Does Inequity Aversion Motivate Punishment? Cleaner Fish as a Model System

In human social interactions, punishment is often directed at cheating individuals. Subjective reports and neuro-imaging studies indicate that the experience of interacting with a cheat produces negative emotions and that the act of punishing a cheat assuages these feelings. However, while negative emotions may elicit punishment, the precise source of these emotions remains obscure. Specifically, it is often very difficult to tease apart whether punishing individuals are inequity averse (upset because cheating partner receives more than they should) or, more simply, whether they might be loss averse (upset because their payoffs did not meet their expectations). We compare results on punishment and inequity aversion in humans with results from a non-human model system, the cleaning mutualism between bluestreak cleaner wrasse (Labroides dimidiatus) and its reef-fish 'clients'. Male cleaner fish are known to punish females that cheat during joint client inspections, but a recent study failed to demonstrate evidence for inequity aversion in this species. We suggest that punishment in cleaner fish may be motivated by loss aversion rather than inequity aversion. Punishment in humans might also often be motivated by loss aversion-and empirical studies that disentangle the two competing motives for punishment are a clear research priority.