Editor’s Note: As we gear up to celebrate 60 years of this tradition with you, let’s take a moment to remember some of the best of our previous Distinguished Alumni with Hall of Fame: Rewind. We hope you’ll enjoy reading about our outstanding alumni honorees from the past who shape the ranks of honorees of the future. Dr. Sarah Brosnan, a 1998 Baylor grad and 2009 Outstanding Young Alumna, made her big break studying fairness and equity in capuchin monkeys as a doctoral researcher. Click here to watch interviews and speeches from previous Hall of Fame events, or click here to learn more about his year’s event and honorees.
Originally published in The Baylor Line’s Summer 2004 issue. Part of the 2025 Hall of Fame: Rewind series.
On the side of a tan brick building at Emory University’s Yerkes National Primate Research Center, where more than two dozen capuchin monkeys live, there’s a green street sign that reads, “Brown Capuchin St.”
“One of the researchers found it on eBay,” behavioral biologist Sarah Brosnan says of the whimsical decoration.
Who says scientists are always serious-minded?
Inside the capuchin lab, Yerkes’s two distinct social groups of sixteen brown capuchins are affectionately and respectively referred to as the “nuts” and the “bolts”–another hint that the monkeys aren’t the only ones around the lab with a sense of humor.
Slightly bigger than large cats, the capuchins have tiny, quick hands and impish, expressive faces. Their devilish appearance is enhanced by horn-like tufts of hair that stick up from the adults’ heads. In their enclosures, they climb handily about on all sides—including the ceiling. They spend time swinging on heavy fire hoses, grooming one another, nibbling on snacks, and grabbing at the springtime bumblebees that buzz unwittingly into their habitats. The capuchins have free access to both food (Purina high-protein monkey chow) and the outdoors, and on this warm Georgia afternoon they are sunning themselves unabashedly.
The monkeys grow somewhat excited to see Brosnan, who knows each of them by name. She admits she has grown attached to all the capuchins, but she has a special eye for the five females who are the stars of a recent study that catapulted Brosnan to scientific stardom–Georgia, Bias, Nancy, Lulu, and Winnie. “You know, Lulu looks a little skinny,” she says worriedly to a graduate student who works in the lab. “I know it seems like they all look the same, but we can tell them apart really easily,” Brosnan adds. “They know who we are, but they don’t interact with us that much. We want them to be bonded to each other, not to us, because that’s how their social groups work in the wild.”
“Fairness is so ubiquitous—it’s such a part of our lives as humans—but nobody had looked at it in another species.” — Sarah Brosnan
Capuchins are instantly recognizable as the organ grinder monkey of storybooks and old movies–a cruel fate, Brosnan explains, since they are intensely social animals and need other capuchins to exist normally. They have even been shown to cooperate with one another and to reciprocate favors: I’ll groom you now, you groom me later. After all, that’s only fair.
The monkeys’ remarkably human-like social behaviors are the focus of groundbreaking research that Brosnan, a 1998 Baylor graduate, has released in collaboration with Frans de Waal, one of the world’s leading primatologists. In a widely noted study published last September in the science journal Nature, Brosnan found that brown capuchin monkeys exhibit a clear “aversion to inequity”—a natural desire for equal treatment. The discovery, which represents the first time non-humans have been shown to demonstrate a sense of fairness, caused an international stir because of its intriguing implications about the nature of human cooperation. Fairness and cooperation, Brosnan says, are probably connecting branches of our evolutionary tree.
“People judge fairness based both on the distribution of gains and on the possible alternatives to a given outcome. Capuchin monkeys, too, seem to measure reward in relative terms, comparing their own rewards with those available, and their own efforts with those of others,” Brosnan writes in the study, which she co-authored with de Waal. “They respond negatively to previously acceptable rewards if a partner gets a better deal. Although our data cannot elucidate the precise motivations underlying these responses, one possibility is that monkeys, similarly to humans, are guided by social emotions.”
The capuchin fairness study was the capstone of a series of research experiments that Brosnan designed for her doctoral dissertation on cooperation in non-human primates. She completed her PhD in population biology, ecology, and evolution this spring at Emory, working with de Waal at Yerkes.
“Cooperation fascinates me because it’s something everybody does, in both human and animal social situations, yet many scientists assume it’s non-adaptive–in other words, if you are helping someone else you are not helping yourself,” Brosnan says. “We assume that everything animals do will benefit them. So that [the idea that cooperation is non-adaptive] did not make sense to me; because so many species cooperate with one another, there must be some gain.”
