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The Coevolution of Individual Behaviors and Social Institutions

Is the remarkable level of cooperation among unrelated humans based in part on the distinctive capacities of humans to construct institutional environments which limit competition and reduce phenotypic variation within groups, allowing individually-costly but ingroup-beneficial behaviors to coevolve with these supporting environments through a process of inter-demic group selection? We model a deme-structured population in which group differences in institutions are culturally transmitted and individual behaviors may be genetically or culturally transmitted. We use simulations of a standard extended fitness accounting framework to identify the parameter space for which this coevolutionary process generates high levels of ingroup-beneficial behaviors.

We show that intergroup conflicts may explain the evolutionary success of both: (a) altruistic forms of human sociality towards non-kin; and (b) 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. Ingroup-beneficial behaviors may evolve if i) they inflict sufficient costs on outgroup individuals and ii) group-level institutions limit the individual costs of these behaviors and thereby attenuate within-group selection against these behaviors.

Our simulations show that if group-level institutions implementing resource sharing or non-random pairing among group members are permitted to evolve, group-beneficial individual traits coevolve 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-beneficial traits evolve only when intergroup conflicts are very frequent, groups are small, and migration rates are low. Thus the evolutionary success of individually-costly but group-beneficial behaviors in the relevant environments during the first 90,000 years of anatomically modern human existence may have been a consequence of distinctive human capacities in social institution-building.