Several years ago an American psychology professor, Sarah Brosnan, carried out an experiment with capuchin monkeys. She trained five pairs to give her a token in return for a slice of cucumber. Then she changed the nature of the trade. When they handed over the token, one of the couple received a juicy grape; the other got the cucumber slice as before.

The effect was remarkable. When the monkeys getting the cucumber saw their partner getting a better gift, many of them refused to co-operate. Rather than eating the cucumber, they decided to go hungry instead. Sometimes they would throw away their tokens, sometimes the cucumber. This irrational response was, Brosnan concluded, evidence of “an early evolutionary origin of inequity aversion”. It hinted at the