Previous research in this area had focused primarily on finding the “ultimate explanation,” or why a particular behavior, such as cooperation, is beneficial to the individual. But Brosnan was interested in studying “proximate mechanisms” —how an animal actually performs the behavior that is ultimately beneficial. “Reciprocity—a basic understanding of give and take—makes sense, but it doesn’t explain why animals first begin to cooperate,” she says. “I was interested in how having a sense of fairness and values could be a proximate mechanism to help animals cooperate more effectively.”
So when people respond negatively to unfair treatment—being paid less for equal work, for instance—is their reaction a learned social response, or a deep-rooted, instinctive affinity for fairness that has played a pivotal role in the evolution of primates—monkeys and humans alike? Brosnan, for one, believes fairness is just in our nature.
Inquiring mind
With long, wavy brown hair, a bright, lilting voice, and an almost girlish enthusiasm about her work, Brosnan doesn’t immediately strike one as a serious scientist. She can be talking about her childhood pets one minute and then, seemingly without effort, eloquently explaining proximate mechanisms the next.
“Sarah had an almost effervescent personality and clearly was not inhibited in showing her enthusiasm for her research in my lab,” says Baylor biology professor Owen Lind, one of Brosnan’s former teachers. “Some of the techniques she used were not routine and required subtle adjustments for each different sample type. She proved quite creative in developing and modifying techniques, and consequently she produced preparations superior to those of us old-timers. Sarah had a combination of intelligence, motivation, creativity, and personality never before seen in my student investigators.”
Brosnan had opportunity aplenty to explore her interest in biology and animal study at Baylor, where she was a National Merit Scholar as well as a University Scholar. She completed her honors thesis under former biology professor Wendy Sera. As she spent countless hours analyzing the mating behavior of prairie voles, Brosnan found she had both a liking for working with animals in the field and the patient diligence necessary for lab research.
“I also learned I was much more interested in large organisms than tiny ones, like cellular bacteria,” she says. “I thought I was going to be an ecologist—I loved the social behavioral aspect of the research. I came to college so certain that I wanted to be a biologist, it was more a question of finding the niche where I wanted to be.”
Brosnan claims she owes that certainty to David Phoebus, her high school biology teacher in Towson, Maryland, whom she credits with her decision to find a career in scientific research. “He was the first really hard teacher I ever had. He was the type who would give you basic information and then turn you loose,” Brosnan recalls. “I remember I wrote a paper about gorillas, and he gave me all this extra time to do additional research. It was the first time I understood that schoolwork was something to be enjoyed and pursued, not just assignments you had to finish.”
Science runs in Brosnan’s family. While she was growing up in some half-dozen different states, her mother taught computer programming and her father worked as an engineer with various transportation companies. Her younger sister is now a computer programmer for IBM. Her husband of two years, Alex Bragg, holds a PhD in bioengineering from Georgia Tech and an MD from Emory, and he is doing his residency in neurobiology at Emory.
The Brosnan family had “tons” of pets—cats, dogs, fish—and Brosnan was drawn to animals and biology at an early age, excelling in her science classes from grade school on. “I was never particularly enamored of primates, though,” she says. “I mean, I liked monkeys just like anybody else.”
Yet it was mainly the monkeys at Yerkes—and a man who has observed and studied them for more than two decades—that brought Brosnan to Emory. One of the world’s leading primatologists and behavioral biologists, Frans de Waal is best known for his research on reconciliation, cooperation, and reciprocity among primates. His studies have shown that capuchin monkeys and chimpanzees habitually exchange goods and services with one another, forming a kind of biological marketplace in which grooming might be traded for food, for instance, or food now for food later on. He also found that capuchins are cooperative and seem to have a basic sense of quid pro quo: If individuals have to work together to earn a reward that only one actually gets, the lucky recipient is much more likely to share the goods with its partner if the partner helped.
These findings—and the questions they raise—intrigue Brosnan, who says she chose Emory for her graduate work in large part because of de Waal’s work and reputation. “I became interested in how primates keep track of the value of these exchanges,” she says. “Can they make value judgments? Do they notice if they are not getting the same thing? Can they compare? To our knowledge, these questions had never been asked before.”
For her dissertation, Brosnan first set out to establish that the capuchins were capable of performing simple conditional association tasks; they could exchange a bolt for a grape and a washer for a piece of melon, eventually figuring out that the bolt was equal to the grape and the washer to the melon. They also could distinguish between a token that was worth a grape and one that was worth a cucumber. Monkeys have a serious sweet tooth, so the grape token was of higher value than the cucumber token.
