Monkeys Show Sense Of Fairness, Study Says

Sean Markey
National Geographic News

If you expect equal pay for equal work, you're not the only species to have a sense of fair play. Blame evolution.

Researchers studying brown capuchin monkeys (Cebus apella) have found that the highly social, cooperative species native to South America show a sense of fairness, the first time such behavior has been documented in a species other than humans.

The question of whether human aversion to unfair treatment—now shown by other primates—is an evolved behavior or the result of the cultural influence of large social institutions like religion, governments, and schools, in the case of humans, has intrigued scientists in recent years.

The new finding suggests evolution may have something to do with it. It also highlights questions about the economic and evolutionary nature of cooperation and its relationship to a species' sense of fairness, while adding yet another chapter to our understanding of primates.

"It looks like this behavior is evolved … it is not simply a cultural construct. There's some good evolutionary reason why we don't like being treated unfairly," said Sarah Brosnan, lead author of the study to be published in tomorrow's issue of the science journal Nature.

Brosnan, a biology Ph. D. candidate schooled in zoology and psychology at the Yerkes National Primate Research Center and Living Links Center at Emory University in Atlanta, Georgia, said her research was inspired, in part, by studies into human cooperation conducted by Swiss economist Ernst Fehr, who found that people inherently reject unfairness.

Monkey Business

To test whether or not such behavior is found in other species, Brosnan designed an experiment for brown capuchin monkeys, a species well-known for strong social bonds and relatively cooperative behavior, particularly in shared food-gathering activities like hunting squirrels and locating fruit trees.

Individuals were drawn from two large, well-established social groups of captive brown capuchins from colonies at the Yerkes National Primate Research Center and paired with a partner. Pairs were placed next to each other and trained to exchange with human handlers a small granite rock within 60 seconds to receive a reward, in most cases, a piece of cucumber.

"That may actually sound simple, but not very many species are willing to relinquish things, especially intentionally," Brosnan said in a telephone interview. (Think of trying to pry a large bone from a dog's mouth.)

Only female capuchins were tested because they most closely monitor equity, or fair treatment, among their peers, Brosnan said.

Partners of capuchins who made the swap either received the same reward (a cucumber slice), or a better reward (a grape, a more desirable food), for the same amount of work or, in some cases, for performing no work at all.

Brosnan said the response to the unequal treatment was astonishing: Capuchins who witnessed unfair treatment and failed to benefit from it often refused to conduct future exchanges with human researchers, would not eat the cucumbers they received for their labors, and in some cases, hurled food rewards at human researchers.

Those actions were significant. They confirmed that not only did capuchins expect fair treatment, but that the human desire for equity has an evolutionary basis.

Susan Perry, a primate expert at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, studies the behavior of Cebus capucinus, a capuchin species closely related to brown capuchin monkeys, in the wild.

Based on her review of a brief summary, Perry described Brosnan's research as a "fascinating paper."

"It is not so surprising to me that the monkeys act in this way," Perry wrote via e-mail. "After all, humans often respond in an apparently irrational way … accepting no reward for both them and their partners rather than accepting unequal rewards … in the ultimatum game," she wrote, referring to the classic laboratory test of inequity aversion.

Primate Culture

The study is the latest in a series of findings on human and primate behavior, culture, and evolution that has spurred new fields of inquiry.

In recent years, researchers have identified an array of unique behaviors found among distinct groups of primate species, including chimpanzees, orangutans, and capuchin monkeys, and associated them with culture. Scientists have sought to explain the social-learning processes by which such behavior is acquired by individual members.

In 2001 Andrew Whiten at St. Andrews University, Scotland, together with Jane Goodall and other researchers analyzed five decades of data on chimpanzees and identified 39 distinct behaviors tied to mating, eating, grooming, and tool use, concluding that chimps have culture.

Researchers are turning their gaze to other primate species. "People are looking at these so-called cultural behaviors, which are behavioral variants between two different groups of the same species that can't be explained by their ecology," said Brosnan. "In other words, how come some [chimpanzees] nut-crack and some [chimpanzees] don't, even though both of them have nuts available that could be cracked?"

"Social learning is believed to be the mechanism by which cultures evolve," said Brosnan, who notes that the ability to socially learn and a species' sense of fairness must be linked, in her view, since both require individuals in a social group to closely observe and monitor the behavior of their peers.

Brosnan's research strengthens the tie between aversion to unfair treatment and cooperation in species. However, scientists have yet to tease an answer from the chicken-and-egg dilemma of which came first, cooperation or a sense of fairness?

"We don't know whether individuals become cooperative and then learn to not like being treated unfairly, or the other way around," said Brosnan. "But that opens up a whole new research field."

Her study and other research leads scientists to ponder just why cooperation evolved and what benefits it bestowed to species.

Cooperation Economics

The finding adds new information to the debate about why species cooperate and the economic decision-making process behind such behavior.

"No one really seems to know why individuals should cooperate," said Brosnan.

Some economists and scientists have argued that cooperation is not a rational, or logical, behavior for species individuals since energy or other resources must be expended in the effort—with no direct benefit to the cooperative individual.

But Fehr, the Swiss economist from the University of Zurich presently based at the Massachusetts Institute of Technology in Cambridge, among others, rejects such thinking. He argues that logic applies not just to the ends but to the means during economic decision-making. "There is nothing irrational in being altruistic," he said in a telephone interview.

Brosnan echoes similar notions. "People often forgo an available reward because it is not what they expect or think is fair," she said in a press statement. "Our findings in nonhuman primates indicate the emotional sense of fairness plays a key role in such decision-making."

Fehr, who has published key research on the economics of human equity, cooperation, and altruism since 1999, observed: "The new finding that even monkeys reject unequal pay is very important, I think, because it suggests that this is a very deeply rooted behavior that we observe among humans."

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