Homo sapiens has been called “the social animal” for a good reason. Living in groups defines a key mode of human existence. Groups contain a bounty of resources critical to survival and reproduction. They afford safety and protection from predators and from other humans. They are populated with potential friends for mutually beneficial social exchange. They contain reproductively valuable mates. And they are inhabited with kin, precious carriers of our genetic cargo, from whom we can receive aid and in whom we can invest. At the same time, group living intensifies competition over precisely those reproductively relevant resources, creating sources of conflict not faced by more solitary creatures. The chapters in this part describe many of the complexities of the evolutionary psychology of group living, focusing on cooperation and conflict.

In Chapter 25, Leda Cosmides and John Tooby provide a comprehensive review of the extensive body of research, much of it conducted by them and their students, on neurocognitive adaptations for social exchange. They elucidate the many design features that such adaptations theoretically should possess and provide compelling arguments that domain-general mechanisms cannot achieve the specific outcomes needed for successful social exchange. They review competing theories to explain the content effects on the Wason selection task and marshal empirical evidence relevant to adjudicating among those theories. In a display of the sort of methodological pluralism advocated by Simpson and Campbell (Chapter 3, this Handbook, Volume 1), Cosmides and Tooby describe cross-cultural studies, studies using traditional methods of cognitive psychology, and studies using neurocognitive techniques.

Martin Daly's chapter (Chapter 26) on interpersonal violence and homicide begins by articulating an evolutionary perspective on conflicts of reproductive interests—a long-standing ingenious strategy pioneered by Daly and his long-time collaborator Margo Wilson. Next, he articulates the rationale for using violence and homicides as assays of social conflicts. Thus, Daly's focus is not so much in explaining violence per se, although key insights into violence do indeed emerge. Rather, his central aim is to exploit patterns of violence to reveal underlying conflicts of evolutionary interests that occur between individuals when they live in groups. He deploys this strategy to make novel scientific discoveries. Kin, for example, who typically have a greater confluence of interest compared to unrelated individuals, display much less violence toward each other, despite the fact that they interact more frequently. Intimate mates, to take another example, can have converging genetic interests, as when they have mutually produced offspring. But conflicts of interest emerge from at least six sources, such as temptations for genetic cuckoldry, temptations to trade up, relationship defection, and channeling pooled resources toward one set of kin at the expense of another (see also Conroy-Beam, Goetz, & Buss, 2015). Violence is more common precisely when these conflicts of interest emerge in intimate mateships.

Anne Campbell's chapter (Chapter 27) provides an overview of theory and research on women's competition and aggression. She explores both the proximate mechanisms (hormones, physiological maturation, neuropsychology) and ultimate selective forces underlying women's competition and aggression. Fear, she argues, acts as a more powerful brake on women's than on men's violent aggression, due to the greater costs of engaging in violent conflict (e.g., costs not only to the woman, but also to her children). But make no mistake, Campbell argues—women's competition, although less ostentatiously violent, can be ferocious. Women compete for the best mates, for example, a form of competition possibly exacerbated by socially imposed monogamy. She argues that appearance (cues to fertility) and fidelity (cues to paternity certainty) become key weapons by which women compete with other women, with tactics that include shunning, stigmatizing, derogating, and ostracizing their rivals. When tactics do escalate to actual violence, they occur in predictable contexts such as resource scarcity and a sex ratio imbalance involving too few men as potential mates. In short, Campbell's excellent chapter provides a detailed analysis of the underlying adaptations for female competition and aggression, the ways in which they are sex-differentiated in design, and the contextual and ecological variables to which they respond.

Prejudice seems to be a ubiquitous feature of human social living. Everywhere, people seem prone to dislike and distrust some others, discriminating against them within groups and even warring with them when they are out-groups. Steven Neuberg and Peter DeScioli (Chapter 28) provide an outstanding chapter on the evolved psychology—threat management systems—designed to deal with adaptive problems arising from within and outside of one's group. These prejudices can cause harm and discrimination in the modern environment, they argue, which makes it all the more important to understand their design features and how they play out in this new world.

Humans are an extraordinarily coalitional species. We form groups, often in competition with other groups. Dominic Johnson's chapter (Chapter 29) on leadership and war focuses on group-on-group conflict. He outlines different hypotheses about the evolution of leader traits in the context of war, or alternatively features of coalitional leadership psychology that could have been coopted for war, and examines the relevant empirical evidence. He makes a compelling case that war has been a major selective force on human psychology, including the evolution of leadership and followership traits—arguments that have critical relevance in a modern world beset with warfare in forms unimaginable in the past, but that exploit the same suite of psychological adaptations.

Group living is what we do as a species. It offers a bounty of benefits through cooperation and an abundance of costs through social conflict. As a consequence, it is reasonable to expect that humans have evolved a large number of specialized adaptations for dealing with other humans, both for within-group interactions and for dealing with other groups. Collectively, these chapters highlight the complexity of human evolutionary psychology for group living and pave the way for the discovery of many more adaptations for grappling with the challenges posed by other humans—challenges centering on cooperation and conflict.