The Algebra of Assortative Encounters and the Evolution of Cooperation

This paper explores the way in which assortative matching can maintain cooperative behavior under evolutionary dynamics. If encounters are random, then in Prisoner's Dilemma games, defectors necessarily get higher payoffs than cooperators and thus will eventually prevail. But if matching is assortative, the cost of cooperating may be repaid by higher probabilities of playing against a cooperating opponent. This paper shows that a simple index of assortativity allows a unifying treatment of the evolutionary dynamics in a wide variety of models of social encounters.