BioEd Online

Chimpanzees will tolerate unfair treatment, as long as it benefits someone they know well, say US researchers. This is the first time such behaviour has been demonstrated outside the human race.

Sarah Brosnan and Frans de Waal, primatologists at Emory University in Atlanta, gave chimpanzees a piece of plastic and rewarded them for giving it back. If a subject is given a paltry payoff, such as a cucumber slice or celery stick, and it can see another getting a grape, the short-changed ape refuses to cooperate.

But the strength of each chimpanzee's response depends on its social life. Those that had lived together for more than 30 years ignored the unequal treatment; whereas animals from a group formed eight years ago and pairs of chimpanzees reacted strongly, the researchers report in the Proceedings of the Royal Society B¹.

Likewise, people in close relationships ignore fairness and "operate on a broader base of trust", says de Waal. "Close people, such as spouses, do not keep track of every little favour they do each other, whereas more distant people do."

An aversion to inequality may have evolved alongside cooperation. "In a highly cooperative society, such as those of humans and chimpanzees, one needs to monitor one's own efforts compared with others, and one's own rewards compared with others to avoid being taken advantage of," says de Waal.