Winners don’t punish

Many theories have been offered to explain the evolution of cooperation in humans. One proposal is that costly punishment can promote cooperation. Everyone benefits on average, the theory goes, despite the cost to those doing the punishing. But most of our interactions are repeated, and in such cases punishment can lead to retaliation. Using a variant of the 'Prisoner's Dilemma' game, Dreber et al. find that punishment increases the frequency of cooperation, but not the average payoff. Costly punishments confer no overall advantage to the group. And players who end up with the highest total payoff ('winners') tend not to use punishment, while those with the lowest payoff ('losers') punish most frequently. It seems that costly punishment may not have evolved to promote cooperation, but for some other purpose.