“They learned to treat tokens as they treated food, like simple currency,” Brosnan says. “We had to show they could associate value with inherently non-valuable objects. More importantly, we determined that after they had learned the values, they could use them in a simple barter situation.”
Next, Brosnan tested the monkeys to see whether they could learn and compare values by watching other monkeys in the same situation, which they demonstrated that they could. The female monkeys proved much more interested in how their peers were faring than the males, prompting Brosnan to choose all females for the fairness study—the penultimate project of her dissertation.
Brosnan conducted the Nature study over six months, testing the monkeys once each day—about four hundred trials in all. Each testing session found her in scrubs, a white lab coat, a surgical mask, gloves, and a plastic face shield—as much for the monkeys’ protection as her own, since they can easily catch illnesses from humans. The animals were tested in pairs inside a clear plastic chamber attached directly to their enclosure, separated by a barrier they could both see and reach through. “The hard part,” Brosnan laughs, “is getting the two you want to come into it, because everyone knows that when you go into the testing chamber you get treats.”
Capitalizing on their fondness for sweets, Brosnan used cucumbers and grapes to measure the capuchins’ reaction to unequal treatment. Since they were already well-versed in the exchange process, Brosnan would give the monkeys a token—a granite rock from outside the lab—which they had to return to her in order to receive a food reward. Giving back the stone, Brosnan explains, qualified as a task successfully performed since few animal species will voluntarily give up anything. The monkeys had to make the swap within a minute in order for it to count.
In some trials, the capuchins would receive a grape for the token; in others, they would get a slice of cucumber while their partner received a grape. Sometimes the partner was presented with a grape for doing nothing at all. In the interest of both fairness and consistent results, each monkey got to be the grape recipient an equal number of times.
The capuchins’ response was almost laughably human. At first, the subjects accepted the cucumbers as a reasonable reward for returning the token. But after repeatedly watching their neighbor get a sweet grape for doing precisely the same thing—or worse, nothing—they became increasingly upset. They then refused to cooperate with Brosnan in one of two ways. In the first scenario, they would return the token and then reject the cucumber, watching Brosnan place it on the chamber floor and then ignoring it completely. At the end of the session, Brosnan says, there might be ten or fifteen spurned slices lying around them. The grape recipient, though, never shared her fruit.
“Often, the partner would finish eating her grape and then reach through and take the cucumber, too,” Brosnan says. “The subject never tried to stop her.”
“The fact that it looks like our sense of fairness might be innate implies that it should be taken into account in all cooperative endeavors–peace treaties, for instance.” – Sarah Brosnan
Their second method of refusal was to take the token or the cucumber and then fling it out of the testing chamber, apparently to show Brosnan just what they thought of her vegetables.
“What you have to realize,” Brosnan says, “is that giving them a grape versus a cucumber is like giving someone a Hershey bar versus a Hershey kiss. They’re both good rewards. We use cucumbers as testing rewards all the time. Their reaction only changed when the other monkey got the grape.”
In some trials, Brosnan worked with just one monkey, exchanging tokens for cucumbers while a lone grape lay in the testing chamber next door. While the animal’s interest in the grape was clear, it would eventually get around to eating the cucumber. This test helped Brosnan distinguish between the motivations of envy and greed—a critical difference for her purposes. Plain greed, she explains, would spur the capuchin to want the grape just because it’s available; envy would cause her to want the fruit because someone else has it. “Greed has nothing to do with fairness,” Brosnan says, “but envy does.”
Brosnan’s study indicated that capuchins are every bit as capable of envy and righteous indignation as humans, and their negative response to unfair treatment could cause them to behave irrationally–to refuse perfectly good food simply because they could see someone else receiving something better.
“This is the first study ever to show any non-human has a sense of fairness,” Brosnan says. “Fairness is so ubiquitous–it’s such a part of our lives as humans–but nobody had looked at it in another species. One species can only tell us so much, but still it’s very interesting.”
One problem with studying such emotional reactions in humans, Brosnan adds, is that people have so many complex psychological layers that it’s difficult to isolate one feeling. “But if you can study it in an animal,” she says, “you can look at the sense of fairness in a really basic form.”
Who’s the fairest?
