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The sense of fairness did not evolve for the sake of fairness but it evolved in order to support long-term co-operation, according to a new study. 

The study essentially is a review of primate behavior data.

"This is the first paper to put forth an evolutionary hypothesis for fairness based on experimental data from animals," said evolutionary biologist Dr Sarah Brosnan of Georgia State University, in the press release.

In a study performed eleven years ago, researchers had noted that a monkey given a cucumber as a reward for performing a task protested by hurling the cucumber back at the researchers if they saw their partner getting a more highly prized grape as a reward. 

"They were perfectly happy to eat the cucumber as long as their partner was getting cucumber, but when the partner started getting grapes they started throwing out their cucumbers," said Brosnan.

Brosnan said tests in nine different species of primates including humans have shown that sensitivity to inequity only occurs in animals that routinely co-operate with others they are not related to.

She added that the evidence supports the hypothesis that having a sense of fairness evolved because it is important for co-operation. It also allows individual to judge the value of their co-operative partner. 

The study is published in the journal Science.