The Ultimatum Game is the gold standard of the human sense of fairness. People have played it all over the world, but until now nonhuman animals have never successfully engaged in this game. One individual needs to propose a reward division to another that the other needs to accept before both can obtain the rewards. Obviously, this procedure is not easily arranged without language.
Researchers at the Yerkes National Primate Research Center, Emory University, in collaboration with colleagues from Georgia State University, are the first to obtain positive results with chimpanzees. They have shown that our closest relatives possess a sense of fairness that has previously been claimed to be uniquely human. The findings are now available in an early online edition of the Proceedings of the National Academy of Sciences (PNAS), suggesting a long evolutionary history of the human aversion to inequity. Possibly, the common ancestor of apes and humans shared a similar preference for fair outcomes.
The Yerkes team wanted to determine how sensitive chimpanzees are to the reward distribution between two individuals if both need to agree on the outcome. According to first author Darby Proctor, PhD, “We used the Ultimatum Game because it is the ultimate test of the human sense of fairness given both partners must agree on a distribution for either to receive rewards.” Humans typically offer generous portions (such as 50%) of the reward to their partners, and that’s exactly what Proctor and her team recorded in their study with chimpanzees.
Co-author Frans de Waal, PhD, adds, “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. We’ve concluded that chimpanzees not only get very close to the human sense of fairness, but that they may actually have exactly the same preferences as our own species.” For purposes of direct comparison, the study was also conducted separately with human children.
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 colored 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 favored 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/children split the rewards equally. However, with a passive partner, who had no chance to reject the offer, they went for 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 behavior.