Near the Knuckle Abstract Irish Travellers constitute a pre-demographic-shift population living among a post-demographic-shift one. Their socio-medico profile identifies them as largely on fast life-history trajectories. In addition, they are strongly religious (typically using no contraception), highly sexually behaviorally dimorphic, with strong traditions of male-male competition (bare-knuckle fighting) and quasi-symbolic bride capture (“grabbing”). Their male-male competitions thus allow for the comparative testing of a number of interesting theories pertaining to the nature and function of types of violence in society. As a pilot study, we used expert raters (some naive to the hypotheses) to analyze a number of real-life bare-knuckle competitions in terms of the support said spectacles offered to theories of this sort of violence as reinforcing ideas of antisociality, hierarchical promotion, intersexual signaling, or maintenance of within-group equality. We found good evidence to support theories of within-group, prosocial hierarchical functions for these contests. Limitations and implications for future research, such as direct measurement of fitness, are discussed.

Complex Sociality of Wild Chimpanzees Can Emerge from Laterality of Manual Gestures Abstract Humans are strongly lateralized for manual gestures at both individual and population levels. In contrast, the laterality bias in primates is less strong, leading some to suggest that lateralization evolved after the Pan and Homo lineages diverged. However, laterality in humans is also context-dependent, suggesting that observed differences in lateralization between primates and humans may be related to external factors such as the complexity of the social environment. Here we address this question in wild chimpanzees and examine the extent to which the laterality of manual gestures is associated with social complexity. Right-handed gestures were more strongly associated with goal-directed communication such as repair through elaboration in response to communication failure than left-handed gestures. Right-handed gestures occurred in evolutionarily urgent contexts such as in interactions with central individuals in the network, including grooming reciprocity and mating, whereas left-handed gestures occurred in less-urgent contexts, such as travel and play. Right-handed gestures occurred in smaller parties and in the absence of social competition relative to left-handed gestures. Right-handed gestures increased the rate of activities indicating high physiological arousal in the recipient, whereas left-handed gestures reduced it. This shows that right- and left-handed gestures differ in cognitive and social complexity, with right-handed gestures facilitating more complex interactions in simpler social settings, whereas left-handed gestures facilitate more rewarding interactions in complex social settings. Differences in laterality between other primates and humans are likely to be driven by differences in the complexity of both the cognitive skills underpinning social interactions and the social environment.

The Proximate Causes of Waorani Warfare Abstract In response to recent work on the nature of human aggression, and to shed light on the proximate, as opposed to ultimate, causes of tribal warfare, we present a record of events leading to a fatal Waorani raid on a family from another tribe, followed by a detailed first-person observation of the behavior of the raiders as they prepared themselves for war, and upon their return. We contrast this attack with other Waorani aggressions and speculate on evidence regarding their hormonal underpinnings. On-the-ground ethnographic observations are examined in light of the neuroendocrinological literature. The evidence suggests a chain of causality in launching lethal violence, beginning with a perceived injury, culminating in a massacre, and terminating in rejoicing. Although no blood or saliva samples were taken at the time of this raid, the behaviors were consistent with a deliberate initiation of the hormonal cascade characterizing the “fight-or-flight” response, along with other hormonal changes. We conclude with observations on the stratified interrelationships of the cognitive, social, emotional, and neuroendocrinological causes of aggression leading to coalitional male homicide.

Fear of Violence among Colombian Women Is Associated with Reduced Preferences for High-BMI Men Abstract Recent studies reveal that violence significantly contributes to explaining individual’s facial preferences. Women who feel at higher risk of violence prefer less-masculine male faces. Given the importance of violence, we explore its influence on people’s preferences for a different physical trait. Masculinity correlates positively with male strength and weight or body mass index (BMI). In fact, masculinity and BMI tend to load on the same component of trait perception. Therefore we predicted that individuals’ perceptions of danger from violence will relate to preferences for facial cues to low-BMI. In two studies in Colombia, men and women from Bogota, Medellin, and surrounding communities were shown pairs of faces transformed to epitomize the shape correlates of men with high or low-BMI. The images were of European, Salvadoran, or Colombian men. Participants were asked to choose the face they considered most attractive. Subsequently, participants answered a survey about their health (e.g., frequency of illnesses the past year), media access (e.g., frequency of Internet use), education level (e.g., graduating from high school), and experiences/perceptions of violence in study 1 and about specific types of violence (public and domestic) in study 2. Results from both studies showed that women who experienced/perceived higher levels of violence preferred faces of low-BMI Salvadoran men. Preferences for low-BMI facial cues were significantly explained by violence (public or domestic), even after controlling for all other variables (including age, education, health, and media access). These results may reflect women’s strategy to avoid male partners capable of inflicting harm.

