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January 23, 2006
A question of fairness

by rachel robertson

It’s a joke in Sarah Brosnan’s family that, as children, she and her sister were so concerned with fairness that when there was something to be divided (a piece of cake, for example), their mother would ask one to cut and the other to choose who got which piece. The punch line is that the cake would be halved so precisely that its division could not have been improved even by caliper measurement.

Now a postdoctoral fellow in anthropology, Brosnan’s early obsession with equity has developed into an impressive body of research examining the evolutionary underpinnings of fairness, cooperation and prosocial behavior (voluntary actions that help another with no cost to oneself).

“I am very, very interested in complex questions,” Brosnan said. “I love questions that you really need to go about answering in a variety of different ways with a variety of different approaches before you have a really good understanding of it.”

Her broad educational background helps Brosnan to examine these questions from several viewpoints. Starting out as a biology major at Baylor University in Waco, Texas, she realized medicine probably was not for her, then turned to ecology. Brosnan was encouraged by her professors and worked in three different research labs, gradually discovering an interest in behavioral ecology which led to her honors thesis on the mating patterns of prairie voles.

Although officially in the biological and biomedical sciences department, Brosnan’s crossover into psychology and anthropology began with her graduate work at Emory, when she started working with capuchin monkeys and chimpanzees under her adviser, Frans de Waal, Charles Howard Candler Professor of Primate Behavior and director of the Living Links Center at Yerkes National Primate Research Center.

Described by de Waal as “outgoing, enthusiastic and full of energy,” Brosnan grabbed the attention of the academic world and the popular press while still a graduate student, when Nature published her article with de Waal on capuchin monkeys’ negative response to unequal rewards.

Tested in pairs, the monkeys’ task was to hand a token to the experimenter for a food reward. When they witnessed a partner receiving a better reward (a grape versus a slice of cucumber, for example), the monkeys were more likely to refuse to participate—indeed, sometimes throwing the token or reward out of the testing area. These results suggested the capuchins were demonstrating certain aspects of “inequity aversion”—important because it is consistent with a model on the development of cooperation put forth by behavioral economist Ernst Fehr.

“If you and another chimpanzee are on a monkey hunt, and you are the one that is actually in there getting bitten by the monkey—the other chimp is not helping you at all—then maybe you need to find another chimp to hunt with,” said Brosnan, whose own aversion to inequity is well documented. “That’s sort of a silly example, but it gets to the point of what [Fehr] meant.

“If you have a sense of inequity aversion, you can use this as a proxy for whether or not you should continue cooperating with this individual,” she continued. “You don’t necessarily have to remember the cost and benefit of every single activity, but you just get a general idea. Based on some of the results we have seen with the primates, I wonder if it’s tied into social emotions—an emotional reaction that is used as a proxy for this fairly complex cognitive calculation.”

Brosnan and de Waal went on to replicate their findings with chimpanzees, discovering more about the social aspects of fairness when they compared different chimp groups. Pairs of chimps from a less established group (together seven years) showed the same negative responses to unequal rewards as the capuchins. However, close-knit chimpanzee pairs who had been together since birth did not react at all when their partner received a better reward, possibly demonstrating a communal orientation seen in humans.

In a recent extension of this work, Brosnan (along with de Waal and undergraduate Cassiopeia Freeman) designed a task in which capuchin monkeys were required to cooperate to receive a food reward. By correctly pulling a bar, monkeys triggered delivery of a food tray. The bar’s weight was adjusted so that it could not be pulled by a single individual. A grape (the more preferred reward) and/or apple slices (the less preferred reward) were set out in two cups so that each monkey received a reward. For some trials, the reward was the same for both monkeys, and for others one monkey got the better reward.

“To my surprise, rather than the reward distribution being the critical factor, what was critical was how the partner reacted to this unequal distribution,” Brosnan said. Specifically, she said, when the dominant monkey consistently took the better reward, the subordinate no longer wanted to participate.
Just a year into her postdoctoral fellowship, Brosnan beat the odds by getting a second publication in Nature. The study in question, which examined prosocial behavior in chimpanzees, was launched by her postdoc adviser, Joseph Henrich, assistant professor of anthropology (along with collaborators Joan Silk at the University of California, Los Angeles and Daniel Povinelli at the University of Louisiana at Lafayette).

Brosnan jumped in with her usual enthusiasm. Looking first at chimpanzees at a center in New Iberia, La., the researchers found the chimps showed no prosocial tendencies in a task where they could choose to deliver food to themselves only or to themselves and a partner (again, using a barpull). After the chimps did not respond as expected, Brosnan’s familiarity with a group of researchers at the University of Texas’ MD Anderson Cancer Center proved useful; it allowed Henrich’s group to replicate their findings with a second population, this time six groups of chimpanzees at MD Anderson who had been together since 1978.

“The fact that the results were consistent between Texas and New Iberia—when you had chimps of different ages and rearing situations, different experimenters, and even a slightly different [research] paradigm—makes the results much stronger,” Brosnan said. The results also were important, she added, because they demonstrated that even though chimpanzees recognize inequity, they appear to lack the regard for others that humans show.

Brosnan’s next mission is to discover what human capacity accounts for this difference. To do that, she has expanded her repertoire of research even further to include another primate: children. Under Henrich’s supervision, she has started testing 3–5-year-olds on the same prosocial tasks.

“We are actually doing as close a replication [to the chimp study] as possible,” Brosnan said. “We built a barpull almost identical to the ones the chimps use—except that it’s very brightly colored and has rounded edges.”

Children in the study are similarly rewarded with food (this time M&Ms instead of bananas). In another task, the children are tested on a cognitive ability (known as “theory of mind”) that enables them to distinguish what they know from what another person knows. By testing children at an age when theory of mind begins to develop, it may be possible to see the influence of this ability on prosocial behavior.

“One hypothesis is that nonhuman primates may not be as prosocial as humans because humans have better theory of mind,” Brosnan said. “It’s possible that the chimps don’t even consider what their partner is getting. They know their partner is there, and they know the food is there, but they seem to be more interested in what they are getting.

“Presumably, as your theory of mind gets better, you would understand that your actions can affect other individuals—you can put yourself in their place and understand that they would like an M&M, too,” she said.

Brosnan certainly has taken her passion for fairness to achievements beyond what her family could have imagined when she was meticulously bisecting pieces of birthday cake.

“To get published in Nature requires that one finds something truly new,” de Waal said of Brosnan’s coup. “To have two such articles at this point in her career is an incredible achievement if one realizes how competitive the top journals are—how few scientists, even in a lifetime, can match this success.”

“I have just been lucky to have such supportive advisers,” Brosnan said. “[De Waal] really pushed me to achieve all that I could.”

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