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. Why is it that some people grow up to want to accumulate as much power and wealth as possible, even if what they already had was good enough, while others are happy with relatively little?

Fairness, a willingness to sacrifice for the sake of greater quality, is an ideal that supports cooperation, resource sharing and sacrifice but that can also lead to competition and greed. It is often talked about as the basis of human civilization, and it affects every aspect of our lives, from gender discrimination to international relations. 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 have become one of the most important issues of our time.

In an effort to understand how much of this concept is hardwired 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 Tuesday 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 a one receives less than a peer. The second, advantageous inequity, happens when one receives more than a peer.

Both are believed to provide part of the glue that holds societies together, and 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 new 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 4 years of age -- and that it's motivated by spite. Advantageous inequity aversion, on the other hand, doesn't appear until closer to age 8.  That seems to indicate that a strong egalitarian preference may be due to the influence of social norms.

The new study, which is believed to be the first to look an 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, Sengal, Uganda and the United States.

Co-author Peter Blake, an assistant professor of psychology at Boston University, explained in an interview with reporters that 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 rules were simple: Two children of the same gender and similar age were seated across from each other and were given some Skittles candy as rewards. Sometimes the allocations were equal and sometimes they were not. One child was given the choice between accepting the allocation or rejecting it.

The experiment was set up to work through a machine that required the child to pull on the green handle to accept the deal -- resulting in the candy being poured into both red bowls -- and on the red handle to reject it -- which dump the sweets into the silver bowl where no 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, children tended to reject the candies when they got less than their peer. That was expected, as there has been some evidence that the psychology of this feeling may have evolutionary roots because it has even been found in nonhuman primates.

"Our prediction is that we will likely find disadvantageous inequity aversion everywhere. This seems to be a basic human response to getting less than someone else," study co-author Katherine McAuliffe, an assistant professor at Boston College, said in a conference call with reporters.

McAuliffe said that whether they were rejecting the candy out of frustration or meanness the children were motivated to deprive others of an advantage.

The reactions to advantageous inequity were more mixed with children in only three countries -- the United States, Canada and Uganda -- having a tendency to reject unequal distributions of candy when they got more than their peers. "In these societies, rejections of advantageous allocations increased with age with AI appearing to emerge by pre-adolescence… Given that Western societies tend to emphasize establishing and enforcing norms of equality, it is possible that children in these communities face social pressures to internalize and enact these norms earlier in development compared to other societies," the researchers wrote, nothing that although Uganda is a non-Western society, the schools they recruited their subjects from tended to have Western teachers.

Blake said the differing results on the two types of inequality shows that "different psychological processes may be at work depending on whether someone is at an advantage or disadvantage."

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