The Co-evolution of Individual Behaviors and Social Institutions

We jointly address two puzzles, namely what accounts for the evolutionary success of both: (a) individually costly and group-beneficial forms of human sociality towards non-kin; and (b) those group-level institutional structures such as food sharing and monogamy which have emerged and diffused repeatedly in a wide variety of ecologies during the course of human history. We show that the frequency and consequences of intergroup conflicts may provide an important part of the answer to both questions: in-group beneficial behaviors may evolve if they inflict sufficient costs on out-group individuals while group-level institutions limit the individual costs of these behaviors. We model a coevolutionary process in which individual traits are transmitted either genetically or culturally and in which the evolutionary trajectories of individual traits and social institutions are mutually determining. Our simulations show that if group-level institutions implementing resource sharing or nonrandom pairing among group members may evolve, group-level selection processes support the coevolution of group-beneficial individual traits along with these institutions, even where the latter impose significant costs on the groups adopting them. In the absence of these group-level institutions, however, group selection pressures support the evolution of group beneficial traits only when intergroup conflicts are very frequent, groups are small, and migration rates are low. Thus under parameter values which may bear some resemblance to the relevant environments during the first 90,000 years of anatomically modern human existence, in-group-beneficial individual traits and group-level institutions of resource sharing and social segmentation could readily evolve, the sociality of humans thus being in part a consequence of human capacities in social institution building.