Winners don't punish - PubMed (original) (raw)

Winners don't punish

Anna Dreber et al. Nature. 2008.

Abstract

A key aspect of human behaviour is cooperation. 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 cooperation. 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 effects. 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.

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Figure 1

Figure 1. Payoff values

A: The game is formulated in terms of unilateral moves. There is the choice between cooperation, C, defection, D, and costly punishment, P. Cooperation means paying a cost, c, for the other person to get a benefit, b. Defection means earning a payoff, d, at a cost, d, for the other person. Punishment means paying a cost, α, for the other person to incur a cost, β. B: The payoff matrix is constructed from these unilateral moves. C and D: The actual payoff matrices of our experiments.

Figure 2

Figure 2. Games people played

There were 1230 pair-wise, repeated interactions each lasting between 1 and 9 rounds. Here are some examples (B, E and G are from T1, the others from T2.) The two players’ moves, the cumulative payoff of that interaction and the final rank of each player (sorted from highest to lowest payoff) are shown. A: All-out cooperation between two top-ranked players. B: Punish and perish. C: Defection for defection can sometimes restore cooperation. D: Turning the other cheek can also restore cooperation. E: Mutual punishment is mutual destruction. F: Punishment does not restore cooperation. Player 1 punishes a defection, which leads to mutual defection. Then player 1 is unsatisfied and deals out more punishment. G: “Guns don’t kill people, people kill people”. (Punishment itself is not destructive, only the people who use it.) Here, an unprovoked first strike destroys cooperation. The option to punish allows irrational people to inflict harm on the undeserving.

Figure 3

Figure 3. Punish and perish

In both treatments T1 (red) and T2 (blue), there is no correlation between average payoff per round and (i) cooperation use (Quantile regression; A, p = 0.33; B, p = 0.21) or (ii) defection use (C, p = 0.66; D, p = 0.36). However, there is a significant negative correlation between average payoff per round and punishment use in both treatments (E, slope = -0.042, p < 0.001; F, slope = -0.029, p = 0.015). Punishment use is the overriding determinant of payoff. G and H: Ranking players according to their total payoff shows a clear trend: players with lower rank (higher payoffs) punish less than players with higher rank (lower payoff).

Figure 4

Figure 4. Tit-for-tat prevails over costly punishment

Lower payoffs are correlated not only with punishment use, but specifically with choosing to punish after the opponent has defected. The probability of punishing immediately after a co-player’s defection is negatively correlated with the average payoff per round, both in T1 and T2 (Quantile regression; A, slope = -0.81, p < 0.001; B, slope = -0.94, p = 0.015). Thus, the lower payoffs of punishers were not caused by the bad luck of interacting with defectors. Winners use a tit-for-tat like approach (D for D), while losers use costly punishment (P for D).

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