Effects of group size and memory on the evolution of cooperation

(INTRODUCTION) Human societies are characterised by high degrees of reciprocal altruism between unrelated individuals. It has even been suggested that humans have evolved a cognitive capacity for effective reasoning about social exchange transactions which does not readily generalise to other, non-social reasoning tasks (Cosmides 1989). Explaining the emergence of this capacity for cooperation is one of the fundamental goals of evolutionary anthropology.(DISCUSSION) Our results appear to resolve the contradiction identified at the start of this paper. Cooperation can evolve as a dominant and stable strategy in very large social groups, provided that certain conditions are met. We have shown that cooperation can be sustained in a large group with a limited number of interactions on each round, if players are able to base decisions about future play on the results of a number of rounds of past play and are permitted to refuse to play with others when necessary. In evolutionary terms the rewards for this cooperation would give a significant advantage to individuals in the group, who would, however, need to store more information to co-ordinate relationships with other players and would require greater processing power to do this effectively