Researchers from Georgia State University and Emory University said that the reason why people can't stand unfairness is because it defies long-term cooperation among humans.

Sarah Brosnan, one of the researchers, said that fairness can't be measured and for a long time has been the basis for wage discrimination and international politics in human society.

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She said that what she and her colleagues are interested in is finding out why humans aren't happy if what they have isn't good enough and someone else has more.

The group has been studying monkey's reaction to injustice and unfairness since 2003.

Their results showed that the brown capuchin monkeys that were part of their study will refuse to perform a task when their partners received a superior reward.

Researchers also found out that their subjects would only respond to inequity if they cooperate routinely with those who are not related to them.

They said that true fairness comes if an individual is willing to give up a benefit to reach an equal outcome.

Brosnan said that giving up an outcome that benefits a person in order to gain a long-term benefit from the relationship requires a willingness to turn down any reward.

She said that this requires a lot of cognitive control, and that's the reason why people respond negatively if they get less than what they deserve.

She said that responding negatively to getting less than what is deserved is the first step to the evolution of fairness.

The ability to turn down a reward is another step that only few can do, Brosnan added.