Reciprocity and shared knowledge structures in the prisoner's dilemma game

A prominent solution to achieving cooperation in prisoner's dilemma situati ons is repeated interaction between players. Although indefinitely repeated play solves the mutual gains problem, it also creates an unsolved coordina tion problem because an infinite number of strategies are possible in equil ibrium. This article explores whether a "shared grammar of strategies," for malized by a knowledge-induced equilibrium, resolves the coordination probl em by prescribing a unique behavioral rule. Applied to the set of strategie s submitted to Axelrod's prisoner's dilemma tournament, tit for tat emerges as that unique coordinating strategy.