Let’s Make a Deal

Do chimpanzees know how to strike an even deal? They just might, according to a recent study in the Proceedings of the National Academy of Sciences. The paper’s findings suggest that chimps understand the concept of fairness, questioning whether the trait is uniquely human.

The researchers used a version of the “ultimatum game,” a test normally reserved for measuring fairness in humans. In a typical game, two human participants must work together to decide how they will share a given amount of money. One proposes how evenly to divide the money, and the other decides whether to accept the proposal or reject it. If the offer is declined—because the recipient considered his share unfair, for instance—neither participant receives anything.

In the recent study, researchers modified the game for six chimps, divided into pairs. One chimp chose between two tokens and offered it to another chimp, called the “responder.” The responder could then exchange the token for a reward—in this case, bananas—that was distributed among both chimps. One token led to an even banana split (an inevitable pun), while the other delivered more fruit to the chimp that had offered the token. The responder could alternatively decide not to cash in the token at all, leaving both chimps with nothing. The game was also played with a group of children to compare how each species behaved.

The researchers found that 75 percent of the time, chimps offered a token that led to equal portions of banana for both participants—in other words, fair shares. But when researchers changed the game so that the responder chimp wasn’t allowed to reject any offers, chimps doled out more tokens that led to unequal fruit distribution. The results aligned with what the researchers observed in the children, suggesting that the animals must have some concept of fairness akin to that in humans, the study concludes.

So why would a chimp—or anyone else, for that matter—make a fair offer? Frans de Waal, a founder of the field of primate behavior and co-author of the paper, has two theories. The first is what he calls the selfish theory, which suggests that an individual will offer a fair deal to peers because they may not be willing to work with him for uneven compensation. “If I want to get the cooperation of my partner, I have to be fair, otherwise he’ll bail on me,” said de Waal in a phone interview with Science Friday. In this case, “bailing” would mean rejecting a token offering unequal banana treats.

The second theory concerns the concept of fairness on a higher level, says de Waal—that is, the idea that it would be immoral to offer an uneven deal. While it is assumed that humans can be motivated by morality, whether chimps have such inclinations is uncertain.

Keith Jensen, a co-author of multiple papers investigating fairness in chimps, isn’t convinced that the study detected fairness in the apes, however. The reason, he says, is because none of the chimps rejected unfair offers when they were permitted to do so. “It’s the power of rejection that’s the test of fairness,” Jensen said in a phone interview. In other words, if the responder chimp had a sense of equality, it should have refused an unequal share so as “to cause the proposer to suffer for his selfishness, even though it is costly to do so,” Jensen wrote in an email exchange. Jensen claims that without rejections, what occurred between the chimps cannot be considered an ultimatum game. “It is the responder’s rejection that makes the ultimatum game the ultimatum game,” he wrote in an email. “The threat of rejection IS the ultimatum. There was no threat of rejection.”

There are a couple of explanations for the lack of rejections, Jensen says. It’s possible that the chimps didn’t understand that refusing an offer was an option. Second, perhaps they just wanted bananas regardless of how many the other chimp got. “If they are concerned just about maximizing rewards, then they should accept any offer at all,” according to Jensen, “And that’s precisely what the responder chimpanzees in the [study] had done.”

But De Waal points out that there was still a difference between the offers made when a participant did or didn’t need the approval of the responder chimp to receive a reward. Further, the chimps performed like the children tested in the experiment. Says de Waal, “The most elegant assumption is, if you find humans and chimpanzees to do basically the same thing under similar circumstances, assume that the motivations are similar also.”

If there’s any consensus, it’s a demand for more research.