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Chimpanzees often share and share alike when cooperating in pairs, suggesting that these apes come close to a human sense of fairness, a controversial new study finds.
Like people, chimps tend to fork over half of a valuable windfall to a comrade in situations where the recipient can choose to accept the deal or turn it down and leave both players with nothing, say psychologist Darby Proctor of Yerkes National Primate Research Center in Lawrenceville, Ga., and her colleagues. And just as people do, chimps turn stingy when supplied with goodies that they can share however they like, the researchers report online January 14 in the Proceedings of the National Academy of Sciences.
“Humans and chimpanzees show similar preferences in dividing rewards, suggesting a long evolutionary history to the human sense of fairness,” Proctor says.
But psychologist Josep Call of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, considers the new results “far from convincing.” In Proctor’s experiments, pairs of chimps interacted little with each other and showed no signs of understanding that some offers were unfair and could be rejected, Call says.
“If anything, Proctor’s study suggests that there is no fairness sensitivity in chimpanzees,” remarks psychologist Keith Jensen of the University of Manchester in England.
Call and Jensen coauthored two earlier studies in which chimps given food generally shared as little as possible with partners, who accepted most offers. Apes on the receiving end affirmed an offer by pulling food within reach using a mechanical device or refused an offer by doing nothing for 30 seconds.
In the new study, chimps and preschool children were tested in a way that Proctor contends is closer to a cooperation task known as the ultimatum game that is used in experiments with adults. In the game’s standard version, one player splits a pot of money with another player. If the receiver accepts the proposer’s offer, both players keep their shares. If the receiver rejects the offer, both players get nothing.
Proposers fork over 40 percent to 50 percent of the pot in most human cultures. A concern with fairness and a fear of retaliation for lowball offers prompts these generous offers, Proctor says.
Her group studied six adult chimps at an outdoor research facility and 20 preschool children ages 2 to 7. Four pairs of chimps and 10 pairs of kids played a modified ultimatum game, in which a proposer can offer one of two tokens to a receiver. Accepted tokens got handed to an experimenter in exchange for rewards. One token represented an even split of six banana slices (for the chimps) or of six stickers (for the children). The other token granted the proposer five of six rewards.
Each ape duo played the game 24 times. Pairs also played a game in which receivers had to accept all offers.
Proposers opted for even splits much more frequently when a partner could reject offers. Two pairs of chimps split banana slices equally substantially more often than expected by chance.
Preschoolers playing these games behaved much as chimps did.
In both species, receivers exchanged all tokens for rewards, even those for unfair deals. Neither chimps nor kids were trained that refusal was an option, but the mere threat of a partner’s retaliation motivated proposers to share equally, Proctor proposes.
Jensen disagrees with that conclusion. Receivers’ acceptance of all offers “suggests that they were not sensitive to unfairness but were motivated only by getting rewards,” he says. That undermines any suggestion that chimp or child proposers assumed that their offers could be rejected.
Psychologist Darby Proctor explains and shows how her research team tested chimps’ willingness to propose fair and unfair food offers to a partner.
Credit: D. Proctor
