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If you give a mouse a cookie, will it give you something back? Researchers have long wondered whether animals play fair. A new study in chimps suggests that they do, although some skeptics are not convinced.

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.”

The study’s “results are clear cut”, says Elisabetta Visalberghi, a primatologist at the Institute of Cognitive Sciences and Technologies in Rome. But she questions why the responder chimps in the new study, like those in the Leipzig experiments, never refused apparently unfair offers, even though humans routinely do so, and why the proposer chimps offered equitable splits even though unfair divisions would not be rejected. “Are they more fair than us?” Visalberghi asks.

For developmental psychologist Michael Tomasello, a leader of the Leipzig team and co-author of the 2007 study, these findings suggest that the experiment didn’t test for chimp fairness. “This is not Ultimatum Game performance at all,” Tomasello argues, because the key participant in the game is supposed to be the responder, not the proposer. “Human responders reject unfair offers, presumably out of a sense of fairness.” And the chimp proposers, Tomasello adds, may have offered more food in fear that their offer would eventually be rejected, rather than out of a sense of fairness on their own part.

James Anderson, a psychologist at the University of Stirling in the United Kingdom, agrees with the authors that “chimpanzees do not always act from a purely selfish perspective.” But he, too, would like to see the mystery of the responders’ failure to reject unfair offers investigated further. “I’d like to see a version of the game in which the responder can actively reject the proposal,” Anderson says, “rather than simply not return the token. For example, if there was an option to put the token into some kind of trash can.” Such a setup, Anderson says, might more closely replicate the human version of the Ultimatum Game.

This story provided by ScienceNOW, the daily online news service of the journal Science.