The researchers suggest that capuchin monkeys, like humans, are guided by social emotions. In the wild, they are a cooperative, group-living species. Such cooperation 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 cpmpletely makes these feelings abundantly clear to other member 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 35 million years ago, is, as yet, an unanswered question.
