What Chimps Understand About Reciprocity

A new study suggests that one of our closest relatives can show intuitions of fairness



July 21, 2017 11:52 a.m. ET

See the column on the Wall Street Journal site


When someone does something nice for you, do you return the favor? Most of us do, and not just because our mothers said we should.

Basic fairness is probably written into our genetic code. Human societies depend on the expectation of reciprocity: We assume that a neighbor will collect our mail if we’ve mowed their lawn, or that drivers will take turns braking at stop signs.

Fundamental as this trait might seem, however, its evolutionary origins are hazy. Previous research has shown that chimpanzees—one of our closest relatives—are less motivated by fairness than by what they immediately stand to gain from a transaction.

A new study shows that chimps can go beyond such reflexive selfishness and cooperate, even if it costs them something. But they don’t just give up what’s theirs, even to their kin. They are particular about when they will share some of their food, according to researchled by University of Vienna biologist Martin Schmelz and just published in Proceedings of the National Academy of Sciences.

Like many of us, the team found, chimps keep score: They’re most likely to allot treats to a partner if that chimp helped them first.

How do we know this, since chimps can’t discuss their intentions? Dr. Schmelz and his colleagues constructed an apparatus in which two chimps face one another with a cage between them. It contains several bowls of food, which the chimps can’t reach.

One chimp can pull on a string, thus lifting a latch that gives the other chimp access to the food in the cage. That chimp then has a decision to make: It can keep a larger portion entirely for itself—with nothing for its partner—or opt for two smaller but equal amounts. The helper chimp had been trained to always pull the string, thus giving its partner access to the bowls of food and the impression of generosity.

In these circumstances, the team found, the chimp given access to the bowls usually chose to reward the string-pulling helper chimp with an equal amount. The decider chimp seemed aware that the other chimp had provided a crucial benefit and wanted to reciprocate.

In other trials, no string was available to the helper chimp. A human experimenter opened the latch, while the helper chimp rocked on its haunches, apparently powerless. In this case, when the helper chimp wasn’t the one opening the hatch, the decider chimp seemed unconcerned about repaying any debt: Sometimes it allocated food to its partner, other times it didn’t.

The chimps were keeping a mental tally—or, to put it more charitably, showing intuitions of fairness. When the helper chimp opened the latch, the decider chimp “chose the option where they both got food more often,” Dr. Schmelz said. “But when I opened it, the [decider chimp] chose randomly.”

The study was small: It involved six chimps from the Leipzig Zoo that were more or less related to one another and took turns playing the role of helper and decider. Some of the chimps at the zoo couldn’t master the apparatus and were cut from the experiment. This raises the question of whether reciprocity surfaces only in more intelligent chimps.

Moreover, the chimps showed not pure generosity but tit-for-tat reciprocity. “They didn’t share spontaneously,” Dr. Schmelz noted. “They only gave a partner food when the partner had assisted them before.”

Still, the chimps had the smarts—and the primitive moral sense—to keep track of who just did what for whom, and they were motivated to reward or punish recent past behavior. The results were strikingly consistent, even in a limited sample, and suggest how more nuanced social exchanges in humans might have evolved.

We don’t usually expect animals to care about debts to each other. But the chimps’ ability to do just that may tell us something about the universal human capacity to form complex societies based on fairness.