Furious Monkeys Rejecting Unequal Pay Explained

Nearly every primate, including humans, hates getting ripped off, finds research.

Monkeys, chimps and other primates go ballistic when they receive unequal pay, much in the way that humans fume under similar circumstances, according to a new study that also helps to explain the reaction.

The angry response to perceived unfairness evolved in order to support long-term cooperation, according to the research, published in the journal Science.

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The research explains a popular video that went viral. The video shows what happens when a brown capuchin monkey figures out that he's being ripped off.

click to play video

Even Sarah Brosnan of Georgia State's department of Psychology and Philosophy, who led the new study, still gets a kick out of this classic experiment.

"The video is definitely the best," she told Discovery News. "The presence of this response across so many species likely indicates that we - and these other species - evolved to respond this way because it was so beneficial in the context of cooperation."

She continued, "We are certainly not the only species that responds to inequity now, and it seems very likely that our ancestors did as well, especially as we know that they cooperated on a large scale."

The video was shot in 2003, and since then, Brosnan, colleague Frans de Waal and others have determined that nine primate species, including humans, have an aversion to inequity. They reviewed the research for the latest study, as well as experiments conducted by other teams.

It's likely that there is a genetic component to the behavior, as well as a learned one. What is clear is that humans aren't the only ones who sometimes feel slighted, and this can be very emotional.

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"We don't know how other species feel about inequity because we can't ask them," Brosnan said. "That being said, it is likely that their responses are underpinned by an emotional reaction to the inequity of a situation, much as ours are. One difference in IA (inequity aversion) in humans as compared to other species is that for us, fairness has become a social ideal, which is unique to humans."

A potential game changer could be the Internet and the global economy, which has us interacting with people from around the world but not necessarily face to face. Brosnan thinks inequity aversion still persists, however.

"It's really interesting to see how fairness continues to be a part of remote cooperation," she said. "Even in the digital age, expectations of equity exist, and if fairness norms are violated, people may seek other partners."

"For instance," she continued, "many online markets utilize a rating system whereby both users and sellers can rate one another. Many things that are commonly discussed relate to fairness: Did you get the product you expected? Is it in good shape? Did it arrive in a timely manner? etc. These virtual systems allow remotely connected humans to choose their partners, which is exactly what we think inequity is used for."

The researchers also found that responding to getting less than a partner is not the only aspect of fairness. For a true sense of fairness, it also matters if you get more.

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Brosnan and de Waal theorize that individuals should be willing to give up a benefit in order to reach equal outcomes and stabilize valuable, long-term cooperative relationships. Thus far, this has only been found in humans and their closest relatives, the 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."

Photo: Two adult female chimps look on as a third eats a preferred food resource. Credit: Sarah Brosnan and the Keeling Center for Comparative Medicine and Research of MD Anderson Cancer Center

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