within-group cooperation and between-group hostility in meerkats make them a valuable point of comparison in attempts to understand the ecological and evolutionary roots of human warfare.
... Recent research also explores the coevolution of inter-group violence with group cooperation, altruism, and the emergence of complex social systems [16,18,19]. Those that propose inter-group violence to be a rare or maladaptive behavior [20][21][22] have been effectively countered in many cases [23][24][25][26], although important debate persists [7]. ...
... Third, as stated above, the 'resource procurement' hypothesis implies the capture or defense of group-level rewards, such as clustered resources or territory, that elicit collective action problems [25,32]. Recent studies have suggested that collective action problems can be overcome when participation in inter-group violence serves to accumulate resource wealth that can be translated into fitness rewards, such as marriage opportunities [12] or status [11], suggesting resource procurement may be a proximate means of gaining social rewards rather than alleviating resource shortages. ...
Article
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Inter-personal violence (whether intra- or inter-group) is a pervasive yet highly variable human behavior. Evolutionary anthropologists suggest that the abundance and distribution of resources play an important role in influencing differences in rates of violence, with implications for how resource conditions structure adaptive payoffs. Here, we assess whether differences in large-scale ecological conditions explain variability in levels of inter-personal human violence. Model results reveal a significant relationship between resource conditions and violence that is mediated by subsistence economy. Specifically, we find that interpersonal violence is highest: (1) among foragers and mixed forager/farmers (horticulturalists) in productive, homogeneous environments, and (2) among agriculturalists in unproductive, heterogeneous environments. We argue that the trend reversal between foragers and agriculturalists represents differing competitive pathways to enhanced reproductive success. These alternative pathways may be driven by features of subsistence (i.e., surplus, storage, mobility, privatization), in which foragers use violence to directly acquire fitness-linked social payoffs (i.e., status, mating opportunities, alliances), and agriculturalists use violence to acquire material resources that can be transformed into social payoffs. We suggest that as societies transition from immediate return economies (e.g., foragers) to delayed return economies (e.g., agriculturalists) material resources become an increasingly important adaptive payoff for inter-personal, especially inter-group, violence.
... This sexual dimorphism suggests that when physical formidability is a desirable trait, males are greatly advantaged over females. Sixth, evidence suggests that throughout history males have been more likely to serve as combatants in wars and other intergroup conflict than females (Keegan, 1993;Goldstein, 2003;Glowacki et al., 2017). This is consistent with research that indicates male leaders are strongly preferred over female leaders and more successfully raise group investment than female leaders during intergroup competition (Van Vugt and Spisak, 2008). ...
... This is consistent with research that indicates male leaders are strongly preferred over female leaders and more successfully raise group investment than female leaders during intergroup competition (Van Vugt and Spisak, 2008). It is also consistent with research that shows groups with greater numbers of males are more likely to win intergroup contests (Glowacki et al., 2017). This review suggests: ...
Article
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Females constitute a far smaller proportion of political leaders than their proportion in the general population. Leading demand- and supply side explanations for this phenomenon account for some of the variance but leave a great deal unexplained. In an effort to account for additional variance, this research evaluates the issue informed by the biological theory of evolution by natural selection, a foundational explanation for the diversity and function of living organisms. It experimentally assesses how varying types of inter- and intragroup threat–a recurring ancestral problem–affect demand for female and male national leaders. This work analyzes data collected from individuals (N = 826) in the U.S. during the 2012 Cooperative Congressional Election Study. The results suggest the predominant preference for male over female leaders in some contexts may be the non-adaptive and non-functional but lingering outcome of an adaptive preference for physically formidable allies that was shaped by natural selection in ancestral environments.
... Current perspectives highlight the need for additional research on conflict resolution and leadership across cultures and contexts. Moreover, the cultural and social diversity of within-group conflict resolution is not well documented, given much of the evolutionary social science on conflict has focused on warfare and between-group conflict (e.g., Glowacki, Wilson, and Wrangham 2017;Sluka 1992;Lopez 2020). ...
... These results are consistent with recent work in anthropology emphasizing heterogeneity, variability, and social complexity among populations such as mobile huntergatherers, often characterized as "small-scale" (e.g., Hill et al. 2011;Bird et al. 2019;Singh and Glowacki 2021). Between-group conflicts, although relatively less represented, are consistently documented across all subsistence types including hunter-gatherers, providing additional evidence for the ubiquity of inter-group conflict across the vast majority of human societies (Glowacki, Wilson, and Wrangham 2020;Hames 2019;Lopez 2016). ...
Article
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Conflicts are ubiquitous between individuals as well as between groups. Effective conflict resolution is essential for individual well-being and group functioning and often involves leadership dynamics. The evolutionary human sciences have suggested conflict resolution is shaped by psychological heuristics, norms, and ecology. There are limited empirical data, however, on conflict resolution across cultures. Using a cross-cultural database of 109 leadership dimensions coded from over 1,200 text records from the eHRAF ethnographic database, exploratory analyses investigated correlates of conflict resolution. Results revealed greater evidence of conflict resolution among kin groups than political groups and greater evidence of within-group conflict resolution than between-group, which did not vary across subsistence strategies or group contexts, with two exceptions – military group conflicts were biased towards between-group contexts and religious groups biased towards within-group contexts. The strongest predictors of conflict resolution services were other prosocial functions and included group representation and providing counsel, protection, and punishment, as well as qualities of interpersonal skills and fairness. Followers received social service benefits and reduced risk of harm. For leaders who resolve conflicts, status and social benefits were potential negative predictors. These results provide a comparative view of the correlates of conflict resolution suggesting diversity across social contexts.
... In contrast, the long chronology accepts the chimpanzee model of low-cost/highbenefit opportunistic coalitional killing as perhaps the most basic form of warfare, upon which humans slowly and over time built greater complexity. In other words, we have been engaging in coalitional violence at least since the split with chimpanzees (Wrangham 1999;Gat 2006;Lopez 2016b;McDonald, Navarrete, and Van Vugt 2012;Glowacki, Wilson, and Wrangham 2017). Both short and long chronologies recognize that developments in the early Holocene brought significant changes to human intergroup violence, but whereas the short chronology sees this as the beginning of "warfare" per se, the long chronology sees it as a period of significant transition in the scale and complexity of warfare (Keeley 1996;Lopez 2019a). ...
... The collective action problem of coordinated violence. Despite significant disagreement regarding the origins and frequency of warfare in our species' history, most agree that at the heart of the puzzle of warfare lies a within-group collective action problem (Kitchen and Beehner 2007;Crofoot and Gilby 2012;Pietraszewski 2016;Glowacki, Wilson, and Wrangham 2017;Lopez 2019b;Wiessner 2019). Large, complex, sedentary societies are often able to solve these problems through the use of coercive institutions, while relatively smaller nomadic societies tend to solve these problems via the implicit or explicit distribution of material rewards or the leveraging of personal reputation and status. ...
Chapter
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The scientific study of the evolution of human coalitional aggression has exploded over the last three decades. In four parts, I explore and integrate many of the useful frameworks that have emerged to describe and explain the human practice of intergroup violence. First, we have a clearer understanding of the general conditions required for the evolution of adaptations for coalitional aggression. Second, given an understanding of these conditions, we can more usefully examine the historic and prehistoric record for evidence of the existence of these conditions. Third, I explore and integrate current lab and field evidence for psychological adaptations for coalitional aggression. This section reveals a core dynamic underlying all forms of coalitional aggression: the form of intergroup engagement is functionally linked with the emergent patterns of intragroup dynamics. In other words, how we fight "abroad" determines how we cooperate "at home," and vice versa. Here I examine five areas of inquiry that suggest special design for coalitional aggression. These are: the collective action problem of coordinated violence; parochial altruism; attacker-defender asymmetries; leader-follower dynamics; sex differences in the costs and benefits of violence. Fourth, and to conclude, I offer speculation on the historical emergence of modern human warfare. I do not use "coalitional aggression" and "warfare" interchangeably; rather, evolved psychological adaptations for small-scale coalitional aggression are what make the historical emergence of large-scale human warfare possible.
... 3 In the last few decades, the evolutionary bases of warfare (see Table 1 for definition of key terms) and its role in human evolution have been hotly debated by scientists. Since a comprehensive review of the literature on warfare is beyond the scope of this manuscript (for in-depth reviews see References 1,5,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] here I briefly present the debate over the evolutionary bases of warfare and focus on some of the main points of the controversy in the following sections. There is no agreement on how we define warfare; some definitions mention the use of weapons, some focus on the intention/occurrence of killing and others on the importance of organized/cooperative actions among members of one social group against members of the opposing group. ...
... Reviewing these advancements and hypotheses is beyond the scope of this manuscript (for in-depth reviews see References 1, [10][11][12][13][14][15][16][17][18][19][20][21][22]24). From a comparative perspective, one key question to address is whether or to what extent group members cooperate during aggressive interactions with other groups. ...
Article
The importance of warfare for human evolution is hotly debated in anthropology. Some authors hypothesize that warfare emerged at least 200,000–100,000 years BP, was frequent, and significantly shaped human social evolution. Other authors claim that warfare is a recent phenomenon, linked to the emergence of agriculture, and mostly explained by cultural rather than evolutionary forces. Here I highlight and critically evaluate six controversial points on the evolutionary bases of warfare. I argue that cultural and evolutionary explanations on the emergence of warfare are not alternative but analyze biological diversity at two distinct levels. An evolved propensity to act aggressively toward outgroup individuals may emerge irrespective of whether warfare appeared early/late during human evolution. Finally, I argue that lethal violence and aggression toward outgroup individuals are two linked but distinct phenomena, and that war and peace are complementary and should not always be treated as two mutually exclusive behavioral responses.
