Humans exhibit an impressive array of social behaviors. We engage in complex cooperative behavior, employ flexibility in social responses, and navigate large social groups effectively. These behaviors are made possible by more fundamental cognitive abilities including facial recognition, communication, storing and accessing concepts about social entities, and processing emotions. All of these abilities have at least part of their neural basis in the temporal lobe (e.g., Deen et al., 2015), one of the major divisions of the cerebral cortex (see Box 6.1). The remarkable human facility for cooperation – and indeed, great conflict – suggests there may be equally remarkable features of the human brain.

Cooperation is the defining feature of societies; in these groups, members work together to achieve something that the individuals alone cannot. We marvel at cooperation in part because it requires communication and coordination, complex behaviors that speak directly to the creative, constructive power of natural selection. Nevertheless, societies can be disrupted by internal conflicts (Hurst et al., 1996; Chapman, 2006; Ratnieks et al., 2006; Burt and Trivers, 2009; Queller and Strassmann, 2018; Sachs et al., 2018). Conflict can be defined in many ways, but it amounts to an incentive to defect because actions that benefit the individual (e.g., do not pay taxes) run counter to those that benefit the group (everyone pays their taxes). In some cases, conflict results in a tragedy of the commons, where cooperation produces goods that are available to all, but some individuals deplete the public good without contributing to its production.

Humans are a remarkably social species. They form and live in groups and recurrently have to decide whether to cooperate or compete with others within and among groups. Cooperation has been essential for group survival and prosperity across human history. In hunter-gatherer societies, people need to form alliances in hunting to alleviate the risks from predator attacks. Likewise, modern societies require groups of people to cooperate in large ventures. Yet, social situations often involve a conflict between one’s short-term personal interest and the long-term collective interest (i.e., social dilemmas; Dawes, 1980; Van Lange et al., 2013). In such mixed-motive situations, what is good for an individual may often harm the collective, and this makes people tempted to free ride and harvest the benefits from others’ cooperation. Indeed, many societal problems and global issues (e.g., traffic problems, environmental pollution, and resource depletion) involve such conflicts of interests. Solving these problems often requires individuals to cooperate by paying a personal cost to benefit another person or the group.

The success of humans in spreading through all of Earth’s ecosystems and transforming them at planetary scale is directly dependent on our capacity to cooperate in large groups and self-organize in complex social structures that sustain such cooperation. One of the main components of such large-scale cooperation is the human capacity and propensity for inventing and following social norms (Ostrom, 2000; Fehr and Schurtenberger, 2018). Social norms influence almost all aspects of human behavior, providing a “grammar of society” (Bicchieri, 2005, 2010) that constrains and enables different kinds of individual behaviors, coordinates collective behavior, and sustains cooperation in the face of conflicts of interests.

The study of prosocial behavior has been an active area of research in social psychology that dates back to the beginnings of the last century. (For a review see Penner et al., 2005,) This large body of literature includes a diverse range of phenomena centering around the origins and tendencies of humans helping other humans, including traits such as empathy. In psychology the term “prosocial behavior” is typically used to indicate a behavior that provides benefit to another person. However, this same term, and all that it implies, has been increasingly applied to nonhuman vertebrate animal behavior and the neural mechanisms regulating these behaviors. It is within this latter context that the term prosocial has been used rather loosely with no clear definitions provided.

The most fundamental questions in international relations are: “Why do states go to war?” “How can interstate conflict be prevented or ameliorated?,” and “What are the pathways to greater international cooperation?” In considering these questions, the dominant paradigm in international relations, political realism, emphasizes the enduring propensity for conflict among self-interested states seeking their security in an “anarchic” international environment, that is, one where there is no central authority to protect states from each other or to guarantee their security. Hence, international cooperation is thought to be rare, fleeting, and tenuous – limited by enforcement problems and each state’s preference for larger relative gains in any potential bargain because of its systemic vulnerability (Morgenthau, 1949; Waltz, 1979). At the extreme, states find themselves in a security condition of mutual distrust that resembles a prisoner’s dilemma game. (See Box 3.1 in Chapter 3 for a description of various games.) Maintaining an equilibrium in the international system through a balance of power and limited cooperation are all that can be hoped for; a situation where war, large-scale violent conflict, is natural and merely “diplomacy by other means” (von Clausewitz, 1989). This is not to argue that international relations are in a constant state of war, rather that they exist within the shadow of war as a final arbiter.

Take any ecology textbook and look up the chapter on interactions between species, and you will find that ecologists distinguish among three outcomes: mutualism, commensalism and parasitism/predation. Mutualisms are mutually beneficial (+/+), commensalisms are beneficial for one partner and neutral for the other (+/0), and parasitism/predation is beneficial for one and detrimental for the other (+/−). Mutualisms are at the core of the world as we know it; the evolution of the eukaryotic cell warranted the mutualistic integration of cell organelles (mitochondria and chloroplasts) into prokaryotic cells, and the radiation of flowering plants as a nutritional basis for the animal food chain is dependent on soil microorganisms for the fixation of nitrogen and phosphate as well as on pollinators (Bronstein, 2015). Therefore, studying mutualism is an integral part of ecological research and one that connects directly to understanding the evolution of cooperation. (See Chapter 4 for a discussion of mutualisms at the cell and genomic levels.)

Finding a balance between cooperative or prosocial behavior – such as social bonding and empathy – and conflict – or competitive-aggressive, self-interested behavior – is the fundamental challenge to the operation of societies and to the behavior of individuals in a social setting. But how do these apparent opposites relate to one other? As would many social or behavioral scientists, we initially approached this with the idea that they are two separate functions that need to be balanced against each other to varying degrees to construct a functioning social entity, and to some extent this holds true. But independently of one another, the contributing authors to this volume advanced a more sophisticated view of the relationship; that the poles of social interaction are in fact interconnected to the extent that what we view as antisocial or aggressive behavior are fundamental to establishing and maintaining positive or prosocial behavior within groups or individuals.

Research on the evolution of social groups has focused substantially on processes that increase cooperative behaviors (Bourke, 2011). Indeed, each major evolutionary transition (the evolution of eukaryotes from prokaryotes, the evolution of multicellular organisms from single-celled organisms, the evolution of eusocial insect societies from solitary species, etc.) involves increasing cooperative behavior to the point where previously independent units now must interact to successfully replicate (Szathmary and Smith, 1995). With the focus on cooperation, the importance of aggression and conflict in societies is typically overlooked, despite the fundamental nature of these processes in giving rise to and maintaining social structures (see Chapters 2, 8, and 9 for other examples). For instance, maternal antipredator aggression is the critical antecedent to a stable social unit that exists in a central place (e.g., a nest or burrow) (Brunton, 1990; Groom, 1992). Within social groups, aggression among individuals establishes dominance hierarchies, manages conflict, and leads to division of labor in contexts of reproduction and offspring care (Ratnieks et al., 2006; Wittemyer and Getz, 2007). Here we discuss the possible roles of aggression and conflict in the evolution and organization of social groups, and explore the underlying molecular and physiological mechanisms associated with these drivers of sociality. We define aggression as behaviors that carry a potential physical cost or otherwise could reduce the direct fitness of the individuals involved, and we define conflict as situations in which the fitness interests of interacting individuals diverge, regardless of the behavioral outcome.