Everybody loves a fat pay rise. Yet pleasure at your own can vanish if you learn that a colleague has been given a bigger one. Indeed, if he has a reputation for slacking, you might even be outraged. Such behaviour is regarded as “all too human”, with the underlying assumption that other animals would not be capable of this finely developed sense of grievance. But a study by Sarah Brosnan and Frans de Waal of Emory University in Atlanta, Georgia, which has just been published in Nature, suggests that it is all too monkey, as well.

　　The researchers studied the behaviour of female brown capuchin monkeys, which have all the necessary ingredients to capture the public imagination. They look cute. They are good-natured, co-operative creatures, and they share their food readily. Above all, like their finicky female human counterparts, they tend to pay much closer attention to the value of “goods and services” than males (although why this is so remains a mystery).

　　Such characteristics make them perfect candidates for Dr Brosnan's and Dr de Waal's study. The researchers spent two years teaching their monkeys to exchange tokens for food. Normally, the monkeys were happy enough to swap pieces of rock for slices of cucumber. However, when two monkeys were placed in separate but adjoining chambers, so that each could observe what the other was getting in return for its rock, their behaviour became markedly different.