... La violencia interpersonal ha caracterizado al ser humano en todos los periodos de nuestra Historia, incluso en especies anteriores del género Homo. En el caso de los chimpancés, se han observado comportamientos violentos, en ocasiones de forma grupal y organizada (Goodall, 1988;Kelly, 2005;Glowacki et al., 2017;Kissel & Kim, 2018). Por tanto, nuestros ancestros más distantes en el tiempo también habrían sido capaces de matar a otros individuos, en ataques individuales o grupales. ...
... "Existe una violencia biológica entre animales de una misma especie y ésta se pone de manifiesto de manera álgida en los conflictos sexuales o alimentarios" (Guilaine & Zammit, 2002: 39). Algunos autores han sugerido que la violencia organizada podría haber sido parte del comportamiento humano desde momentos mucho anteriores al Holoceno, incluso jugando un papel importante en la evolución biológica de los homínidos (Wrangham, 1999;Glowacki et al., 2017). Una vez que el género Homo alcanzó una capacidad cognitiva suficiente, formas complejas de comunicación y una organización social sofisticada habría sido capaz de llevar a cabo actos de violencia cooperativa hacia los individuos considerados como "otros" o "extraños", constituyendo el origen de la violencia interpersonal, e incluso del concepto de "guerra" (Kissel & Kim, 2018). ...
Thesis
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[Master's Thesis] The site of Cova Foradada (Calafell, Tarragona) presents two differentiable accumulations of human remains corresponding to the use of this cavity as a sepulchral cave during recent Prehistoric times. A total of 1332 unburned osteological remains and 1424 burned remains have been analysed from the perspective of Forensic Taphonomy, consisting in a combination of taphonomic, forensic anthropological and paleopathological analysis. The main objective of this study is to reconstruct the taphonomic history of the assemblage, from an approximation to the lifestyle of the individuals to their state of health, the cause of death, funerary treatment received and the formation and subsequent alteration of the site. Between the two accumulations, a total of 23 individuals of all ages have been calculated. All of them received the same funerary treatments, consisting in the cremation of the bodies or the collective burial. While palaeopathological analysis was unable to reveal any serious global health issues, the frequency of prehistoric interpersonal violent episodes is an important feature presented within this site. These intentionally produced traumas have been identified as the cause of death for some individuals. Further taphonomic data reveals a multitude of different agents to have intervened in the postdepositional alteration of this accumulation, including anthropic activities, and the reuse of the cavity in distinct moments of time, throughout the Late Neolithic – Chalcolithic and towards the beginning of Late Bronze Age. There is, therefore, a repeated use of this cavity with the same sepulchral conception by different funerary practices: inhumation and cremation.
... Looking at war across human history and cultures, it is, therefore, justified to view warfare as 'a strategy by which coalitions of males (our emphasis) cooperate to acquire and defend resources necessary for reproduction' [5, p. 963]. Even though warfare may have shaped key aspects of human psychology [6,7], including fundamental gender differences in social psychology [8], the underlying evolutionary origins of this gender bias in participation in intergroup violence has always been taken for granted, and comparative perspectives have rarely extended beyond chimpanzees [5,6]. Here, we offer a broader comparative perspective that examines the nature and potential drivers of sex biases in participation in intergroup conflict across diverse mammalian societies [9]. ...
... Our findings are consistent with the notion that male bias in participation in intergroup conflict (as seen in human warfare) has deep evolutionary roots within the mammalian lineage. Men are generally the main participants in warfare in both small-scale societies and large, industrialized societies [5,142] and the 'male warrior hypothesis' [8,143,162] was thus proposed to explain male-dominated initiation, planning and participation in intergroup aggression and its effect on psychology [40,54]. On average, men are more prejudiced against and openly hostile towards members of outgroups, for example, immigrant groups in society, and outgroup men are more likely to be the target of intergroup prejudice [8]. ...
Article
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Intergroup conflict is a major evolutionary force shaping animal and human societies. Males and females should, on average, experience different costs and benefits for participating in collective action. Specifically, among mammals, male fitness is generally limited by access to mates whereas females are limited by access to food and safety. Here we analyse sex biases among 72 species of group-living mammals in two contexts: intergroup conflict and collective movements. Our comparative phylogenetic analyses show that the modal mammalian pattern is male-biased participation in intergroup conflict and female-biased leadership in collective movements. However, the probability of male-biased participation in intergroup conflicts decreased and female-biased participation increased with female-biased leadership in movements. Thus, female-biased participation in intergroup conflict only emerged in species with female-biased leadership in collective movements, such as in spotted hyenas and some lemurs. Sex differences are probably attributable to costs and benefits of participating in collective movements (e.g. towards food, water, safety) and intergroup conflict (e.g. access to mates or resources, risk of injury). Our comparative review offers new insights into the factors shaping sex bias in leadership across social mammals and is consistent with the ‘male warrior hypothesis' which posits evolved sex differences in human intergroup psychology. This article is part of the theme issue ‘Intergroup conflict across taxa’.
... Such participation is clearly strategic and responsive to the fitness benefits of collective action.784In particular, as emphasized byGlowacki et al. (2017), "warfare is a strategy by which coalitions of 785 males cooperate to acquire and defend resources necessary for reproduction. This strategy is not the 786 result of a single 'instinct' for war, but is instead an emergent property resulting from evolved psy-787 chological mechanisms (such as xenophobia and parochial altruism). ...
... Humans exhibit complex and flexible inter-group relationships. Groups, or individuals from different groups, are sometimes strongly hostile to each other, even engaging in "warfare," but may also participate in intimate and cooperative relationships (Hill, Barton, & Hurtado, 2009;Glowacki, Wilson, & Wrangham, 2017;Kissel & Kim, 2019). The understanding of the differences in inter-group relationships and underlying factors, which produce these differences in the two Pan species, would provide crucial information leading to explanation of the evolution of inter-group relationships in humans. ...
Article
Objectives: Although conflicts between groups over valuable resources are common in the animal kingdom, an individual's strategy toward out-group individuals may differ according to the benefits and costs received from inter-group interactions. Groups of bonobos encounter each other frequently and may mingle and range together from a few hours to a few days. During these inter-group associations, individuals across groups exhibit both aggressive and affiliative interactions. This study aimed to examine the strategies that bonobos employ with other groups, by comparing the patterns of within- and inter-group aggression. Materials and methods: We observed the aggressive interactions within a group of wild bonobos and between the group and three neighboring groups in Wamba, Luo Scientific Reserve, DR Congo. Results: Bonobos increased the level of cooperation to attack out-group individuals more than they do to attack within-group individuals. Additionally, they reduced the aggression between within-group members during inter-group associations, compared to that when not associated with other groups. Males selectively and cooperatively attacked out-group males. Inter-group aggression among females was rare. Furthermore, females sometimes formed coalitions with out-group individuals to attack a common target. Discussion: Our results support the hypothesis that inter-group competition occurs in bonobos, with males across groups competing over mates. Females across groups were tolerant and even cooperative with each other. Regardless of the ideal male strategy, female tolerant and cooperative relationships across groups and female within-group superiority over males could preserve tolerant inter-group relationships in bonobos.
... However, the possibility that warfare, through its effect on demography, might shape human social behaviours more widely than just those connected with success in war has been largely neglected. Warfare can affect with whom individuals mate and whether or not they reproduce at all (Glowacki et al. 2017), thus possibly altering patterns of relatedness and competition. Patterns of relatedness and competition, in turn, are known to modulate incentives to perform social behaviours (Frank 1998) and, in this way, warfare demographies could influence other forms of within-group altruismnot just those that result in greater success in battle and that have been the focus of mathematical analyses so far (also Bowles 2006Bowles , 2009Choi and Bowles 2007;Garcia and van der Bergh 2011;Lehmann and Feldman 2008;Micheletti et al. 2017Micheletti et al. , 2018. ...
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Recent years have seen great interest in the suggestion that between-group aggression and within-group altruism have coevolved. However, these efforts have neglected the possibility that warfare – via its impact on demography – might influence human social behaviours more widely, not just those directly connected to success in war. Moreover, the potential for sex differences in the demography of warfare to translate into sex differences in social behaviour more generally has remained unexplored. Here, we develop a kin-selection model of altruism performed by men and women for the benefit of their groupmates in a population experiencing intergroup conflict. We find that warfare can promote altruistic, helping behaviours as the additional reproductive opportunities winners obtain in defeated groups decrease harmful competition between kin. Furthermore, we find that sex can be a crucial modulator of altruism, with there being a tendency for the sex that competes more intensely with relatives to behave more altruistically and for the sex that competes more intensely with non-relatives in defeated groups to receive more altruism. In addition, there is also a tendency for the less-dispersing sex to both give and receive more altruism. We discuss implications for our understanding of observed sex differences in cooperation in human societies.
... However, despite evidence of this behavioral flexibility, much of the existing literature on intergroup behavior in primates emphasizes the release of selection pressures favoring aggression (e.g., the Dear Enemy Effect 16 ), which allows for either "random" 17 or tolerant encounters ( Figure 1); for example, other reviews have provided thorough treatment of the selection pressures favoring (or disfavoring) aggressive intergroup behavior in nonhuman primates and in humans. 3,5,15,18 Our approach differs in that we focus on individual-level selection pressures that, given selection pressures disfavoring intergroup aggression, favor intergroup encounter and association over random encounter. When incentives for contest with extragroup conspecifics are low, optimality theory would predict that (a) if there are low benefits to encounter, an individual should randomly encounter extra-group conspecifics 17 (d, Figure 1) and (b) if there are high benefits to encounter, an individual should encounter extra-group conspecifics at a rate higher than chance (b, Figure 1). ...