Brosnan’s discovery suggests that humans’ sense of fairness may have its roots in evolution rather than social or cultural development, as many theorize. And like cooperation, the desire for equity may have come about for good reason. As cooperation evolved, Brosnan explains, it may have become important for humans to compare their own efforts and rewards with those of others. If expectations of equity were not met, established patterns of cooperation–hunting or planting together, for instance–could crumble, putting an entire group in jeopardy. So it benefitted the species to be both cooperative and fair.
The notion that our sense of fairness is inborn could prove a critical point in the ongoing debate about why people cooperate with each other even when the advantage to individuals is not immediately apparent. Economists, in particular, have long tried to make sense of such seemingly irrational human behavior. But Ernst Fehr, a Swiss economist, has studied cooperation and found that emotions play a key role in economic decision-making.
Brosnan says Fehr’s studies were a significant influence on her research, particularly a 1999 theoretical paper in which he argued that a sense of fairness could exert a stabilizing influence on human cooperation. Unfairness, on the other hand, may be a significant disincentive to cooperate.
“Her study suggests that the human sense of justice has deep evolutionary roots,” Fehr says of Brosnan’s work. “My belief is that both cultural and genetic evolution contributed to the human sense of fairness.”
“The big argument in economics has been maximization,” Brosnan says, referring to the theory that people do what they believe will yield the maximum benefit to them. “But there is a new movement in economics to incorporate emotions into decision-making, which can mean people are not completely rational. If you consider fairness, it explains a lot. The ultimatum game shows that humans will cut off their nose to spite their face if they are not being treated fairly, and the capuchins are definitely doing the same thing. It’s non-rational if you think about maximizing returns.”
Over time, however, a stubborn insistence on fair treatment could actually raise a capuchin’s “relative fitness” in a social colony, in contrast to boosting its “absolute fitness,” Brosnan says. In other words, if a capuchin refuses to hunt with the others because he didn’t receive his fair share from the last kill, he raises his own relative fitness relative to everyone else even if the group is not able to kill a squirrel without his help. Better to go hungry the same as everyone else than lose status by being cheated again.
“I think this opens up a whole new way of looking at primate behavior,” de Waal says. “Economists have believed we humans are rational optimizers, meaning we try to get as much as we can out of the deals that we make. But economists have always struggled with the fact that we are so emotional that sometimes the way we act is not rational at all. If you are a capuchin and you refuse cucumbers and throw them out of a cage, that is not rational. That’s emotional. If you were rational, you would just eat what you get. Humans show the same sort of reaction. The theory is that, in the long run, these decisions would be beneficial. In other words, if you keep an eye on what others get, in the long run you will be better off.”
The application of this knowledge to human situations could be far-reaching, from finessing family dynamics to resolving global conflict.
“The fact that it looks like our sense of fairness might be innate implies that it should be taken into account in all cooperative endeavors—peace treaties, for instance,” Brosnan says. “If you want something to succeed without bitterness, you need to make sure people are treated equitably.”
Just rewards
Brosnan’s fairness study made her something of an overnight sensation. On the day that the study came out, she heard from fifteen reporters; that week, she gave some seventy interviews to media outlets including the New York Times, Newsweek, the BBC, and National Geographic. She appeared on TV twice and was on the radio ten times. The story stayed in the number two slot on the popular website Google News for twenty-four hours straight, until it was displaced by the discovery of a giant rodent fossil in South Africa.
“It got a ton of attention, and no one really prepares you for that,” she says with a laugh. “When I did the interview with National Public Radio, I didn’t realize it would be on All Things Considered at 5:15 p.m., when so many people would hear it. Suddenly all these people were calling me, saying they had heard me on the radio. It was kind of neat to see people get so enthusiastic about science.”
The Nature study may have been the scientific equivalent of a pop album hitting platinum, but it probably won’t be the last discovery that puts Brosnan in the news. She and de Waal are currently conducting follow-up research to further examine the findings, while Brosnan begins her post-doctoral work at Emory, working with an anthropologist to study the positive influence of fairness on social interaction in groups of chimpanzees, children, and human adults. She also is teaching science courses at Emory and two other Atlanta-area colleges, and plans to continue to teach and conduct research.
“Sarah was a very enterprising student who always did more than what was regular, and she has a very outgoing personality, which helps a lot in that she easily initiates contacts with other people,” de Waal says. “She is extremely persistent. It is not unusual for academic journals to refuse papers, but she never got depressed or discouraged. What I really appreciated about Sarah was that she just kept going because she had enough belief in herself. The Nature paper was a nice reward for her persistence.”
Which, of course, is only fair.
Paige Parvin is associate editor of Emory Magazine and a freelance writer living in Atlanta.
