In a new study, researchers have shown for the first time that chimpanzees possess a sense of fairness that has previously been attributed as uniquely human.
Working with colleagues from Georgia State University, researchers at the Yerkes National Primate Research Center, Emory University, played the Ultimatum Game with the chimpanzees to determine how sensitive the animals are to the reward distribution between two individuals if both need to agree on the outcome.
According to the researchers, the findings of the study suggest that a long evolutionary history of the human aversion to inequity as well as a shared preference for fair outcomes by 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,” first author Darby Proctor said.
“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,” Proctor said.
In the study, researchers tested six adult chimpanzees (Pan troglodytes) and 20 human children (ages 2 – 7 years) on a modified Ultimatum Game. One individual chose between two differently coloured tokens that, with his or her partner''s cooperation, could be exchanged for rewards (small food rewards for chimpanzees and stickers for children).
One token offered equal rewards to both players, whereas the other token favoured the individual making the choice at the expense of his or her 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.
Both the chimpanzees and the children responded like 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, chimpanzees and children chose the selfish option.
Chimpanzees, who are highly cooperative in the wild, likely need to be sensitive to reward distributions in order to reap the benefits of cooperation.
Thus, this study opens the door for further explorations into the mechanisms behind this human-like behaviour.
The study has been published online in Proceedings of the National Academy of Sciences (PNAS).