New research explores how humans develop a sense of fairness, and whether that quality is innate or learned socially. Photograph by Harry Zernike / Vault

A pair of brown capuchin monkeys are sitting in a cage. From time to time, their caretakers give them tokens, which they can then exchange for food. It’s a truth universally acknowledged that capuchin monkeys prefer grapes to cucumbers. So what happens when unfairness strikes—when, in exchange for identical tokens, one monkey gets a cucumber and the other a grape?

When Sarah Brosnan and Frans de Waal carried out just this experiment, in 2003, focussing on female capuchin monkeys, they found that monkeys hate being disadvantaged. A monkey in isolation is happy to eat either a grape or a slice of cucumber. But a monkey who sees that she’s received a cucumber while her partner has gotten a grape reacts with anger: she might hurl her cucumber from her cage. Some primates, Brosnan and de Waal concluded, “dislike inequity.” They hate getting the short end of the stick. Psychologists have a technical term for this reaction: they call it “disadvantageous-inequity aversion.” This instinctual aversion to getting less than others has been found in chimpanzees and dogs, and it occurs, of course, in people, in whom it seems to develop from a young age. The psychologists Alessandra Geraci and Luca Surian have found, for example, that babies as young as twelve months prefer fair-minded cartoon animals to unfair ones.

And yet, for humans, an aversion to getting less is just one aspect of unfairness. Unlike other animals, we sometimes balk at receiving more than other people. Technically speaking, we experience “advantageous-inequity aversion.” In some situations, we’ll even give up something good because it’s more than someone else is getting. In those moments, we seek to insure that the distribution of goods remains fair. We don’t want the long end of the stick, either.

It seems likely that our aversion to being disadvantaged is innate, because we share it with other animals. The question for psychologists is whether our aversion to benefitting from inequality is innate, too—or, alternatively, if it’s learned through some form of socialization. In December, the psychologists Peter Blake, Katherine McAuliffe, Felix Warneken, and their colleagues published the results of experiments designed to answer this question. Their research spanned seven nations—India, Uganda, Peru, Senegal, Mexico, Canada, and the United States—and looked at close to nine hundred children, aged four to fifteen. They examined whether advantageous-inequity aversion—A.I., as they call it—emerges in all cultures, and, if it does, whether it emerges in the same way everywhere.

Their method was relatively simple. They sat two children down at a table, each in front of an empty bowl. Above each bowl was a tray, onto which the experimenter placed candy. Often, she distributed candy unfairly: she might place four candies on one tray and only one on the other. The child being tested then faced a choice. She could pull a green handle to accept the presented candies, causing them to fall into their respective bowls—or she could pull a red handle to reject them, causing all the candies to fall into a third, off-limits bowl, in the center.

The researchers found that, all over the world, children tended to reject the candies when the split favored the other child. (That is, they rejected disadvantageous inequity, or D.I.) They also found that some, older kids would reject advantageous offers. None of that is surprising. A.I. has been documented among adults many times in the past; in one early study, from behavioral economist George Loewenstein and his colleagues, as many as sixty-six per cent of participants disliked getting more than someone else. The surprising part is that the kids only displayed A.I. in three countries: Canada, the United States, and Uganda. In the other countries—Mexico, India, Senegal, and Peru—they enjoyed the sweet taste of inequality.

These results raise some fascinating questions. Why were kids from only certain countries bothered by having an unfair advantage? And were they rejecting those unfair offers because they cared about fairness—or for some other, less obvious reason?

It’s helpful to start by stepping back from the more complicated case of A.I. to the simpler case of D.I. There are lots of reasons to object to disadvantageous inequity, and some are more obvious than others. D.I. is bad substantively, of course, because you get less candy. But it’s also bad socially, because it signals a demotion in status. In fact, when kids reject disadvantageous offers, they’re often most concerned about their social status, rather than with the candy itself, or with abstract ideas like equality. It’s not about right or wrong. It’s all about me: how do I come off in this scenario?

The importance of social hierarchy in the rejection of disadvantageous unfairness has been cleverly demonstrated in several experiments. In one study, the psychologists Mark Sheskin, Paul Bloom, and Karen Wynn had kids choose between getting one token and giving one to another child, or getting two tokens and giving three to the other kid. “You might think that the latter is the better choice because both children get more,” Bloom writes, in his book “Just Babies.” Often, though, the children chose the first option—just one token each—insuring that they wouldn’t get less than someone else.

In another version of the study, Bloom and his colleagues offered a choice between two tokens all around, or one for the subject and none for her counterpart. Five- and six-year-olds preferred the second option: that is, they gave up a reward in exchange for having more than their peers. “We have a natural aversion to getting less—not to inequity,” Bloom told me. The kids’ behavior isn’t principled; on the contrary, Bloom believes, it seems motivated by something very much like spite. And the message is clear: I want to emerge on top. The absolute number of candies matters less than my relative status.

If D.I. is really about status rather than fairness, could A.I. be about status, too? Rejecting an advantageous offer, after all, also sends a social signal. If you live in a society where ideas of fairness and equality hold a privileged position, then it becomes meaningful to show yourself as embracing those ideals, even at personal cost. Those around you might feel that, since you’re the type of person who believes in equity no matter what, you’re valuable to society, and worthy of respect. From this perspective, both D.I. and A.I. achieve the same end: making sure you maintain status. Perhaps, for older kids who are transitioning into adolescence, status doesn’t always come from having more. It could also flow from being an admirable role model.

If status really is the driving force behind both D.I. and A.I., it would explain one of the study’s relative outliers: Mexico. When the experiment was run there, very few of the children exhibited A.I.; moreover, D.I. appeared to develop far later than in other societies. The Mexican children, in other words, tended to accept all offers, however unequal in any direction. The authors point out that, in that particular sample, most of the kids already knew each other. Perhaps they had already developed reputations, and, as a result, what happened in the experiment had no real implications for their social hierarchy. They were free to enjoy the candy all by itself, without social signals on the side.