Your sense of what's fair and unfair is actually a human trait that evolved from the need people, and primates, had through generations to promote long-term cooperation, say researchers from two universities in Georgia.

Fairness, explains Dr. Sarah Brosnan of Georgia State's departments of Psychology and Philosophy, the Neuroscience Institute and the Language Research Center, is a social ideal that cannot be measured easily.

Nonetheless, Bronsan spent the last decade studying behavioral responses to equal versus unequal reward division in other primates.

Along with colleague Dr. Frans de Waal of the Yerkes National Primate Research Center and the Psychology Department at Emory University, Bronsan reviewed literature from their own research on the responses to primates to inequity, as well as studies from other researchers.

Noting how the awareness of fairness is central to the human experience, Brosnan and de Waal hypothesized it evolved, and therefore has elements that can be detected in other species.

"This sense of fairness is the basis of lots of things in human society, from wage discrimination to international politics," Brosnan said in a news release. "What we're interested in is why humans aren't happy with what we have, even if it's good enough, if someone else has more. What we hypothesize is that this matters because evolution is relative. If you are cooperating with someone who takes more of the benefits accrued, they will do better than you, at your expense. Therefore, we began to explore whether responses to inequity were common in other cooperative species."

Brosnan explained she and de Waal began their studies of fairness in monkeys in 2003, becoming the first in the field to study the subject in a non-human species.

In that earlier paper, titled "Monkeys Reject Unequal Pay" and published in the journal Nature, brown capuchin monkeys grew agitated and refused to perform a task when a partner received a superior reward for doing the same task.

Since then, Brosnan has tested responses to inequity in nine different species of primates, including humans, and discovered species only appear to respond to inequity when they routinely cooperate with those who are not related to them.

The team, however, push their study forward by further suggesting that responding to getting less than a partner is not the only aspect of fairness.

For a true sense of fairness, the team said, "it also matters if you get more." Such responses have only been observed in humans and apes.

"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. "These both require a lot of cognitive control. Therefore, we hypothesize that lots of species respond negatively to getting less than a partner, which is the first step in the evolution of fairness, but only a few species are able to make the leap to this second step, which leads to a true sense of fairness."