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Primate individuals use a variety of strategies in intergroup encounters, from aggression to tolerance; however, recent focus on the evolution of either warfare or peace has come at the cost of characterizing this variability. We identify evolutionary advantages that may incentivize tolerance toward extra‐group individuals in humans and nonhuman primates, including enhanced benefits in the domains of transfer, mating, and food acquisition. We highlight the role these factors play in the flexibility of gorilla, chimpanzee, bonobo, and human behavior. Given humans have an especially broad range of intergroup behavior, we explore how the human foraging ecology, especially large spatial and temporal fluctuations in resource availability, may have selected for a greater reliance on tolerant between‐community relationships—relationships reinforced by status acquisition and cultural institutions. We conclude by urging careful, theoretically motivated study of behavioral flexibility in intergroup encounters in humans and the nonhuman great apes.
... First, although attackers can use shock weapons to inflict blunt trauma during hand-to-hand combat, projectiles, such as spears, arrows, and darts, enable raiders to injure or kill their rivals from a safe distance (Keeley, 1997). Second, raiders tend to choose solitary victims or smaller groups that are unlikely to successfully fend off an attack (Glowacki, Wilson, & Wrangham, 2017). Third, if competing groups share an ethnolinguistic background and maintain a minimum level of open communication (as in lowland Amazonian societies, in which the frequency of treachery tactics was 9.5 times higher during internal conflicts relative to external clashes), the killing party could host a meeting or feast with the sole intention of eliminating their rivals (Walker & Bailey, 2013). ...
Chapter
This chapter will provide a synthesis of the current evolutionary literature concerning lethal coalitional aggression in small-scale societies. Attacks, raids, skirmishes, ambushes, and other forms of intergroup aggression present significant risk of injury or death, irrespective of group size, though the means of differentiation among groups, like the mechanisms of ensuring coordination within groups, change as a function of group size. Human coalitional violence is often explained via kin selection and reciprocal altruism, such that an individual’s assumption of risk is compensated by fitness-enhancing benefits to relatives and allies. These explanations become increasingly inapplicable in progressing from bands and tribes to chiefdoms and states. The growth of larger social aggregations compelled the emergence of institutions enforcing intragroup cooperation above and beyond the effects of underlying social networks based on kinship and direct reciprocity. Perspectives reviewed herein, such as cultural group selection, consider the cultural evolution of such institutions in generating between-group variance and facilitating lethal intergroup competition. According to these theories, cultural transmission, group differentiation, symbolic ornamentation, punishment of defectors, and ethnocentrism are integral components of intergroup competition, with lethal coalitional aggression being an extreme manifestation of between-group rivalry. Furthermore, due to the significant fitness costs imposed upon defeated factions, the study of lethal coalitional aggression in small-scale societies provides fertile ground for examining the interaction between group-level and individual-level selective pressures.
... These egalitarian leaders did not have any special authority or power to enforce their opinion or will on others (Boehm, 1999;Glowacki & von Rueden, 2015;Smith et al., 2016). However, these groups also faced frequent inter-group competition for scarce resources, escalating the level of coordination and cooperation challenges (Bowles, 2009;Gat, 2006;Glowacki, Wilson, & Wrangham, 2017;Keeley, 1997). Under such situations a second form of leadership, known as authoritarian leadership, emerged to help a group address its challenges (Chagnon, 1997;Earle, 1997;Lowie, 1948;Meggitt, 1967;Price, 1981). ...
... Since unequal division of costs and benefits is also a feature of complex warfare in humans (59), it is possible that similar processes could contribute to the destructive nature of human warfare. Consequently, while there are many ecological and social factors that may have shaped patterns of human intergroup violence [including kinship, subsistence strategy, military technology, and systems of political organization (11,60,61)], our findings highlight the value of exploring when and how leaders may become decoupled from the costs that they incite. ...
Article
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Collective conflicts among humans are widespread, although often highly destructive. A classic explanation for the prevalence of such warfare in some human societies is leadership by self-serving individuals that reap the benefits of conflict while other members of society pay the costs. Here, we show that leadership of this kind can also explain the evolution of collective violence in certain animal societies. We first extend the classic hawk−dove model of the evolution of animal aggression to consider cases in which a subset of individuals within each group may initiate fights in which all group members become involved. We show that leadership of this kind, when combined with inequalities in the payoffs of fighting, can lead to the evolution of severe intergroup aggression, with negative consequences for population mean fitness. We test our model using long-term data from wild banded mongooses, a species characterized by frequent intergroup conflicts that have very different fitness consequences for male and female group members. The data show that aggressive encounters between groups are initiated by females, who gain fitness benefits from mating with extragroup males in the midst of battle, whereas the costs of fighting are borne chiefly by males. In line with the model predictions, the result is unusually severe levels of intergroup violence. Our findings suggest that the decoupling of leaders from the costs that they incite amplifies the destructive nature of intergroup conflict.
... Chimpanzees and other organisms have been suggested to engage in warfare and lethal conspecific violence (Gomez et al. 2016). But within historical times, no other species has employed war on the scale, complexity, or lethality as humans (Glowacki, Wilson, and Wrangham 2017;Majolo 2019;Wrangham 1999;Wrangham and Peterson 1996). Indeed, modern human warfare threatens not only all people but the viability of all life on the entire planet (Ehrlich et al. 1983). ...
... Evolutionary anthropologists have used a variety of approaches and methods to understand the prevalence of intergroup conflict in the face of such costs (reviewed in ref. 8). For example, behavioral ecologists highlight the importance of reproductive benefits that can be acquired by members of successful groups (9). ...
Article
Conflict between groups of individuals is a prevalent feature in human societies. A common theoretical explanation for intergroup conflict is that it provides benefits to individuals within groups in the form of reproduction-enhancing resources, such as food, territory, or mates. However, it is not always the case that conflict results from resource scarcity. Here, we show that intergroup conflict can evolve, despite not providing any benefits to individuals or their groups. The mechanism underlying this process is acculturation: the adoption, through coercion or imitation, of the victor’s cultural traits. Acculturation acts as a cultural driver (in analogy to meiotic drivers) favoring the transmission of conflict, despite a potential cost to both the host group as a whole and to individuals in that group. We illustrate this process with a two-level model incorporating state-dependent event rates and evolving traits for both individuals and groups. Individuals can become “warriors” who specialize in intergroup conflicts, but are costly otherwise. Additionally, groups are characterized by cultural traits, such as their tendency to engage in conflict with other groups and their tendency for acculturation. We show that, if groups engage in conflicts, group selection will favor the production of warriors. Then, we show that group engagement can evolve if it is associated with acculturation. Finally, we study the coevolution of engagement and acculturation. Our model shows that horizontal transmission of culture between interacting groups can act as a cultural driver and lead to the maintenance of costly behaviors by both individuals and groups.
... Elders, on the other hand, may benefit less from status than they would from access to new trading partners, creating a dynamic where elders and youth are often in opposition to each other. Variation in the payoffs of war and peace contribute to large age-and gender-based differentials in war participation (Glowacki, Wilson, and Wrangham 2017). ...
Article
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Drawing on data from the Enga of Papua New Guinea, I (1) compare the challenges in organizing collective action for warfare in small-scale societies with those for peacemaking; (2) identify the many different channels of appeal that are interwoven to elicit the cooperation of individuals with different agendas: the rational or pragmatic, the emotional and the ritual; (3) propose that warfare is a dynamic process involving continual change in response to internal group conflicts of interest that generate new institutions, rules, and morals to facilitate collective action; and (4) show how the juxtaposition of war and active peacemaking is an effective strategy for building social complexity. This raises the question of why active peacemaking was relatively rare in small-scale societies or if it indeed was in the past.
... Arkush & Tung, 2013;Devakumar et al., 2014;Gat, 2008;Glowacki et al., 2017;Gómez et al., 2016;Human security report, 2005;Keeley, 1996;Otterbein, 1999;Pinker, 2011;Walker, 2001;Walker & Bailey, 2013;Wrangham, 1999;Wrangham et al., 2006). Despite the significance of violence, our picture of conflict throughout the human career remains highly fragmented. ...
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Objectives This study uses osteological and radiocarbon datasets combined with formal quantitative analyses to test hypotheses concerning the character of conflict in the Nasca highlands during the Late Intermediate Period (LIP, 950–1450 C.E.). We develop and test osteological expectations regarding what patterns should be observed if violence was characterized by intragroup violence, ritual conflict, intermittent raiding, or internecine warfare. Materials and methods Crania (n = 267) were examined for antemortem and perimortem, overkill, and critical trauma. All age groups and both sexes are represented in the sample. One hundred twenty‐four crania were AMS dated, allowing a detailed analysis of diachronic patterns in violence among various demographic groups. Results Thirty‐eight percent (102/267) of crania exhibit some form of cranial trauma, a significant increase from the preceding Middle Horizon era. There are distinct trauma frequencies within the three subphases of the LIP, but Phase III (1300–1450 C.E.) exhibits the highest frequencies of all trauma types. Males exhibit significantly more antemortem trauma than females, but both exhibit similar perimortem trauma rates. Discussion There was chronic, internecine warfare throughout the Late Intermediate Period with important variations in violence throughout the three temporal phases. Evidence for heterogeneity in violent mortality shows a pattern consistent with social substitutability, whereby any and all members of the Nasca highland population were appropriate targets for lethal and sublethal violence. We argue that by testing hypotheses regarding the targets and types of conflict we are better able to explain the causes and consequences of human conflict.
... The outcome of intergroup encounters among conspecifics has been shown to determine prior access to valuable food sources (banded mongoose, Mungos mungo, Thompson, Marshall, Vitikainen, & Cant, 2017; spotted hyaenas, Crocuta crocuta, Boydston, Morelli, & Holekamp, 2001;bonobos, Pan paniscus, Lucchesi et al., 2020) but also to mates (humpback whales, Megaptera novaeangliae, Clapham, Mattila, & Vasquez, 1992; red deer, Cervus elaphus, Carranza, Alvarez, & Redondo, 1990). Anticipating the potential occurrence of encounters at specific locations has strong defensibility advantages in territorial animals, which might aid in minimizing the incursion of neighbouring groups (Christensen & Radford, 2018;Glowacki, Wilson, & Wrangham, 2017). Thus, memorizing strategic locations possibly contributes to the long-term stability of home ranges' demarcation as a cognitive adaptation of intergroup competition. ...
