Humans are uniquely adept at cultivating positive, xenophilic relationships beyond their immediate group, forming bonds through various mechanisms such as trade, exogamy, and shared defense. Despite the significance of intergroup cooperation, synthetic approaches to understanding the evolutionary, ecological, and institutional drivers of xenophilia remain underdeveloped compared to the wealth of research on intergroup conflict. Here, I synthesize ethnographic, archaeological, psychological and comparative evidence to argue that xenophilia is supported by recurrent human cognitive and social capacities whose expression and stability depend on ecological, economic and institutional contexts. The aims of this paper are threefold: first, to clarify the selective pressures and incentives that favour peaceful intergroup relations, with particular attention to factors contributing to amicable intergroup ties and expanding the circle of tolerance and cooperation, including economic interdependence through trade and reciprocity, information exchange, exogamy, and the necessity of collective defense against external threats; second, to examine how these different drivers of xenophilia interact and under what conditions they generate, stabilize or fail to sustain intergroup tolerance; and third, to place human xenophilia in comparative perspective by highlighting both parallels and distinctions with non-human primates in their strategies for fostering intergroup peace. By distinguishing between mechanisms that generate cooperation and those that stabilize it in the face of freeriding and defection, this synthesis highlights how humans uniquely scale and institutionalize intergroup peace. This perspective is particularly timely, as uncovering the evolutionary roots of tolerance and cooperation across group boundaries remains crucial for addressing pressing global challenges such as climate change, poverty, famine, conflict, and disease.
Cyril C. Grueter (2026) studied this question.