Undiscounted costs and socially discounted benefits modulate cooperation in one-shot and iterated prisoner's dilemma games.

Cooperation involves an individual's choice that benefits both themself and others -in contrast to selfishness, which benefits the individual only-and has been suggested to be more likely when the benefit to others, discounted as a function of their social distance (i.e., social discounting), exceeds the undiscounted cost to the cooperator. To test this hypothesis, we exposed 126 participants to eight, one-shot reward matrices of prisoner's dilemma games, among which socially discounted benefits and undiscounted costs systematically varied. Increasing benefits and increasing costs increased and decreased, respectively, the percentage of cooperators across the matrices. Then, 111 participants from the original sample completed one of five iterated, 40-trial reward matrices programmed to play a tit-for-tat strategy, among which benefits and costs varied. Overall, increasing benefits and increasing costs increased and decreased, respectively, cooperation across trials. This tendency, however, was more clearly observed in later than earlier trials. Both in one-shot and in iterated games, the effect of costs was greater than that of benefits and the effects of both benefits and costs decreased at extreme values. These findings suggest that cost-benefit balance, modulated by social discounting, determines degree of cooperation in both one-shot and repeated-trial cooperation tasks.