Intergroup conflict,ostracism, and the evolution of cooperation under free migration

Theoretical investigations and quasi-experimental evidence from modern conflict areas suggest that intergroup conflict and ostracism play a pivotal role in the evolution of cooperation. However, there is little direct evidence about the influence of intergroup conflict on human social behavior in the presence of endogenous group formation and unrestricted migration. This study introduces an experiment to examine the impact of intergroup conflict and ostracism on group formation, and human cooperation in a dynamic environment where group size, the occurrence of intergroup hostility and the threat of ostracism are endogenously determined. Here, we show that intergroup conflict may inhibit merging into single large human coalitions. The threat of ostracism is shown to increase the average group size and cooperation within the society. In addition, we find that competitive pressure between groups decreases the likelihood of social exclusion. Our results suggest that free migration between groups suppresses the impact of between-group competition on within-group cooperation. Moreover, our results stress the role of ostracism as a means to repress competition within groups and enhance group success in competition against other groups.