Article
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Memories linked to specific locations provide information for animals to plan future actions and anticipate the occurrence of events. Here, we examined goal-directed travel towards locations where biologically meaningful events had previously taken place to elucidate the relative importance of past social and ecological information for route planning. We inferred goal-directed travelling by detecting long sections of straight-line travel, followed by significant directional changes along travel trajectories of five neighbouring groups of black howler monkeys, Alouatta pigra, at Palenque National Park, Mexico. Post hoc, we determined at the approached locations: (1) the behaviour of the group; (2) the occurrence of previous intergroup interactions; and (3) the ecological properties of the feeding tree (i.e. importance in diet, level of phenological synchrony). The likelihood of goal-directed travel towards a location to engage in loud calling increased after having experienced an encounter with a neighbouring group at that same location in the past. Additionally, the likelihood of goal-directed travel towards a location to forage on fruits increased when the approached tree was considered important in the diet with highly synchronous phenological cycles. Our results indicate that route planning in wild animals involves an integration of both social and ecological variables.
... For all seven of the societies where hunting disguises were used, the methods involved animal mimicry or camouflage with clothing or pigments, and this was also true for four of the five societies where disguises were used in the context of interpersonal violence (Table 3). War disguises were most commonly used when scouting or preparing for an ambush, consistent with perspectives on hunter-gatherer warfare emphasizing the common pattern of surprise attacks under the cover of secrecy (Buckner, 2019;Buckner & Glowacki, 2019;Gat, 2006;Glowacki & Wrangham, 2013;Glowacki et al., 2017;Kelly, 2005;Lopez, 2020;Otterbein, 2009;van der Dennen, 1995;Wrangham & Glowacki, 2012). Disguises consisted of grass headdresses as a masquerade among the Ojibwa; wolfskin and bearskin disguises for animal mimicry among the Blackfoot and the Tlingit, respectively; use of pigments for disguising identity or concealment (disruptive coloration) among both the Ona and the Tlingit; use of leaves and branches to conceal children when under attack (masquerade) among the Ona; and use of shoes to disguise footprints and impersonate a spirit among the Arunta. ...
Article
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Thermoregulation is often thought to be a key motivating factor behind the origins of clothing. Less attention has been given, however, to the production and use of clothing across traditional societies in contexts outside of thermoregulatory needs. Here I investigate the use of disguises, modesty coverings, and body armor among the 10 hunter-gatherer societies in the Probability Sample Files (PSF) within the Human Relations Area Files (HRAF) World Cultures database, with a particular focus on disguise cases and how they compare with strategies of deception across other taxa. The employment of disguises-defined as altering one's appearance for purposes of deceiving conspecifics or other animals-is noted for eight of the 10 societies, with their use occurring in contexts of hunting, religious or cult practices, and war or interpersonal violence. Most hunter-gatherer disguises demonstrated clear similarities to cases of visual deception found in other species, with the majority of examples fitting categories of animal mimicry, masquerading as plants, disrup-tive coloration (camouflage), or background matching (camouflage), while disguises unique to humans involved the impersonation of culture-specific "spirit-beings." Clothing for modesty purposes (nine societies) and body armor (six societies) are also noted. I propose that strategic initiatives by individuals or groups to disguise or conceal themselves represents one possible initial pathway to the cultural evolution of clothing. There are likely multiple potential (nonexclusive) social and functional pathways to the emergence of clothing outside of thermoregulatory needs.
... Evolutionary social scientists have similarly argued that coalitions and alliances can substantially enhance coalition members' abilities to physically defend and acquire valuable resources like mates, food and territory [5,[41][42][43][44][45][46]. In humans, coalitional psychology has been linked to an evolutionary history of warfare, an overwhelmingly male activity [5,[47][48][49][50][51]. Studies have indeed shown a male bias in coalitional psychology (e.g. ...
Article
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Those with better reputations often obtain more resources than those with poorer reputations. Consequently, gossip might be an evolved strategy to compete for valuable and scarce material and social resources. Influenced by models of non-human primate competition, we test the hypotheses that gossip: (i) targets aspects of reputation relevant to the domain in which the competition is occurring, (ii) increases when contested resources are more valuable, and (iii) increases when resources are scarcer. We then test hypotheses derived from informational warfare theory, which proposes that coalitions strategically collect, analyse and disseminate gossip. Specifically, we test whether: (iv) coalitions deter negative gossip, and (v) whether they increase expectations of reputational harm to competitors. Using experimental methods in a Mechanical Turk sample ( n = 600), and survey and ego network analysis methods in a sample of California sorority women ( n = 74), we found that gossip content is specific to the context of the competition; that more valuable and scarcer resources cause gossip, particularly negative gossip, to intensify; and that allies deter negative gossip and increase expectations of reputational harm to an adversary. These results support social competition theories of gossip. This article is part of the theme issue ‘The language of cooperation: reputation and honest signalling’.
... Violence between human groups has caused vast losses of lives through history and prehistory (Allen & Jones, 2014;Durrant, 2011;Glowacki et al., 2017;Kissel & Kim, 2019; but see Kelly, 2000). One of the most widespread forms of intergroup lethal and severe violence in humans is gang attacks during raids (Keeley, 1996;Otterbein, 2004). ...
Article
(Full manuscript available for free up to the 19th October 2021 here: https://authors.elsevier.com/a/1dfmfmjLwpaL ) Lethal gang attacks, in which multiple aggressors attack a single victim, are among the most widespread forms of violence between human groups. Gang attacks are also frequent in some other social mammals, such as chimpanzees, Pan troglodytes, wolves, Canis lupus, spotted hyaenas, Crocuta crocuta, and meerkats, Suricata suricatta. So far, species in which gang attacks have been observed share one or more of these socioecological features: territoriality, fission–fusion, cooperative breeding or coalitionary bonds. However, the scarcity of data in other taxa makes it challenging to determine whether one/all of these socioecological features is necessary and sufficient to drive the evolution of gang attacks. Here we describe the first reports of intergroup gang attacks in the crested macaque, using data on three groups collected over 13 years, with the joint observation times for the three groups summing to 37 years. Crested macaque gangs attacked outgroup conspecifics when aggressors were numerically superior to victims. Adult females were the most frequent age/sex category to attack outgroup conspecifics. The victims were mostly adult females and infants. We propose that coalitionary bonds, hostility towards outgroup individuals and the ability to estimate numerical odds may suffice to trigger intergroup gang attacks when the conditions favour an imbalance of power between victims and attackers.
... From economics, prosociality and parochialism are often theorized to emerge as a form of social insurance to protect personal security and property rights in the face of threats (Greif, 1989;Meier, 2007). Evolutionary theories view prosociality and parochialism as selective responses to intergroup competition that enhance group fitness (Bowles, 2006;Choi and Bowles, 2007;Rusch, 2014;Glowacki, Wilson, & Wrangham, 2020). Finally, psychological theories point to positive and negative adaptive responses to war-time experiences that are often characterized respectively as post-traumatic growth or post-traumatic stress (Tedeschi and Calhoun, 2004;Shai, 2021;Kienzler, 2008;Rusch et al, 2016;Bellucci et al., 2020). ...
Article
How does conflict affect prosocial and parochial preferences within a society? Our research considers the case of recent violence in Donbas, Ukraine where ethnic Russian separatists are battling the Ukrainian military. To evaluate social preferences, we utilize a non-costly dictator game with ethnic treatments among young ethnic Ukrainian male combatants and noncombatants in the eastern city of Kharkiv, which borders the Donbas region. At the onset of violence, we find no differences in how these men treat ethnic Russians in their local community compared to their own in-group. However, after a year of intense fighting with separatists in the nearby Donbas region, we find evidence of the erosion of fairness preferences and increased bias against ethnic Russians, especially among noncombatant civilians, underscoring how parochial responses to violence may extend beyond direct combat exposure mechanisms. Our results point to the short-term destabilizing effects of conflict on prosocial preferences with potential long-term consequences for entrenching parochial divisions.
... The origin of inter-group violence, occasionally referred to as 'warfare', has been a pivotal question in anthropology, archaeology, and other disciplines since the seventeenth century, one often involved in arguments about human nature (Hobbes [1651] 2014; Rousseau [1755] 2009). In addition to a general bioarchaeological interests in violence (e. g., Arkush and Allen 2006;Carman and Harding 1999;Cybulski 1992;LeBlanc 1999;Martin and Harrod 2012;Redfern 2016), among scholars assuming a warlike predisposition in humans, many have argued the presence of various triggers for inter-group violence, and while not mutually exclusive, a large number of such hypotheses have been proposed: a change in subsistence strategies from hunting and gathering to agriculture (e.g., Guilaine and Zammit 2004;Nolan, 2003), the development of weapons (e.g., Bingham 2000;Otterbein 2004), ecological constraints (e.g., Allen et al., 2016;Ember and Ember 1992;Hsiang et al., 2011;Keeley 1996;Otterbein 2004;Scheffran et al., 2012), phylogeny (e.g., Wrangham and Peterson, 1996;Wrangham and Glowacki 2012;Glowacki et al., 2020), population pressure (e.g., Carneiro 1970Gat 2008;Nolan 2003;Oka et al., 2017;Turchin and Korotayev 2006), social hierarchy or structure (e.g., Kelly 2000;Flannery and Marcus 2003;Zefferman and Mathew 2015), and so on. ...
