Classic game theory considers defection to be the rational choice in the Prisoner’s Dilemma. Although defection maximizes a player’s gain regardless of the opponent’s choice, many players cooperate. We suggest that cooperation can be explained in part by expectations of reciprocal behavior, that is, by the belief that players facing the same situation probably will come to the same decision. In Experiment 1, expectations of reciprocity were experimentally manipulated. As predicted, cooperation increased monotonically with these expectations. In Experiment 2, experimentally manipulated expectations of opponent cooperation were not associated with higher rates of cooperation. These findings are interpreted in light of a general model of payoff maximization, and implications for other models (e.g., social value orientation) are discussed.