Fig. 1.

In the PD, players can either cooperate (C) or defect (D). Although mutual cooperation yields the socially optimal outcome of four tokens per player, each subject has an incentive to defect as a dominant strategy. Defection of both players is the Nash equilibrium of the game, yielding a payoff of two for each player. The first (second) number in each cell indicates player 1’s (player 2’s) payoff.