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A sense of fairness separates apes and humans from other animals and evolved to promote the benefits of long-term co-operation, experts claim.

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.

A paper co-authored by Dr Sarah Brosnan from Georgia State University reviewed research into the sense of fairness in animals. They found only humans and apes have a true understanding of justice. Both disadvantage themselves to the benefit of others to avoid negative confrontations (stock image shown)

The trait is not entirely self-less, the researchers note, but rather it is the result of wanting to avoid negative 'pay back' from a dissatisfied partner and ensure future co-operation.

The paper reviewing research into the trait was published in the journal Science and was co-authored by psychologist Dr Sarah Brosnan from Georgia State University

'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,' she said.

'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.'

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

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 routinely co-operated 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 to the benefit of a partner despite suffering short-term disadvantage was something unique to humans and great apes.

A paper published in Nature featured a study of brown capuchins (stock image shown) which became agitated and uncooperative when a partner received a better reward for performing the same task. Animals only responded to unfairness when they routinely co-operated with non-related strangers, the scientists found

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 co-operation.

'Humans' unprecedented brain enlargement allows for greater understanding of the benefits of self-control in the context of resource division. 

'Additionally, the development of language enabled communication about third parties, which may have enhanced the role of reputation building.

'Despite these differences, many of the basic emotional reactions and calculations underlying our sense of fairness seem rooted in our primate background.'