In the world of capuchins, grapes are luxury goods (and much preferable to cucumbers). So when one monkey was handed a grape in exchange for her token, the second was reluctant to hand hers over for a mere piece of cucumber. And if one received a grape without having to provide her token in exchange at all, the other either tossed her own token at the researcher or out of the chamber, or refused to accept the slice of cucumber. Indeed, the mere presence of a grape in the other chamber (in the absence of an actual monkey able to eat it) was enough to induce sullen behaviour in a female capuchin.

　　Dr Brosnan and Dr de Waal report that such behaviour is unusual in their trained monkeys. During two years of bartering prior to these experiments, failure to exchange tokens for food occurred in fewer than 5% of trials. And what made the behaviour even more extraordinary was that these monkeys forfeited food that they could see—and which they would have readily accepted in almost any other set of circumstances.

　　The researchers suggest that capuchin monkeys, like humans, are guided by social emotions. In the wild, they are a co-operative, group-living species. Such co-operation is likely to be stable only when each animal feels it is not being cheated. Feelings of righteous indignation, it seems, are not the preserve of people alone. Refusing a lesser reward completely makes these feelings abundantly clear to other members of the group. However, whether such a sense of fairness evolved independently in capuchins and humans, or whether it stems from the common ancestor that the species had 35m years ago, is, as yet, an unanswered question.