How do kids learn to play fair? What researchers found may surprise you

Life isn't fair.

That universal truth is something that children seem to understand almost intuitively at a young age, but the path through which they develop a sense of what's fair and what isn't — and how they act on injustices — is something that has been a puzzle for social scientists.

Fairness, a willingness to sacrifice for the sake of greater quality, is an ideal that supports cooperation, resource sharing and sacrifice. But it also can lead to competition and greed.

It is often talked about as the basis of human civilization, and it affects every aspect of our lives. As the gap between the world's top 1 percent and the rest has increased to historic highs in recent years, fairness in material payoffs or inequality has become one of the most important issues of our time.

In an effort to understand how much of this concept is hard-wired into our biology and how much of it is cultural, a team of psychologists and anthropologists led by Harvard University professor Felix Warneken traveled to seven countries to study how different groups of children play fair.

Their work, which was published in the journal Nature, was focused on the children's reaction to two types of scenarios that are unfair. The first, disadvantageous inequity, occurs when one receives less than a peer. The second, advantageous inequity, happens when one receives more than a peer.

The theory has been that these are two distinct concepts that emerge at different ages and use different parts of the brain. But little has been known about environmental influences until this study.

An aversion to disadvantageous inequity "can provide long-term benefits by preventing competitors from attaining a relative advantage and signaling that one will not tolerate being exploited," Warneken, a social sciences professor, and his co-authors wrote.

Advantageous inequity aversion "entails a larger immediate sacrifice by rejecting a relative advantage. It may signal that one is a good cooperative partner who will not exploit others."

Previous studies have found that a distaste for disadvantageous inequity develops in children by the time they are 4. Advantageous inequity aversion, on the other hand, doesn't appear until closer to 8. That seems to indicate the influence of social norms.

The new study, believed to be the first to look at inequity aversion across societies in children, was seeking to find out more about which aspects of fairness might be universal and which might be culturally driven. To that end, the researchers designed an "inequity game" that they used to test 866 pairs of children ages 4 to 15 in Canada, India, Mexico, Peru, Senegal, Uganda and the United States.

Co-author Peter Blake, an assistant professor of psychology at Boston University, explained that the experiment was specifically designed to see how children would respond to two sides of inequality and how they made decisions that affected both themselves and a peer.

The experiment was set up to work through a machine that required the child to pull on one handle to accept the deal — resulting in the candy being poured into a bowl for each child — and a different handle to reject it — dumping the sweets into a third bowl where neither one would get to eat it.

In all seven societies — which ranged from small villages with a subsistence economy to large industrialized cities — the results indicated a rejection of disadvantageous inequity. That is, when the children were allocated less candy than their peers, they tended to route all the treats into the bowl that no one could access.

That was expected. "This seems to be a basic human response to getting less than someone else," said study co-author Katherine McAuliffe, an assistant professor at Boston College.

Whether they were rejecting the candy out of frustration or meanness, the children were motivated to deprive others of an advantage, she said.

The reactions to advantageous inequity were more mixed. Children in only three countries — the United States, Canada and Uganda — had a tendency to reject unequal distributions of candy when they got more than their peers.