The role of social structure in the maintenance of cooperative regimes

We analyze the role of social structure in maintaining cooperation within a population of adaptive agents for whom cooperative behavior may be costly in the short run. We use the example of a collection of agents playing pairwise Prisoner's Dilemma. We call sustained cooperative behavior in such circumstances a 'cooperative regime'. We show that social structure, by channeling which agents interact with which others, can sustain cooperative regimes against forces that frequently dissolve them. We show in detail the process through which structured interaction in a population creates a 'shadow of the adaptive future', allowing even a small set of cooperative strategies to grow into a cooperative regime, a coherent, self-sustaining entity that is something more than the sum of the pairwise interactions among its members.