A sense of fairness separates apes and humans from other animals and evolved to promote the benefits of long-term cooperation, experts claim. Two US scientists came to the conclusion after studying the ‘evolutionary puzzle’ for more than 10 years.

While monkeys and dogs became upset when treated unfairly, only humans and their great ape cousins voluntarily allowed themselves to be disadvantaged in the interests of fair play.

This is not a purely altruistic trait, according to the researchers. Rather, it is the result of wanting to avoid negative ‘pay back’ from a dissatisfied partner and ensure future cooperation.

Psychologist Sarah Brosnan, from Georgia State University, who co-authored a review of the research published in the journal Science, said: “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.

“These both require a lot of cognitive control. Therefore, we hypothesise 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.”

Brosnan and Frans de Waal, from the Yerkes National Primate Research Centre at Emory University, began their studies of fairness in monkeys in 2003.

This is not a purely altruistic trait

A paper published in Nature journal entitled ‘Monkeys reject unequal pay’ featured a study of brown capuchins which became agitated and uncooperative when a partner received a better reward for performing the same task.

Since then further tests on responses to inequity were conducted on nine different species of primate, including humans.

Animals only responded to unfairness when they routinelycooperated with non-related strangers, the scientists found.

Beyond primates, domestic dogs showed evidence of “first-order inequity aversion” (IA) – the sense of being treated unfairly – and the trait has even been seen in some members of the crow family. But the desire to equalise outcomes despite suffering short-term disadvantage was something unique to humans and great apes.

The researchers concluded: “Our closest relatives, the anthropoid apes, show evidence of second-order IA (inequity aversion), an essential component of human fairness because it seeks to equalise outcomes. Thus, humans and other species seem to share basic reactions to inequity, which serve the need for sustained cooperation.”