Inequity aversion in capuchins?

Brosnan and de Waal1 have shown that capuchin monkeys are more likely to reject a cucumber slice after seeing that another capuchin has received a more attractive grape. In interpreting this finding, the authors make a link to work in humans on ‘inequity aversion’ and suggest that capuchins, like humans, may reject rewards because they are averse to unequal pay-offs. Here I argue that this interpretation suffers from three problems: the results contradict the predictions of the inequity-aversion model that Bosnan and de Waal cite2; experimental results indicate that humans do not behave like capuchins in similar circumstances; and the available evidence does not suggest that inequity aversion is cross-culturally universal3,4,5.