Article
The causes of prehistoric inter-group violence have been a subject of long-standing debate in archaeology, anthropology, and other disciplines. Although population pressure has been considered as a major factor, due to the lack of available prehistoric data, few studies have directly examined its effect so far. In the present study, we used data on skeletal remains from the middle Yayoi period of the Japanese archipelago, where archaeologists argued that an increase of inter-group violence in this period could be explained by a population-pressure hypothesis. We quantitatively examine the effect of population pressure on the frequency of inter-group violence by compiling an exhaustive data set. We collected demographic information based on burial jars (kamekan) and the frequency of violence based on the ratio of injured individuals. The results are consistent with the hypothesis, i.e., high population density can promote inter-group violence.
... He makes the following proposal: "By making possible the belief that a supernatural entity knows the outcome of all actions and can influence such outcomes, that one's 'self' (i.e., 'soul') is not tied to one's physical body, and that if one is killed in battle, one's essential self (i.e., soul) will go to a better 'place' (e.g., heaven, Valhalla, etc.) the capacity for religious experience can tip the balance toward participation in warfare." Surveys of ancient burial sites indicate as many as 20-30% of prehistoric deaths resulted from war-related trauma, suggesting tremendous evolutionary pressures to adapt to the exigencies of intertribal conflict [42][43][44]. Anthropological accounts have repeatedly shown that shamans tend to be fixated with the intangible threat of neighboring tribes [11], and persecutory delusions commonly associated with schizophrenia may be a vestigial remnant of such adaptive paleolithic cognitive processes. One problem in the evolutionary study of behaviors is that there is often a failure to separate incidental actions from evolutionary effective activities. ...
Article
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Ever since the first detailed descriptions of shamans by pioneering anthropologists of the 19th century, researchers have pondered the similarities between shamanism and schizophrenia. Where some theorists have seen compelling parallels, others have dismissed such similarities as coincidence. This review draws upon the latest knowledge in medical genetics, evolutionary science, religious studies, psychology, anthropology and medical history, in order to catalogue all of the possible links between the institution of shamanism and the medical condition of schizophrenia. Major discrepancies between the two phenomena are also examined. It is concluded that schizophrenia could have possibly originated in Upper Paleolithic shamanism-a constitutional behavioral trait that may have once been adaptive for modern hominids.
... In human societies, women and men often differ in the structure and function of their same-sex coalitions. Men have been more likely to value, build and participate in large coalitions, often involving non-kin, in the service of intra-group coalitional competition (Low, 1992;Smuts, 1995) and intergroup warfare (McDonald et al., 2012;Glowacki et al., 2017). Several studies in industrialized societies suggest that women's same-sex coalitions tend to be smaller in size, less ostensibly hierarchical owing to greater enforcement of egalitarianism and slower to reconstitute once broken apart (Benenson, 2019;David-Barrett et al., 2015;Liesen, 2013;Vigil, 2007). ...
... The excavation at Wassenaar changed the then prevailing view of the Bronze Age as an Era of Peace [2][3][4]. Extant work in evolutionary archaeology and anthropology strongly suggests that intergroup conflict has been a constant throughout human history [5][6][7][8][9][10][11] (but see [12]). Moreover, and in parallel, it has become clear that intergroup conflict and warfare 1 are far from unique to the human species. ...
Article
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Although uniquely destructive and wasteful, intergroup conflict and warfare are not confined to humans. They are seen across a range of group-living species, from social insects, fishes and birds to mammals, including nonhuman primates. With its unique collection of theory, research and review contributions from biology, anthropology and economics, this theme issue provides novel insights into intergroup conflict across taxa. Here, we introduce and organize this theme issue on the origins and consequences of intergroup conflict. We provide a coherent framework by modelling intergroup conflicts as multi-level games of strategy in which individuals within groups cooperate to compete with (individuals in) other groups for scarce resources, such as territory, food, mating opportunities, power and influence. Within this framework, we identify cross-species mechanisms and consequences of (participating in) intergroup conflict. We conclude by highlighting crosscutting innovations in the study of intergroup conflict set forth by individual contributions. These include, among others, insights on how within-group heterogeneities and leadership relate to group conflict, how intergroup conflict shapes social organization and how climate change and environmental degradation transition intergroup relations from peaceful coexistence to violent conflict. This article is part of the theme issue ‘Intergroup conflict across taxa’.
... Past ages and the present abound with instances of such conicts. They have shaped many of the boundaries of those pieces of land we call`countries', our family trees, our institutions, our cultural memories, and maybe even some of the ways in which we think and behave [19,39,58,102,103,104]. ...
Article
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Microeconomic modelling offers a powerful formal toolbox for analysing the complexities of real-world intergroup relations and conflicts. One important class of models scrutinizes individuals’ valuations of different group memberships, attitudes towards members of different groups and preferences for resource distribution in group contexts. A second broad class uses game theoretical methods to study strategic interactions within and between groups of individuals in contest and in conflict. After a concise discussion of some essential peculiarities of microeconomic modelling, this review provides an overview of the pertinent literatures in economics, highlights instructive examples of central model types and points out several ways forward. This article is part of the theme issue ‘Intergroup conflict across taxa’.
... They help to give birth and maintain the strong interpersonal links within "fictive kinship" units. These tightly cohesive and combative cells ready to act as offensive/ defensive coalitions (Wrangham, 1999, Glowacki et al., 2020. ...
... Due to their phylogenetic proximity, studies exploring the ecological and evolutionary correlates of chimpanzee intercommunity conflict could provide researchers with a deeper understanding of human between-group aggression (Glowacki et al. 2020;Wrangham and Glowacki 2012;Wilson and Glowacki 2017). Wrangham et al. (2006) gathered data on human and chimpanzee lethal aggression. ...
... A second approach has been to not rely on vague, psychological terms at all, but rather to operationalize the concept of a group as something concrete enough that it can be modeled or studied-which typically involves describing objective behaviors among multiple agents. This formalized study of n-person dynamics has been underway for several decades now-originating from a diverse set of fields including artificial intelligence (Sandholm et al., 1999), evolutionary biology (Koykka & Wild, 2017;Rusch & Gavrilets, 2017), the evolutionary social sciences (Böhm, Rusch, & Baron, 2018;Glowacki, Wilson, & Wrangham, 2017), primatology (Harcourt & de Waal, 1992), international relations (Schelling, 1966), and economics and decision-making (Schelling, 1956). These n-person dynamics have been studied in the lab (e.g., Bornstein, 2003;Gonzalez et al., 2015;Yamagishi, Jin, & Kiyonari, 1999) and in the field (Bissonnette et al., 2015;De Dreu & Van Vianen, 2001), and have been informed by a decades-long enterprise of game theoretic and optimality analyses (e.g., Burani & Zwicker, 2003;Gamson, 1961;Mesterson-Gibbons et al., 2011;Shehory & Kraus, 1998 The problem with this second approach, however, is that these concrete implementations have not informed the issue raised by the first approach: namely, what-literally-is being represented in the mind when people are perceiving and reasoning about groups? ...
Article
We don't yet have adequate theories of what the human mind is representing when it represents a social group. Worse still, many people think we do. This mistaken belief is a consequence of the state of play: Until now, researchers have relied on their own intuitions to link up the concept social group on the one hand, and the results of particular studies or models on the other. While necessary, this reliance on intuition has been purchased at considerable cost. When looked at soberly, existing theories of social groups are either (i) literal, but not remotely adequate (such as models built atop economic games), or (ii) simply metaphorical (typically a subsumption or containment metaphor). Intuition is filling in the gaps of an explicit theory. This paper presents a computational theory of what, literally, a group representation is in the context of conflict: it is the assignment of agents to specific roles within a small number of triadic interaction types. This "mental definition" of a group paves the way for a computational theory of social groups-in that it provides a theory of what exactly the information-processing problem of representing and reasoning about a group is. For psychologists, this paper offers a different way to conceptualize and study groups, and suggests that a non-tautological definition of a social group is possible. For cognitive scientists, this paper provides a computational benchmark against which natural and artificial intelligences can be held.
... These models suggest cultural conformity and learning biases lead to the evolution of wellstructured groups and better equip groups to compete with others groups [11,74,75]. Such between-group competitive dynamics can occur through altruistic provisioning of group members or through intergroup violence [76,77] and can in turn, further support within-group cooperation [78,79]. ...
Article
Reputations are an essential feature of human sociality and the evolution of cooperation and group living. Much scholarship has focused on reputations, yet typically on a narrow range of domains (e.g. prosociality and aggressiveness), usually in isolation. Humans can develop reputations, however, from any collective information. We conducted exploratory analyses on the content, distribution and structure of reputation domain diversity across cultures, using the Human Relations Area Files ethnographic database. After coding ethnographic texts on reputations from 153 cultures, we used hierarchical modelling, cluster analysis and text analysis to provide an empirical view of reputation domains across societies. Findings suggest: (i) reputational domains vary cross-culturally, yet reputations for cultural conformity, prosociality, social status and neural capital are widespread; (ii) reputation domains are more variable for males than females; and (iii) particular reputation domains are interrelated, demonstrating a structure consistent with dimensions of human uniqueness. We label these features: cultural group unity , dominance , neural capital , sexuality , social and material success and supernatural healing . We highlight the need for future research on the evolution of cooperation and human sociality to consider a wider range of reputation domains, as well as their social, ecological and gender-specific variability. This article is part of the theme issue ‘The language of cooperation: reputation and honest signalling’.
