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It is that time of the year when appraisals take place and if you don't like the heavy look of your colleague's pay package, it’s possibly your evolutionary origin at play.

A recent study by researchers in the US shows brown capuchin monkeys refused to play along when they saw another monkey get a better payoff for performing the same work. This suggested evolutionary origin has something to do with human aversion to unfair treatment.

Explaining the background to the study, Sarah Brosnan, a biology PhD candidate schooled in zoology and psychology at the Yerkes National Primate Research Center and Living Links Center at Emory University in Atlanta told yours truly, "It is because of our interest in cooperation."

Brosnan and her colleague Frans de Waal conducted the research on monkeys, drawn from two large well-established social groups of captive brown capuchins from colonies at the Yerkes National Primate Research Center. Pairs were placed next to each other and trained to exchange with human handlers a small granite rock within 60 seconds to receive a reward.  

The researchers taught the brown capuchin monkeys (Cebus apella ) to swap plastic tokens for food. Normally, monkeys were happy to exchange a token for some cucumber.  

But it was a different story when one monkey was given a more coveted food item such as grapes. The other monkey would throw a fit or take offence, either throwing the token with disdain, refusing to eat the cucumber or giving it to the other monkey.

"We found that  capuchins monkeys, when presented with a simple exchange task in which their partner receives a superior food reward for the same amount of work, refuse to exchange. This negative response is exacerbated when the partner receives the superior reward for less work," she said.  

"The animal’s umbrage was even greater if the other monkey was rewarded for doing nothing," Brosnan notes. "Thus, it appears that capuchins are able to compare their rewards to those of others, and react negatively when they are treated unfairly."  

About 80% of the monkeys rebelled in some way in this situation. Previous experiments with humans have shown that they become less cooperative if treated unfairly, and punish uncooperative people even if their own reward declines as a result. This is akin to a monkey throwing away the cucumber that it has already worked for.  

Interestingly only female monkeys showed this pique. Males were much less sensitive to inequality, the researchers found. According to researchers their minds may have been on other things: "Males care about sex, and females care about food. The males might not consider the food differences worth worrying about."

It has recently been argued that human economic decision-making, particularly about cooperative enterprises, is driven as much by an emotional aversion to unfair situations as to rational considerations, but it is unknown to what degree this response is a result of evolution versus culture.

CR Jayachandran in New Delhi

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