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.

One of the foundational approaches to the evolution of behavior is the comparative approach. The underlying logic is to use similarities and differences across species to draw conclusions about the evolution of the trait in question, with the assumption that similar selective pressures lead to similar traits. When using this approach to understand humans, a natural starting place is the other primates, as we are primates ourselves. But this is not the only approach; we may also wish to know what the impact of a specific feature is, in which case we will focus on other species that have the same feature independent of phylogeny, or share a particular ecological or social niche. Convergences across disparate taxa may suggest the ways in which the trait in question is linked to a specific behavioral outcome.

Primates engage in a variety of complex social behaviors. Broadly speaking, these social behaviors can range from agonistic to affiliative depending on the context of a given interaction and a variety of other factors such as the sex, age, familiarity, and rank of individuals. Social interactions of any kind – whether cooperative or “prosocial,” as they is often termed, or conflict- and aggression-based, often termed “antisocial” – are based on the individual’s personality and cognitive traits and are manifest in their communication and behaviors directed toward others. (Chapter 5 discusses the problems associated with this terminology.) In other words, similar to humans, within different primate groups there are individual differences in the frequency of behaviors that reflect the range of social behaviors that are expressed during social interactions.  Understanding how or why this cluster of traits varies among individuals is therefore important for understanding social interactions.  It is now clear that one source of individual variation in both competitive and cooperative behavior is genes. Two of the most widely studied are genes that regulate the receptor distribution of oxytocin (OXTR) and vasopressin (AVPRA, AVPR1B and AVPR2). (See Box 7.1 for an overview of terminology and concepts associated with genetic variation.)

The evolutionary paradox of animals helping others in their social group, rather than living independently, has fascinated researchers for many decades. Ultimately, this cooperation is hypothesized to have evolved because the benefits of cooperation outweigh the costs (Hamilton, 1964), and considerable empirical evidence supports this hypothesis (see Koenig and Dickinson, 2016). However, one major cost of cooperation, particularly for territorial cooperative breeders, is conflict (Shen et al., 2017; Nelson-Flower et al., 2018a). This conflict may occur both within and between groups; such conflict includes access to reproductive opportunities, social status, territory, water, or food. We suggest that a consideration of the influence of both intergroup and intragroup levels of conflict in individual decisions to cooperate is essential, since intragroup conflict may lead to decisions to disperse if intergroup opportunities to mate are high, whereas high levels of intergroup conflict may promote intragroup cooperation in order to defend existing resources (see Chapter 10 for review of inter- vs. intragroup aggression in social insects). Conflict is therefore a natural outcome of cooperation, and the level of conflict may define the point at which cooperation is no longer a beneficial strategy for individuals. This possibility, that conflict at both inter- and intragroup levels define the stability of cooperation over time, remains relatively under-explored despite its potential importance in understanding the evolution of cooperation.