Inter-group cooperation in humans and other animals

Social interactions are often characterized by cooperation within groups andconflict 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 andpresent examples with particular focus on groups in two well-studied butdissimilar taxa: humans and ants. Drivers for the evolution of inter-groupcooperation include overarching threats frompredators, competitors or adverseconditions, and group-level resource asymmetries. Resources can differbetween 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 forsocial coherence; we consider the proximate roles of identity effects in shapinginter-group cooperation and allowing membership of multiple groups. Finally,we identify numerous valuable avenues for future research that will improveour understanding of the processes shaping inter-group cooperation.