Fairness is one of the traits that separates Homo sapiens from other animals. We’re willing to sacrifice money, time, and energy in order to make things fair and equal for everyone. But a comprehensive study using different species of monkey recently proved that fairness, while intrinsic, is motivated by selfish desires for specific outcomes.

The study, conducted by Sarah Brosnan of Georgia State University and Frans de Waal of Emory University, showed that humans and chimpanzees displayed fairness in order to ensure long-term cooperation.

Nine species of primate, including humans, were included in the study. Other types of primates, such as brown capuchin monkeys, refused to complete tasks if other participants were given larger rewards. Only humans and chimpanzees showed a willingness to give up rewards for the sake of fair outcomes and group harmony.

According to Brosnan, “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.”

Other monkey species clearly showed signs that they understood inequity, but only humans and chimpanzees were able to not only understand, but think about how fairness would help them in the future.

“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.

This study is important in discovering the evolutionary history of the human mind and how fairness evolved as a trait in order to ensure harmony and cooperation among groups. It represents a leap from average primate cognition to a more advanced structure.

The study was published on Sept. 18 in the magazine Science.