When economists and psychologists want to test fairness in humans, they turn to the Ultimatum Game. Typically, one subject, called the “proposer,” is given a sum of money to divide with a “responder.” If the responder accepts the proposer’s offer, both are rewarded. But if the responder rejects the offer, both get nothing. Although results vary, human proposers usually offer about 40 percent to 50 percent of the money, and responders reject offers that are under 20 percent — even though they will end up with nothing for doing so. Researchers interpret both behaviors as evidence for a basic sense of fairness.
In 2007, a team at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, tried the Ultimatum Game on chimps. In a simplified version of the game, the team gave the chimps set options about how to divvy up dishes of raisins, such as a 50-50 split or an 80-20 split. The proposer, in a cage next to that of the responder, made an offer by pulling a tray halfway toward the responder. The responder had to choose between pulling it the rest of the way so that both chimps could get the food or refusing to pull it at all, in which case both chimps got nothing. In contrast with humans, chimps rarely rejected offers of 20% of the food; they only rejected such tenders in experiments where the proposer had the additional choice of taking all of the food and leaving the responder with nothing. The team concluded that the responders would accept even the meanest split as long as they got something, suggesting that unlike humans, they were not offended by blatantly unfair offers.
Some researchers pounced on the study, however, suggesting that chimps couldn’t be expected to play fair given the conditions of their captivity, which had taught chimps that they had little control over how much they got to eat, or that they might not be able to understand the complicated tray-pulling apparatus.
To try to resolve the debate, a team led by primatologist Frans de Waal of Emory University in Atlanta has developed what it considers a more chimp-friendly version of the Ultimatum Game. In previous work, de Waal and his co-workers had established that chimps could be trained to exchange tokens for food. In the new study, published online this week in the Proceedings of the National Academy of Sciences, de Waal and his colleagues trained six chimps to recognize that such colored tokens, made of short pieces of plastic pipe, represented one of two ways of dividing six banana slices: an equal 3-3 split or an unequal 5-1 split (see video). The proposer chimp chose one of the tokens and then handed it through the wire mesh of its cage to a responder chimp, which had to give it to a human experimenter for both chimps to get the food. In essence, the tokens served as money that could be exchanged for the banana slices.
The chimps seemed to be playing fair. The percentage of equitable (3-3) offers ranged from 58% to 92%, much higher than in the Leipzig study which averaged about 25% for such 50-50 splits. However, as in the Leipzig study, the chimps never rejected “unfair” offers of 5-1 splits.
The team concluded that a sense of fairness arose sometime before the chimp and human lines split some 5 million to 7 million years ago, and that doing right by others has a long evolutionary history. “When we see this kind of behavior in humans, we call it fairness,” says the study’s lead author, psychologist Darby Proctor, now of Emory University, who adds that researchers shouldn’t “hesitate to call it fairness in chimps.”