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Researcher examines the evolution of cooperation and inequity in primates

Thursday, May 26, 2011 – Jeremy Craig, University Relations

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Scientists have studied how behaviors in primates have evolved into how humans behave today, such as reactions to unequal rewards. But as Georgia State University scientists and their collaborators have found out, our frustration at getting something less than our neighbors isn’t something that emerged in all of our primate relatives.

Kate Talbot, a doctoral student under GSU Assistant Professor Sarah Brosnan, found that unlike another monkey species – capuchins -- squirrel monkeys do not respond negatively when given rewards that are not equal to others. The research article was recently published online in Biology Letters of The Royal Society.

“What we found was that overall, the squirrel monkeys did not respond to inequitable outcomes, contrasting with capuchins, which show the response,” Talbot said. “Capuchins are highly social and have been shown to cooperate in the wild and in captivity. Squirrel monkeys are social, but cooperation is rare.”

The research indicates that this behavior in primates is something that developed on a path of convergent evolution — where different species of primates developed (or did not develop) reactions to unequal rewards separately — rather than from a common ancestor, where all subsequent species would share the same behavior.

“The research supports a hypothesis that responses to inequity evolved in conjunction with cooperation as a mechanism for evaluating one’s payoffs relative to another,” Talbot said. “It evolved in unrelated species.”

In Talbot’s study, squirrel monkeys exchanged a token with an experimenter, and there were several separate conditions that they received: following the exchange task, one subject received a high value reward while the partner received a lower value reward; both subjects received the same value reward for completing the task; a condition to see if there was frustration over not receiving the better reward; and to evaluate the role of a task, a condition where both subjects were given rewards for free.

The high value reward, preferred by all monkeys, was generally a mealworm, while the lower value reward was a piece of cereal or granola.

The findings have applications when looking at human behavior. Other studies have examined responses to inequity in humans, Talbot said, and while repsonses vary across cultures, being paid less for doing the same amount of work yields a reaction in most of them.

“What this study is telling is that our responses to inequity did not really emerge from a common ancestor but likely emerged in tandem with cooperation,” Talbot said. “Humans are a highly cooperative species.”

Talbot’s research was conducted at the Michale E. Keeling Center for Comparative Medicine and Research of the University of Texas M.D. Anderson Cancer Center. Talbot’s advisor, Brosnan, has a joint appointment with M.D. Anderson as an adjunct assistant professor and is the director of Georgia State’s Laboratory for Comparative Economics and Behavioral Studies.

Talbot’s research was supported by a National Science Foundation grant to Brosnan and the National Institutes of Health. She is also working on studies at Georgia State’s Language Research Center.

The article, “Squirrel monkey’s response to inequitable outcomes indicates a behavioural convergence within the primates,” is online in Biology Letters at