In this special issue on human conflict, we consider the deep evolutionary roots of violent confrontation. We trace the trajectory of violence and war throughout history, exploring racism, ethnic conflicts, the rise of terrorism, and the possible future of armed conflicts. We also consider our innate capacity to mediate conflict and our ability to achieve—and live in—peace.

Competition and conflict both among and within species, for food or a place to live or a mate, are implicit in the process of evolution and thus intrinsic to our biology. But like many other animals, we are also social beings, and, like them, we have evolved behaviors to avoid the detrimental effects of excessive intraspecies violence. These include ritual singing or fighting displays, acts of submission or conciliation, and simple spatial avoidance seen in diverse species as birds, ants, and our primate relatives.

The groups we consider ourselves to be part of, our “ingroups,” are critical to our understanding of “self ” and categorization of “other” or “outgroups”; these groups often form the basis for conflict. Groups can have myriad identities. The first were certainly those of kin, and then ethnicity, but as societies grew, groups identified themselves in more complex ways. Our sense of a “group self ” influences the way we see and feel about the world around us: We are more likely to empathize with those of our own group and more likely to dehumanize outgroups. Groups based around religious or sacred values can increase ingroup reliance and cooperation through costly rituals, and they can also inspire nonrational sacrifices (suicide bombing, for example), whereas reasoned social contracts seem less able to inspire such fervent loyalty. But although we are ready to see the world as made up of coalitions, it is also true that our perception of “us” and “them” is not primordial because people have constantly shifted affiliations throughout history.

This special section looks at the past and future of conflict and the kinds of data needed to analyze it. However, our interest in conflict must ultimately be to facilitate peace. There are many examples of neighboring societies that do not war against each other, and they appear to have worked in part by expanding the “us.” It seems that encompassing a common identity with other groups and transforming outsiders into insiders provides an effective way to overcome conflict. Although some systems (the European Union) still war against outside entities, others (the Orang Asli societies in Malaysia) do not. We hope that understanding how human societies have overcome the odds and developed peaceful relations will help chart a path to a less violent future.

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