CHIMPS recognise unfairness, even when it involves individuals other than themselves. This sense of unfairness towards others may be a rudimentary form of the social justice that characterises human societies.

In earlier studies several apes, monkeys and even dogs responded negatively when they received a meagre reward for the same task that earned others a more lavish pay-off. But none of these animals apparently recognised unfairness directed at others.

Sarah Brosnan, a primate behaviourist at Georgia State University in Atlanta, and her colleagues trained captive chimps to exchange tokens for a food reward, then tested how same-sex pairs of chimps reacted to various levels of reward. As expected, chimps were more likely to reject a boring carrot when their partner got a yummy grape for the same token. Surprisingly, the chimps were also more likely to reject a grape if their partner only got a carrot (Animal Behaviour, DOI: 10.1016/j.anbehav.2010.02.019).

In previous experiments, other groups of chimps showed no sensitivity towards unfairness directed at others. Perhaps Brosnan's animals rejected their "undeserved" grapes in part because they sat right next to their less fortunate partner and may have feared retaliation for their windfall, the researchers suggest.

But the chimps' awareness of the mistreatment of others as well as themselves also lays the groundwork for complex social interactions more like those of human groups, they note.