Roles of Technical Reasoning, Theory of Mind, Creativity, and Fluid Cognition in Cumulative Technological Culture Abstract Cumulative technological culture can be defined as the progressive diversification, complexification, and enhancement of technological traits through generations. An outstanding issue is to specify the cognitive bases of this phenomenon. Based on the literature, we identified four potential cognitive factors: namely, theory-of-mind, technical-reasoning, creativity, and fluid-cognitive skills. The goal of the present study was to test which of these factors—or a combination thereof—best predicted the cumulative performance in two experimental, micro-society conditions (Communication and Observation conditions; n = 100 each) differing in the nature of the interaction (verbal, visual) allowed between participants. The task was to build the highest possible tower. Participants were also assessed on the four aforementioned cognitive factors in order to predict cumulative performance (tower height) and attractiveness. Our findings indicate that technical-reasoning skills are the best predictor of cumulative performance (tower height), even if their role may be restricted to the specific technological domain. Theory-of-mind skills may have a facilitator role, particularly in the Communication condition. Creativity can also help in the generation of novel ideas, but it is not sufficient to support innovation. Finally, fluid cognition is not involved in cumulative technological culture. Taken together, these findings suggest that domain-specific knowledge (i.e., technical-reasoning skills) remains critical for explaining cumulative technological culture.

Review of Nam C. Kim and Marc Kissel’s Emergent Warfare in Our Evolutionary Past

Exploring Warfare and Violence from a Cross-Cultural Perspective Abstract This special issue of Human Nature presents selected works from the 2015 and 2017 “Warfare, Environment, Social Inequality, and Pro-Sociability” (WESIPS) conferences held at the Center for Cross-Cultural Study in Seville, Spain. These investigations explore the manifestations of indigenous warfare and violence from a host of theoretical perspectives. Topics range from the origins of warfare to the psychological repercussions of combat, the relationship between warfare and status, as well as the documentation of peace processes among warring groups. This issue also examines the effects of militarization and coercive conservation on indigenous peoples.

Prosocial Emotion, Adolescence, and Warfare Abstract Examining the costs and motivations of warfare is key to conundrums concerning the relevance of this troubling phenomenon to the evolution of social attachment and cooperation, particularly during adolescence and young adulthood—the developmental time period during which many participants are first recruited for warfare. The study focuses on Samburu, a pastoralist society of approximately 200,000 people occupying northern Kenya’s semi-arid and arid lands, asking what role the emotionally sensitized, peer-driven adolescent life stage may have played in the cultural and genetic coevolution of coalitional lethal aggression. Research in small-scale societies provides unparalleled opportunities for sharply defined variables, particularly in age generation societies in which all young men are initiated into “warriorhood.” Proposing an epigenetic and component behavior approach, we examine whether raiding activities such as number of raids, killing, and sparing enemy lives associate with DNA methylation in two candidate genes: MAOA, linked to mood and arousal, and NR3C1, linked to stress and immune response. We report statistically significant associations between the epigenetic variables and the combat (exposure) variables of overall raiding activity and reportedly showing mercy to enemies. In contrast, epigenetic variables did not associate with post-traumatic stress disorder (PTSD) symptom scores (a potential outcome measure), and the only combat variable associated with PTSD (but not DNA methylation) was losing one’s own livestock in a raid. These findings raise important questions concerning the mechanisms driving warfare’s paradoxical mix of violent and altruistic behaviors.

Is War in Our Nature? Abstract The Seville Statement on Violence rejected the view that violence and war were in any way rooted in human nature and proclaimed that they were merely a cultural artifact. This paper points out both the valid and invalid parts of the statement. It concludes that the potential for both war and peace is embedded in us. The human behavioral toolkit comprises a number of major tools, respectively geared for violent conflict, peaceful competition, or cooperation, depending on people’s assessment of what will serve them best in any given circumstance. Conflict is only one tool—the hammer—in our diverse behavioral toolkit. However, all three behavioral strategies are not purely learned cultural forms. This naive nature/nurture dichotomy overlooks the heavy and complex biological machinery that is necessary for the working of each of them and the interplay between them. They are all very close under our skin and readily activated because they have all been very handy during our long evolutionary past. At the same time, they are variably calibrated to particular conditions through social learning, which means that their relative use may fluctuate widely. Thus, state authority has tilted the menu of human choices in the direction of the peaceful options in the domestic arena, and changing economic, social, and political conditions may be generating a similar effect in the international arena.

Pacifying Hunter-Gatherers Abstract There is a well-entrenched schism on the frequency (how often), intensity (deaths per 100,000/year), and evolutionary significance of warfare among hunter-gatherers compared with large-scale societies. To simplify, Rousseauians argue that warfare among prehistoric and contemporary hunter-gatherers was nearly absent and, if present, was a late cultural invention. In contrast, so-called Hobbesians argue that violence was relatively common but variable among hunter-gatherers. To defend their views, Rousseauians resort to a variety of tactics to diminish the apparent frequency and intensity of hunter-gatherer warfare. These tactics include redefining war, censoring ethnographic accounts of warfare in comparative analyses, misconstruing archaeological evidence, and claiming that outside contact inflates the intensity of warfare among hunter-gatherers. These tactics are subject to critical analysis and are mostly found to be wanting. Furthermore, Hobbesians with empirical data have already established that the frequency and intensity of hunter-gatherer warfare is greater compared with large-scale societies even though horticultural societies engage in warfare more intensively than hunter-gatherers. In the end I argue that although war is a primitive trait we may share with chimpanzees and/or our last common ancestor, the ability of hunter-gatherer bands to live peaceably with their neighbors, even though war may occur, is a derived trait that fundamentally distinguishes us socially and politically from chimpanzee societies. It is a point often lost in these debates.