Abstract
Social interactions are often characterized by cooperation within groups and conflict or competition between groups. In certain circumstances, however, cooperation can arise between social groups. Here, we examine the circumstances under which inter-group cooperation is expected to emerge and present examples with particular focus on groups in two well-studied but dissimilar taxa: humans and ants. Drivers for the evolution of inter-group cooperation include overarching threats from predators, competitors or adverse conditions, and group-level resource asymmetries. Resources can differ between groups in both quantity and type. Where the difference is in type, inequalities can lead to specialization and division of labour between groups, a phenomenon characteristic of human societies, but rarely seen in other animals. The ability to identify members of one's own group is essential for social coherence; we consider the proximate roles of identity effects in shaping inter-group cooperation and allowing membership of multiple groups. Finally, we identify numerous valuable avenues for future research that will improve our understanding of the processes shaping inter-group cooperation.