Although cooperation is widespread from amoebas to humans, the underlying mechanisms are still not well understood, which precludes a full understanding of how cooperation evolved, particularly the complex forms seen in both nonhuman and human primates. The diversity of forms and expressions of cooperation seen across species complicates this, a challenge that has been addressed empirically with studies of cooperation into the lab, where similar methods can be used across species, allowing us to determine what mechanisms are, or are not, shared across species. In the case of cooperation, these methods include joint-action tasks (such as the cooperative barpull) and economic games. With data from standardized lab tests, we can make predictions about how each species should respond in more species-typical, natural contexts. This process allows us to understand not only when mechanisms are shared that might not be obvious (i.e., because they manifest in different ways), but when similar outcomes are underpinned by dissimilar mechanisms. For instance, many primates coordinate, but results from economic games suggest that they do so using a variety of different mechanisms. In addition, we can use these results to identify situations in which cognitive abilities are present, but may not manifest, and to look for the environmental pressures that may inhibit their expression. For example, chimpanzees show evidence of many of the mechanisms necessary for trade and barter, but they do not manifest in all contexts, possibly due to the absence of third-party enforcement mechanisms. Ultimately, understanding cooperation requires recognizing the interplay between cognitive mechanisms and ecology, such that we identify not only how and in what contexts other primates cooperate, but also those situations in which primates do not cooperate, but might be expected to. In so doing, we also move closer to understanding both how humans cooperate, and why it sometimes breaks down so spectacularly.

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.

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.

In this essay, I first discuss the necessity of examining the history of peacebuilding from an evolutionary standpoint. In so doing, I compare my model to other paradigms and demonstrate that there are insights to be garnered from the peacebuilding behavior of nonhuman species, particularly when it comes to avoidance of conflict and reconciliation. However, I suggest that, at its base, the struggle for power between nonhuman species drives the pursuit of cooperation, the dominance hierarchy (in the case of the primates), a reflection of power as a means to an end, which in this case is group survival.I conclude with some thoughts about the implications my theory has for understanding other schools of thought in IR theory as well as peacebuilding policy and practice.