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Parochial altruism-individual sacrifice to benefit the in-group and harm an out-group-undermines inter-group cooperation and is implicated in a plethora of politically-significant behaviors. We report new experimental findings about the impact of variation in social distance within the in-group together with variation in social distance between the in-and out-groups on parochial altruism. Building from a minimal group paradigm setup , we find that differential social distance has a systematic effect on individual choice in a setting of potential inter-group conflict. In particular, parochial altruism is stimulated when individuals' distance to both their in-and out-group is high. A long-standing finding about behavior in contexts of inter-group conflict is that low social distance facilitates collective action. Our results suggest that the effects of high social distance may create a potential additional pathway to group-based individual action. Research on inter-group conflict and collective action can be advanced by investigating such effects.
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Purpose The purpose of this paper is return to some findings and approaches typical of behavioral sciences and evolutionary anthropology that will allow us to link the process of self-domestication that can be seen in our evolutionary past, the primate tendency to enter into conflicts through patterns of signal exchange rather than direct aggressions, and the development of the persuasive dimension of language, with the possible evolutionary origin of both cultural violence and structural violence. Design/methodology/approach The approach has been, at all times, multidisciplinary insofar as it has sought to elucidate how the inquiries made from the behavioral sciences can help to understand human violence. Findings What was found is the possibility of understanding conflicts as a mechanism of evolutionary pressure that has been involved not only in social restructuring but also in the evolutionary origin of the human being. Research limitations/implications More empirical evidence should be found in this regard. Originality/value This study is a multidisciplinary approach that seeks to understand both the phenomenon of violence and peace from an evolutionary perspective.
Article
Parochial altruism – individual sacrifice to benefit the in-group and harm an out-group –undermines inter-group cooperation and is implicated in a plethora of politically-significant behaviors. We report experimental evidence about the impact of variation in individuals' distance to in-group members and to out-group members on parochial altruism in a setting in which inter-group conflict is made possible. We find that distance has systematic effects on individual choice. Parochial altruism is stimulated particularly when individuals’ distance to both their in- and out-group is high. Our results suggest that high distance may create a pathway to group-based individual action. Existing research focuses on the interaction between close-knit in-groups and proximate, threatening out-groups as the typical setting in which parochial altruism occurs. Our study indicates the need for future research to investigate how parochial altruism can occur across a wider range of in-group and out-group configurations.
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War, in human and animal societies, can be extremely costly but can also offer significant benefits to the victorious group. We might expect groups to go into battle when the potential benefits of victory ( V ) outweigh the costs of escalated conflict ( C ); however, V and C are unlikely to be distributed evenly in heterogeneous groups. For example, some leaders who make the decision to go to war may monopolize the benefits at little cost to themselves (‘exploitative’ leaders). By contrast, other leaders may willingly pay increased costs, above and beyond their share of V (‘heroic’ leaders). We investigated conflict initiation and conflict participation in an ecological model where single-leader–multiple-follower groups came into conflict over natural resources. We found that small group size, low migration rate and frequent interaction between groups increased intergroup competition and the evolution of ‘exploitative’ leadership, while converse patterns favoured increased intragroup competition and the emergence of ‘heroic’ leaders. We also found evidence of an alternative leader/follower ‘shared effort’ outcome. Parameters that favoured high contributing ‘heroic’ leaders, and low contributing followers, facilitated transitions to more peaceful outcomes. We outline and discuss the key testable predictions of our model for empiricists studying intergroup conflict in humans and animals. This article is part of the theme issue ‘Intergroup conflict across taxa’.
Chapter
This chapter is the first of five comprising Part III. Though, as in Chap. 10.1007/978-3-030-49520-6_6, we have previously allowed some lexical analyses to interpolate Part II’s historical-empirical thrust, Part III is predominately statistical, even as it continues to review relevant literature and history. Though, consistent with the mandate of this monograph, we aim ultimately to establish the reality of human group selection, this initial chapter alone treats chimpanzees. To thoroughgoing evolutionists, the relevance will be self-evident; we only add that establishing evidence of group selection in such a highly related species foundationally supports the empirical argument for human group selection, as presented in the four subsequent chapters constituting Part III of this volume.
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An extensive literature in political science shows how citizens' evaluations of politicians—as well as their electoral behavior—are affected by trait impressions of these politicians. However, deeper, interdisciplinary theory building that seeks to address when and for whom specific trait impressions come to guide candidate evaluations remains absent. In this article, I outline the theory of adaptive followership that seeks to address this shortcoming. Grounded in evolutionary psychology, I argue that leadership evolved as a solution to problems of intragroup coordination in ancestral small‐scale societies. In order to understand the traits that drive followers' and voters' evaluations of leaders and politicians, one should therefore focus on problems related to group coordination and ask how these problems might regulate followers' prioritizations of various traits in leaders. On this basis, I outline an analytical framework consisting of three predictions that simultaneously formulate how (1) contexts and (2) individual differences of relevance to a given group‐coordination problem regulate trait preferences, and (3) how such preferences differ between leaders and nonleaders (i.e., other social categories). The analytical framework is applied for structuring two reviews (including new empirical studies) of the ways through which intergroup conflict and disease threat, respectively, affect followers' trait preferences in leaders. Finally, directions and suggestions for future research on trait‐based candidates and leader evaluations are discussed.
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This article examines the evolutionary stability of other-regarding preferences in a group contest for a prize, which is endogenously determined. In a destructive contest, such as war, contest efforts of all groups decrease the value of the prize. In contrast, in a productive contest, such as a patent race, contest efforts of all groups increase the value of the prize. The indirect evolutionary approach allows to endogenize players’ preferences, that is, the utility weights given by a group member, in her subjective utility function, to the material payoffs of in-group and out-group members. After characterizing the set of evolutionarily stable preference types, I show that the evolutionarily stable degree of in-group altruism is always stronger when the group contest is destructive than when it is productive. Moreover, when the group contest is strongly productive, preference evolution leads to in-group spite. However, a smaller group size and a larger number of competing groups makes this outcome less likely.
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Primate individuals use a variety of strategies in intergroup encounters, from aggression to tolerance; however, despite the prevalence of tolerance in humans, recent focus on the evolution of intergroup contest has come at the cost of characterizing the role of tolerance in human sociality. Can we use the selection pressures hypothesized to favor tolerance in intergroup encounters in the non-great ape primates to explain the prevalence and plasticity of tolerance in humans and our closest living relatives? In the present paper, we review these candidate ecological and social factors and conclude that additional selection pressures are required to explain the prevalence of tolerance in human intergroup encounters; we nominate the need to access non-local resources in the human foraging ecology as a candidate pressure. To better evaluate existing hypotheses, additional, targeted data are needed to document the prevalence and plasticity of tolerance during intergroup encounters in some great ape species.
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Parochial altruism – individual sacrifice to benefit the in-group and harm an out-group –undermines inter-group cooperation and is implicated in a plethora of politically-significant behaviors. We report experimental evidence about the impact of variation in social distance within the in-group together with variation in social distance between the in- and out-groups on parochial altruism in a setting in which inter-group conflict is made possible. We find that differential social distance has a systematic effect on individual choice. In particular, parochial altruism is stimulated when individuals’ distance to both their in- and out-group is high. A long-standing hypothesis about behavior in contexts of inter-group conflict is that low social distance facilitates collective action leading to greater cooperation and greater conflict. Our results suggest that the effects of high social distance in a homogenous population may create a potential additional pathway to group-based individual action.
Article
This first part of a two-part article illustrates how research in evolutionary biology and psychology illuminates questions arising in philosophy—specifically questions about the origins of severe, systemic aggression that arise in the mimetic theory of René Girard. Part I looks at: (i) how old the systemic practice of severe aggression is, (ii) how much results from humanity's mimetic/social and competitive nature and how much from ecological, resource, and cultural conditions, and (iii) if ecological and cultural conditions are important, might we adapt them toward greater cooperativity today? After briefly reviewing mimetic theory, the article looks at evolutionary psychology and biology, including fossil and archeological evidence. Findings suggest that severe, systemic aggression might be relatively recent and that its occurrence depends on ecological/resource/socioeconomic conditions additional to our mimetic/social nature. Thus, distinguishing the conditions that prod aggression from those that support pro-social behavior might aid us in structuring society today.
Thesis
As the past decade built toward today’s crisis, Scandinavian social democracy was frequently suggested as a model that could reform capitalism. The Nordic region’s income equality, gender equality, low-conflict politics, and prosperous economies with generous benefits contribute to high levels of happiness and social cohesion. Leading politicians on the American Left, as well as a majority of young Americans, express that they would prefer such outcomes. But is the Nordic Model suitable for cross-cultural export? This study examines the cultural origins of Scandinavian egalitarianism by applying an evolutionary perspective to ten influential works of fiction. These criticisms—ranging from an Icelandic saga to Swedish posthumanist TV—align to trace the emergence of modernity in the Nordic region. These works illustrate how fiction can be an evolutionary tool when environmental change requires that communities update the story they live by, their master-narrative. This study analyzes the ideological evolution from the polytheistic beliefs of Viking kinship societies, to Christian monotheism, to religious and later secular humanism, the master-narrative of the modern world. With each of these transitions, communities face a narrative abyss. The unquestionable story that had provided meaning, informed their cooperation, and dictated which future to strive for becomes transparent to them. Homo sapiens need such stories, or their larger communities come unglued. Reading these Nordic case studies through an evolutionary lens illuminates the brain mechanisms that make us factionalize, fall prey to anxiety, double down on orthodoxy, or even kill our neighbors during these master-narrative transitions. This study proposes that the West entered into such a transition in the 2010s. No longer convinced by liberal humanism, some populations have reverted to older stories of nationalism or tribal belonging. Until people are able to unite around a new master-narrative, things could get worse. The tenth work of fiction points to the leading candidate for becoming tomorrow’s uniting story: dataism. This study concludes that social democracy arose from uniquely Nordic experiences, which makes the political model unlikely to work well elsewhere. But insights from this journey through a millennium of cultural change illuminate the challenges humanity faces in the twenty-first century.
