A key aspect of human behaviour is cooperation1,2,3,4,5,6,7. We tend to help others even if costs are involved. We are more likely to help when the costs are small and the benefits for the other person significant. Cooperation leads to a tension between what is best for the individual and what is best for the group. A group does better if everyone cooperates, but each individual is tempted to defect. Recently there has been much interest in exploring the effect of costly punishment on human cooperation8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23. Costly punishment means paying a cost for another individual to incur a cost. It has been suggested that costly punishment promotes cooperation even in non-repeated games and without any possibility of reputation effects10. But most of our interactions are repeated and reputation is always at stake. Thus, if costly punishment is important in promoting cooperation, it must do so in a repeated setting. We have performed experiments in which, in each round of a repeated game, people choose between cooperation, defection and costly punishment. In control experiments, people could only cooperate or defect. Here we show that the option of costly punishment increases the amount of cooperation but not the average payoff of the group. Furthermore, there is a strong negative correlation between total payoff and use of costly punishment. Those people who gain the highest total payoff tend not to use costly punishment: winners don’t punish. This suggests that costly punishment behaviour is maladaptive in cooperation games and might have evolved for other reasons.
Support from the John Templeton Foundation, the National Science Foundation (NSF)–National Institutes of Health joint program in mathematical biology, the Jan Wallander Foundation (A.D.) and an NSF grant (D.F.) is gratefully acknowledged. The Program for Evolutionary Dynamics is sponsored by J. Epstein.
The file contains Supplementary Figures S1-S6 and Supplementary Data, and Supplementary Notes with experimental instructions.