Punishment is a costly behaviour that is often aimed at individuals who cheat during social interactions. Although punishers make an initial investment to harm cheats, the investment may be repaid if the cheat behaves more cooperatively in future interactions [1
]. Identifying the motives underpinning human punishment is crucial as punishment plays an important role in the maintenance of cooperation in human societies [3
]. Several recent studies have shown that players experience negative emotions, such as anger or disgust, when they interact with cheats and that the intensity of these emotions is positively associated with the desire to reciprocally harm cheating partners [4
]. The act of administering punishment provides relief from negative emotions as it activates reward centres in the brain [7
]. In this way, punishment can be subjectively rewarding. Although negative emotions motivate punishment, it is not yet clear why these emotions are produced during interactions with cheats. One possibility is that negative emotions are caused by disadvantageous inequity aversion (hereafter ‘inequity aversion’), or the disutility associated with experiencing lower payoffs than a cheating partner [8
]. However, a simpler alternative is that victims of cheats experience negative emotions because cheats violate cooperative norms, thereby imposing losses on cooperative partners [4
]. Thus, in some contexts, punishment may be motivated by the desire to reciprocally harm cheating partners, even if the cheating partner did not experience higher payoffs than the victim. Thus, experiencing losses without simultaneously experiencing unequal payoffs may suffice to motivate punishment, although this possibility has not been tested.
The concepts of inequity aversion and of reciprocity both predict that punishment decisions will be influenced by the partner's behaviour. However, inequity aversion predicts that individuals are sensitive to how their payoffs compare with those of interaction partners, whereas the concept of reciprocity predicts that punishment decisions are influenced by how payoffs compare with individual expectations and are therefore independent of the relative payoffs gained by cheating partners. It is hard to disentangle whether punishment is motivated by a desire to reciprocate losses or by inequity aversion because players involved in interactions with cheats often simultaneously experience losses and inequity [9
]. For example, evidence from laboratory public goods games (where contributions to a communal account are altruistic in the sense that they yield benefits to other group members at a cost to the donor; [11
]) has shown that players experience anger and disgust when interacting with non-contributing (‘free-riding’) group members. The intensity of these emotions correlates with the propensity to administer costly punishment to reduce free-riders' incomes [4
]. However, in such games, cooperative individuals (those who contribute to the communal account) experience absolute losses and lower payoffs relative to those of free-riders, meaning that it is not clear whether the negative emotions produced from interactions with cheats arise from a desire to reciprocate losses or from inequity aversion [10
]. A more recent study used a random income game (where players were randomly allocated different-sized earnings) to show that punitive behaviour can be motivated by inequity, even in the absence of losses [12
]. Low-earning players in this game experienced negative emotions targeted towards higher-earning counterparts and were willing to pay a cost to reduce the income of high earners [12
]. Thus, punitive behaviour can arise even when higher-earning players do not impose losses on lower-earning individuals and when there is no cooperative norm to be enforced. In a subsequent study, it was shown that the tendency to reduce the income of high earners in a random income game is positively associated with the propensity to punish free-riders in public goods games [13
]. While these findings indicate that punishment may be motivated by inequity aversion, neither study asked whether punishment might also be motivated by losses in the absence of inequity. We tested this possibility here.
We designed an experiment based on a simplified version of the moonlighting game [14
] to determine whether human punishment is motivated by a desire to reciprocate losses or by inequity aversion. Subjects were assigned to one of two roles, player 1 (P1) or player 2 (P2), and were allocated money according to one of three treatments (A–C). In treatment A, P1 was given $0.70 and P2 was given $0.10. In treatment B, P1 was given $0.70 and P2 was given $0.30. In treatment C, P1 was given $0.70 and P2 was given $0.70. The game consisted of two stages: in the first stage, P2 could choose to ‘cheat’ by taking $0.20 of P1's endowment and in the second stage P1 could choose to punish P2 (pay $0.10 to reduce P2's income by $0.30). In treatment A, P1 maintained a higher payoff than P2 when P2 cheated ($0.50 versus $0.30). In treatment B, cheating by P2 produced equal outcomes ($0.50 each). In treatment C, cheating by P2 meant that P1 got $0.50 while P2 got $0.90. Thus, P1 experienced the same losses in all three treatments and always finished with a payoff of $0.50 if P2 cheated. Crucially, however, only in treatment C did cheating by P2 result in P1 experiencing lower payoffs than P2 (disadvantageous inequity). This setup therefore allowed us to disentangle the effect of experiencing losses from the effect of experiencing inequity as motivators for punishment.