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In humans and chimpanzees, most intraspecific killing occurs during coalitionary intergroup conflict. In the closely related genus Gorilla, such behavior has not been described. We report three cases of multi-male, multi-female wild mountain gorilla (G. beringei) groups attacking extra-group males. The behavior was strikingly similar to reports in chimpanzees, but was never observed in gorillas until after a demographic transition left ~25% of the population living in large social groups with multiple (3+) males. Resource competition is generally considered a motivator of great apes’ (including humans) violent intergroup conflict, but mountain gorillas are non-territorial herbivores with low feeding competition. While adult male gorillas have a defensible resource (i.e. females) and nursing/pregnant females are likely motivated to drive off potentially infanticidal intruders, the participation of others (e.g. juveniles, sub-adults, cycling females) is harder to explain. We speculate that the potential for severe group disruption when current alpha males are severely injured or killed may provide sufficient motivation when the costs to participants are low. These observations suggest that the gorilla population’s recent increase in multi-male groups facilitated the emergence of such behavior, and indicates social structure is a key predictor of coalitionary aggression even in the absence of meaningful resource stress. Observations of severe and lethal coalitionary attacks in wild mountain gorillas. Available from: https://www.researchgate.net/publication/310475080_Observations_of_severe_and_lethal_coalitionary_attacks_in_wild_mountain_gorillas/stats [accessed Nov 22, 2016].
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The origin of human violence and warfare is controversial, and some scholars contend that intergroup conflict was rare until the emergence of sedentary foraging and complex sociopolitical organization, whereas others assert that violence was common and of considerable antiquity among small-scale societies. Here we consider two alternative explanations for the evolution of human violence: (i) individuals resort to violence when benefits outweigh potential costs, which is likely in resource poor environments, or (ii) participation in violence increases when there is coercion from leaders in complex societies leading to group level benefits. To test these hypotheses, we evaluate the relative importance of resource scarcity vs. sociopolitical complexity by evaluating spatial variation in three macro datasets from central California: (i) an extensive bioarchaeological record dating from 1,530 to 230 cal BP recording rates of blunt and sharp force skeletal trauma on thousands of burials, (ii) quantitative scores of sociopolitical complexity recorded ethnographically, and (iii) mean net primary productivity (NPP) from a remotely sensed global dataset. Results reveal that sharp force trauma, the most common form of violence in the record, is better predicted by resource scarcity than relative sociopolitical complexity. Blunt force cranial trauma shows no correlation with NPP or political complexity and may reflect a different form of close contact violence. This study provides no support for the position that violence originated with the development of more complex hunter-gatherer adaptations in the fairly recent past. Instead, findings show that individuals are prone to violence in times and places of resource scarcity.
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Intergroup violence is common among humans worldwide. To assess how within-group social dynamics contribute to risky, between-group conflict, we conducted a 3-y longitudinal study of the formation of raiding parties among the Nyangatom, a group of East African nomadic pastoralists currently engaged in small-scale warfare. We also mapped the social network structure of potential male raiders. Here, we show that the initiation of raids depends on the presence of specific leaders who tend to participate in many raids, to have more friends, and to occupy more central positions in the network. However, despite the different structural position of raid leaders, raid participants are recruited from the whole population, not just from the direct friends of leaders. An individual’s decision to participate in a raid is strongly associated with the individual’s social network position in relation to other participants. Moreover, nonleaders have a larger total impact on raid participation than leaders, despite leaders’ greater connectivity. Thus, we find that leaders matter more for raid initiation than participant mobilization. Social networks may play a role in supporting risky collective action, amplify the emergence of raiding parties, and hence facilitate intergroup violence in small-scale societies.
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The psychological, sociological and evolutionary roots of conspecific violence in humans are still debated, despite attracting the attention of intellectuals for over two millennia. Here we propose a conceptual approach towards understanding these roots based on the assumption that aggression in mammals, including humans, has a significant phylogenetic component. By compiling sources of mortality from a comprehensive sample of mammals, we assessed the percentage of deaths due to conspecifics and, using phylogenetic comparative tools, predicted this value for humans. The proportion of human deaths phylogenetically predicted to be caused by interpersonal violence stood at 2%. This value was similar to the one phylogenetically inferred for the evolutionary ancestor of primates and apes, indicating that a certain level of lethal violence arises owing to our position within the phylogeny of mammals. It was also similar to the percentage seen in prehistoric bands and tribes, indicating that we were as lethally violent then as common mammalian evolutionary history would predict. However, the level of lethal violence has changed through human history and can be associated with changes in the socio-political organization of human populations. Our study provides a detailed phylogenetic and historical context against which to compare levels of lethal violence observed throughout our history.
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Whether man is predisposed to lethal violence, ranging from homicide to warfare, and how that may have impacted human evolution, are among the most controversial topics of debate on human evolution. Although recent studies on the evolution of warfare have been based on various archaeological and ethnographic data, they have reported mixed results: it is unclear whether or not warfare among prehistoric hunter–gatherers was common enough to be a component of human nature and a selective pressure for the evolution of human behaviour. This paper reports the mortality attributable to violence, and the spatio-temporal pattern of violence thus shown among ancient hunter–gatherers using skeletal evidence in prehistoric Japan (the Jomon period: 13 000 cal BC–800 cal BC). Our results suggest that the mortality due to violence was low and spatio-temporally highly restricted in the Jomon period, which implies that violence including warfare in prehistoric Japan was not common.
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There is significant heterogeneity within and between populations in their propensity to engage in conflict. Most research has neglected the role of within-group effects in social networks in contributing to between-group violence and focused instead on the precursors and consequences of violence, or on the role of between-group ties. Here, we explore the role of individual variation and of network structure within a population in promoting and inhibiting group violence towards other populations. Motivated by ethnographic observations of collective behavior in a small-scale society, we describe a model with differentiated roles for individuals embedded within friendship networks. Using a simple model based on voting-like dynamics, we explore several strategies for influencing group-level behavior. When we consider changing population level attitude changes and introducing control nodes separately, we find that a particularly effective control strategy relies on exploiting network degree. We also suggest refinements to our model such as tracking fine-grained information spread dynamics that can lead to further enrichment in using evolutionary game theory models for sociological phenomena.
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Intergroup contests determine access to resources and influence the evolution of group living in social species. Asymmetries in resource-holding potential and payoffs should influence the outcome and intensity of such contests. We evaluated predictors of contest outcome and intensity using data collected over 40 months from 6 groups of wild blue monkeys, Cercopithecus mitis. We found increased odds of winning when a group was larger and used the contest site more than its opponent, and when contests occurred closer to the group's home range centre while farther from the opponent's centre. However, a larger difference in group size (across five pairs of opposing groups) did not predict a greater proportion of contests won by the larger group. Some evidence suggested increased odds of a draw when group sizes were more similar. In addition, contests were longer and more aggressive when groups were more similar in size and when the contest site was similarly central in both groups' home ranges. Contests were also more aggressive when the opposing groups' use of the contest site was more similar. Overall, asymmetries in resource-holding potential (i.e. group size) and/or payoffs related to the contest's location influenced a group's competitive advantage, the likelihood of a draw and the intensity of intergroup contests. Although comparable data are limited, it seems clear that both types of asymmetries can play a role in determining the outcome and intensity of intergroup contests, and that the relative power of each may vary across species.
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The nature of inter-group relations among prehistoric hunter-gatherers remains disputed, with arguments in favour and against the existence of warfare before the development of sedentary societies. Here we report on a case of inter-group violence towards a group of hunter-gatherers from Nataruk, west of Lake Turkana, which during the late Pleistocene/early Holocene period extended about 30 km beyond its present-day shore. Ten of the twelve articulated skeletons found at Nataruk show evidence of having died violently at the edge of a lagoon, into which some of the bodies fell. The remains from Nataruk are unique, preserved by the particular conditions of the lagoon with no evidence of deliberate burial. They offer a rare glimpse into the life and death of past foraging people, and evidence that warfare was part of the repertoire of inter-group relations among prehistoric hunter-gatherers.
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Chimpanzees, bonobos, and human foragers share a fission-fusion social system and a mating system of joint male resource defense polygyny. Within-community skew in male strength varies among and within species. In this study, we extend a mathematical model of within-group male coalition formation among primates to derive the conditions for between-community conflicts in the form of raids. We show that the main factor affecting the presence of successful raiding is the likelihood of major discrepancies in party strength, which are set by party size distributions (and thus community size) and the skew in strength. This study confirms the functional similarities between the raiding of chimpanzees and human foragers, and it supports the "imbalance of power" hypothesis for raiding. However, it also proposes two amendments to this model. First, the absence of raiding in bonobos may be attributable more to potential female involvement in defense against raids, which increases the size of defensive coalitions. Second, the model attributes some of the raiding in humans to major contrasts in instantaneous fighting ability created by surprise raids on unarmed victims; it also draws attention to the distinction between minor raids and major raids that involve multiple bands of the same community.
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Evidence for territoriality is usually correlative or post hoc as we observe the results of past selection that are challenging to detect. Wolves (Canis lupus) are considered territorial because of competition for food (resource defense), yet they exhibit classic intrinsic behaviors of social regulation (protection against infanticide). This emphasis on prey and infrequent opportunity to observe wild wolf behavior has led to little investigation into the causes of or competitive underpinnings in the evolution of wolf territoriality. We report 6 cases of territorial wolf packs attacking neighboring packs at or near their den; 2 attacks were observed in detail. In all cases, except perhaps one, the attacking pack killed adult wolves either at the den or near it; in 4 cases, pups were probably lost. Loss of pups led to future loss of territory and in one case pack cessation. Intraspecific killing (measured in collared adults only) peaked in April, the month when pups were born and helpless in dens, even though aggressive interactions were at their seasonal low. Twelve of 13 (92%) of the wolves killed during the denning season (March, April, May) were reproductive (males and females), and 8 of 12 were dominant individuals (highest ranking wolf for that sex in the pack). Wolf-wolf killings were also high in October and December, the beginning and middle of the nomadic season, respectively. Aggressive interactions were more frequent during the nomadic season when wolves were roaming their territory as a group compared to the denning season when wolf activity was centered on the den and pack members less cohesive. We conclude that attacks on dens are a more effective form of interpack competition than interference during the breeding season, the current best-supported hypothesis, and that protected pup-rearing space is the primary cause of wolf territoriality.
