Where does our moral sense come from? Those of a fundamentalist persuasion say “God,” and leave it at that, but a more interesting answer comes from research on the nonhuman primates we share ancestors with.

Economists used to say that people are self-interested and rational, maximizing whatever payday is within reach. But recent studies have blown that idea to smithereens. When people are given the choice of accepting or rejecting the split of some spoils that a partner offers—say, how to divide the $10 that researchers have given them in an experiment—they reject offers perceived as unfair. So if you offer me $2 and propose to keep $8 for yourself, I’ll walk away and leave us each with nothing—stupid, considering that I’m rejecting $2 in free money, but consistent with the emerging idea that humans have a strong, evolved sense of fairness that trumps immediate self-interest. Something like this probably underlies people’s tendency to punish cheaters, free-riders and noncooperators. The game has been played uncounted times in labs, and the basic finding is that proposers typically offer 40 to 50 percent of the pot, and responders walk away from any offer less than 20 percent.

Now there is evidence that a fine-tuned sense of justice—insisting on fairness at the expense of tangible rewards—may be uniquely human, rather than something our closest living relatives also have.

In a study being reported today in Science, researchers had 11 chimpanzees at the Wolfgang Köhler Primate Research Center in Germany play this “ultimatum” game. One chimp, the “proposer,” sat beside the “responder.” The proposer pulled out a tray as far as he could. The tray held two dishes with raisins, separated by a see-through divider: one for the proposer and the other for the responder. The proposers first chose which tray to pull out; if the responder liked what he saw—and he could see how many raisins he and the proposer would each get, by seeing how many raisins were on each side of the divider—he accepted the offer by pulling the tray the rest of the way out. Both chimps would then chow down. If the responder did not like the offer, he refused to pull the tray the rest of the way out, and neither chimp got a snack.

If the dishes held the same number of raisins, the responder chimp almost always accepted a 50-50 offer and rejected a 100-0 offer. Unlike people, though, they rarely rejected 80-20 offers—only 5 to 14 percent of the time. And unlike people, who fume when confronted with unfair offers, the chimps almost never took umbrage, throwing a tantrum at an unfair offer a mere 2 percent of the time.

There has long been a debate over whether chimps are able to sense fairness, much less tolerate unfairness. These results suggest that chimps behave “according to traditional economic models of self-interest, unlike humans, and that this species does not share the human sensitivity to fairness,” the scientists write. As scientists find fewer and fewer fundamental human traits to be unique (see the previous post on tool-using animals), at least we can keep hold of this one.