New York, Sep 19 (IANS): Ever wondered why it bothers you when your colleague earns more even when both of you do the same job? This is because humans have a sense of fairness that has evolved over time to favour long-term cooperation, says a study.

The human response to unfairness evolved in order to support long-term cooperation, the findings showed.

"This sense of fairness is the basis of lots of things in human society, from wage discrimination to international politics," said Sarah Brosnan from Georgia State University in the US.

Fairness is a social ideal that cannot be measured, so to understand the evolution of fairness in humans, the scientists had earlier studied the behavioural responses to equal versus unequal reward division in other primates.

In this study, brown capuchin monkeys became agitated and refused to perform a task when a partner received a superior reward for that same task.

For the current study, the researchers reviewed literature from their own research regarding responses to inequity in primates, as well as studies from other researchers.

However, responding to getting less than a partner is not the only aspect of fairness. For a true sense of fairness, it also matters if you get more.

"Giving up an outcome that benefits you in order to gain long-term benefits from the relationship requires not only an ability to think about the future, but also the self-control to turn down a reward," Brosnan said.

The study appeared in the journal Science.