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The use of evolutionary theory for explaining human warfare is an expanding area of inquiry, but it remains obstructed by two important hurdles. One is that there is ambiguity about how to build an evolutionary theory of human warfare. The second is that there is ambiguity about how to interpret existing evidence relating to the evolution of warfare. This paper addresses these problems, first by outlining an evolutionary theory of human warfare, and second by investigating the veracity of four common claims made against the use of evolutionary theory for explaining warfare. These claims are: (1) ancestral warfare was not frequent or intense enough to have selected for psychological adaptations in humans for warfare; (2) the existence of peaceful societies falsifies the claim that humans possess adaptations for fighting; (3) if psychological adaptations for warfare exist, then war is an inevitable and universal component of the human condition; (4) modern warfare and international politics is so qualitatively different from ancestral politics that any adaptations for the latter are inoperative or irrelevant today. By outlining an evolutionary theory of war and clarifying key misunderstandings regarding this approach, international relations scholars are better positioned to understand, engage, and contribute to emerging scholarship on human warfare across the social and evolutionary sciences.
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Reputations are a ubiquitous feature of human social life, and a large literature has been dedicated to explaining the relationship between prosocial reputations and cooperation in social dilemmas. However, humans form reputations in domains other than prosociality, such as economic competency that could affect cooperation. To date, no research has evaluated the relative effects of multiple reputation domains on cooperation. To bridge this gap, we analyse how prosocial and competency reputations affect cooperation in two Latin American communities (Bwa Mawego, Dominica, and Pucucanchita, Peru) across a number of social contexts (Dominica: labour contracting, labour exchange and conjugal partnership formation; Peru: agricultural and health advice network size). First, we examine the behavioural correlates of prosocial and competency reputations. Following, we analyse whether prosocial, competency, or both reputation domains explain the flow of cooperative benefits within the two communities. Our analyses suggest that (i) although some behaviours affect both reputation domains simultaneously, each reputation domain has a unique behavioural signature; and (ii) competency reputations affect cooperation across a greater number of social contexts compared to prosocial reputations. Results are contextualized with reference to the social markets in which behaviour is embedded and a call for greater theory development is stressed. © 2015 The Author(s) Published by the Royal Society. All rights reserved.
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Life abounds with examples of conspecifics actively cooperating to a common end, despite conflicts of interest being expected concerning how much each individual should contribute. Mathematical models typically find that such conflict can be resolved by partial-response strategies, leading investors to contribute relatively equitably. Using a case study approach, we show that such model expectations can be contradicted in at least four disparate contexts: (i) bi-parental care; (ii) cooperative breeding; (iii) cooperative hunting; and (iv) human cooperation. We highlight that: (a) marked variation in contributions is commonplace; and (b) individuals can often respond positively rather than negatively to the contributions of others. Existing models have surprisingly limited power in explaining these phenomena. Here, we propose that, although among-individual variation in cooperative contributions will be influenced by differential costs and benefits, there is likely to be a strong genetic or epigenetic component. We then suggest that selection can maintain high investors (key individuals) when their contributions promote support by increasing the benefits and/or reducing the costs for others. Our intentions are to raise awareness in—and provide testable hypotheses of—two of the most poorly understood, yet integral, questions regarding cooperative ventures: why do individuas vary in their contributions and when does cooperation beget cooperation?. © 2015 The Author(s) Published by the Royal Society. All rights reserved.
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Asymmetries in resource-holding potential between opposing groups frequently determine outcomes of intergroup contests. Since both numerical superiority and high intergroup dominance rank may confer competitive advantages, group members should benefit from assessing the relative strength of rivals prior to engaging in defensive displays. However, differences in individual assessment may emerge when cost–benefit trade-offs differ among group members. We examine the influence of numerical superiority and intergroup dominance relationships on individual participation in intergroup encounters in black howler monkeys (Alouatta pigra) and tufted capuchin monkeys (Sapajus nigritus). Black howlers responded with longer vocal displays during encounters with neighbours with an equal number of resident males, while tufted capuchins increased their participation with increasing relative male group size. Within each species, males and females responded similarly to varying numerical odds, suggesting that despite pay-off asymmetries between males and females, both sexes were similarly influenced by numerical asymmetries in deciding to participate in collective group defence. Whereas the outcome of contests among tufted capuchins was determined by relative male group size, reflected in a pronounced intergroup dominance hierarchy, the absence of dominance relationships among black howler groups may have provoked prolonged vocal displays in order to assess rival groups with matching competitive abilities.
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Models of collective action infrequently account for differences across individuals beyond a limited set of strategies, ignoring variation in endowment (e.g. physical condition, wealth, knowledge, personality, support), individual costs of effort, or expected gains from cooperation. However, behavioural research indicates these inter-individual differences can have significant effects on the dynamics of collective action. The papers contributed to this theme issue evaluate how individual differences affect the propensity to cooperate, and how they can catalyse others' likelihood of cooperation (e.g. via leadership). Many of the papers emphasize the relationship between individual decisions and socio-ecological context, particularly the effect of group size. All together, the papers in this theme issue provide a more complete picture of collective action, by embracing the reality of inter-individual variation and its multiple roles in the success or failure of collective action.
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Observation of leadership in small-scale societies offers unique insights into the evolution of human collective action and the origins of sociopolitical complexity. Using behavioural data from the Tsimane forager-horticulturalists of Bolivia and Nyangatom nomadic pastoralists of Ethiopia, we evaluate the traits of leaders and the contexts in which leadership becomes more institutional. We find that leaders tend to have more capital, in the form of age-related knowledge, body size or social connections. These attributes can reduce the costs leaders incur and increase the efficacy of leadership. Leadership becomes more institutional in domains of collective action, such as resolution of intragroup conflict, where collective action failure threatens group integrity. Together these data support the hypothesis that leadership is an important means by which collective action problems are overcome in small-scale societies.
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Even when hunting in groups is mutually beneficial, it is unclear how communal hunts are initiated. If it is costly to be the only hunter, individuals should be reluctant to hunt unless others already are.We used 70 years of data from three communities to examine how male chimpanzees ‘solve’ this apparent collective action problem. The ‘impact hunter’ hypothesis proposes that group hunts are sometimes catalysed by certain individuals that hunt more readily than others. In two communities (Kasekela and Kanyawara),we identified a total of five males that exhibited high hunt participation rates for their age, and whose presence at an encounter with red colobus monkeys increased group hunting probability. Critically, these impact hunters were observed to hunt first more often than expected by chance.We argue that by hunting first, these males dilute prey defences and create opportunities for previously reluc- tant participants. This by-product mutualism can explain variation in group hunting rates within and between social groups. Hunting rates declined after the death of impact hunter FG in Kasekela and after impact hunter MS stopped hunting frequently in Kanyawara. There were no impact hunters in the third, smaller community (Mitumba), where, unlike the others, hunting probability increased
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Classic socio-ecological theory holds that the occurrence of aggressive range defence is primarily driven by ecological incentives, most notably by the economic defendability of an area or the resources it contains. While this ecological cost-benefit framework has great explanatory power in solitary or pair-living species, comparative work on group-living primates has always found economic defendability to be a necessary, but not sufficient condition to account for the distribution of effective range defence across the taxon. This mismatch between theory and observation has recently been ascribed to a collective action problem among group members in, what is more informatively viewed as, a public goods dilemma: mounting effective defence of a communal range against intrusions by outgroup con-specifics. We here further develop this framework, and report on analyses at three levels of biological organization: across species, across populations within a single lineage and across groups and individuals within a single population. We find that communal range defence in primates very rarely involves collective action sensu stricto and that it is best interpreted as the outcome of opportunistic and strategic individual-level decisions. Whether the public good of a defended communal range is produced by solitary, joint or collective action is thus the outcome of the interplay between the unique characteristics of each individual, local and current socio-ecological conditions, and fundamental life-history traits of the species.
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In many social species, group members cooperate to defend a communal home range. Fighting in between-group conflicts carries an opportunity cost, a risk of injury or death, and the possibility of exploitation by free-riding group members. As a result, it is rare that all group members fight in a given between-group conflict, and individual participation in range defence is often highly variable. Thus, to understand the patterns of behaviour observed at the group level, we must first understand the causes of within-and between-individual variability. Although sex differences have been well studied, our understanding of the relative importance of the various mechanisms promoting between-group aggression within a sex is limited. We observed the participation of 22 male vervet monkeys, Chlorocebus aethiops pygerythrus, in 126 between-group conflicts, and then partitioned aggressive acts according to the context in which they occurred. Using this approach, we found evidence that two mechanisms drive male between-group aggression and, therefore, that individual variability is in part driven by the multiple selective benefits of participation. First, males that were likely to have sired offspring tended to exhibit defensive aggression and were more active when infants were present in the group, suggesting they fight to defend probable offspring. Second, males were more likely to support females in initiating between-group aggression just prior to, and during, the mating season. Female vervet monkeys are able to exert female choice, and males that frequently supported female instigators tended to enjoy the highest mating success. These results indicate that males probably use between-group aggression to improve their reputation with choosy females and subsequently maximize their mating success. Our findings indicate that a greater understanding of the evolutionary mechanisms promoting cooperative home range defence can be gained if we consider the context in which acts of between-group aggression occur.
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