Chimps have the same sense of fairness as human beings, say researchers who played the Ultimatum game with them - the first time the trait's been observed in a non-human species.
The researchers say their findings suggest that as a species we've had an aversion to unfairness for a long time - indeed, since the common ancestor of humans and apes.
"We used the Ultimatum Game because it is the gold standard to determine the human sense of fairness. In the game, one individual needs to propose a reward division to another individual and then have that individual accept the proposition before both can obtain the rewards," says Darby Proctor of the Yerkes National Primate Research Center at Emory University.
"Humans typically offer generous portions, such as 50 percent of the reward, to their partners, and that's exactly what we recorded in our study with chimpanzees."
The team tested six adult chimpanzees (Pan troglodytes) and 20 human children aged between two and seven years old with a modified version of the Ultimatum Game.
One individual chose between two differently colored tokens that, with the partner's cooperation, could be exchanged for rewards. One token offered equal rewards to both players, whereas the other favored the individual making the choice at the expense of the partner.
The chooser then needed to hand the token to the partner, who needed to exchange it with the experimenter for food. This way, both individuals needed to be in agreement for either to get the reward.
Both the chimpanzees and the children responded in the same way as adult humans typically do. If the partner's cooperation was required, the chimpanzees and children split the rewards equally. However, with a passive partner, who had no chance to reject the offer, both groups chose the selfish option.
"Until our study, the behavioral economics community assumed the Ultimatum Game could not be played with animals or that animals would choose only the most selfish option while playing," says Yerkes's Frans de Waal.
"We've concluded that chimpanzees not only get very close to the human sense of fairness, but the animals may actually have exactly the same preferences as our own species."