Nonhuman primates respond negatively when their fellow animals receive better rewards, but the reaction is based on fairness and not on awareness that better rewards are available, say researchers at Yerkes National Primate Research Center and the Living Links Center, Emory University. The scientists report results of a recent study in the current issue of the Proceedings of the National Academy of Sciences.
In 2003 researchers led by Sarah Brosnan, PhD, and Frans de Waal, PhD, found nonhuman primates respond negatively to unequal reward distribution, a reaction often seen in humans. In a follow-up study, Drs. Brosnan and de Waal attempted to determine whether negative reactions to inequity could be based on seeing better possible rewards, expecting better rewards based on past performances or, as suggested in the first study, a simple inequity between the rewards individuals received for completing the same task.
The researchers concluded a negative response is based on unequal reward distribution and that neither the presence of better rewards nor expectations of better rewards were factors.
In both studies, researchers made food-related exchanges with brown capuchin monkeys. The subjects refused previously acceptable rewards (cucumbers) if they witnessed their partners receiving higher-value rewards (grapes) for equal or less work. This is similar to the negative response humans display when they see other individuals receiving a better deal.
In the most recent study the scientists tested to ensure individuals were not reacting based on better rewards being available or having received better rewards in the past. The study included a condition in which possible food rewards were shown to the individual prior to completing a task. Researchers also tested whether individuals would be less likely to perform a task if the individual received cucumbers, a less valuable reward, following sessions in which the reward was a highly valued grape. Neither of the conditions, however, solicited a negative response.
"What if the negative reaction was not about what kind of reward the other got, but about holding out for something better? If the researcher has grapes, why work for cucumber?" says Dr. de Waal. "However, we could reject this argument because when we gave both monkeys equal cucumber slices, yet waved grapes around to show that we had them, they still reacted very much the same as they had in normal equity tests. This means their negative reaction to inequity is not so much about the presence or expectation of a better reward, but about the fact that the other is getting a better deal," he says.
The study also demonstrated individuals worked harder when rewards were fairly distributed, and the greatest sensitivity to inequity came from individuals who worked harder but were not fairly rewarded. The results of this study, coupled with data from the previous study, further demonstrate a direct link between nonhuman primate behavior and that of humans.
These findings continue to support the idea that economic decision-making is based as much on an emotional sense of fairness as on rational considerations. Identifying similar reactions in nonhuman primates as in humans offers insight into how such emotional reactions developed, providing researchers and economists new perspective on why humans make certain economic decisions in relation to efforts, gains and losses of others.
For more than seven decades, the Yerkes National Primate Research Center, Emory University, has been dedicated to advancing science and to improving human health and well-being. Today, the center, as one of only eight National Institutes of Health–funded national primate research centers, provides specialized scientific resources, expertise and training opportunities. Recognized as a multidisci¬plinary research institute, the Yerkes Research Center is making landmark disco veries in the fields of microbiol¬ogy and immunology, neuroscience, psychobiol¬ogy and sensory-motor systems. Research programs are seeking ways to: develop vaccines for infectious and noninfectious diseases, such as AIDS and Alzheimer's disease; treat cocaine addiction; interpret brain activity through imaging; increase understanding of progres¬sive illnesses such as Parkinson's and Alzheimer's; unlock the secrets of memory; determine behavioral effects of hormone replacement therapy; address vision disorders; and advance knowledge about the evolutionary links between biology and behavior.
The goal of the Living Links Center at the Yerkes National Primate Research Center is to view great apes as a window to the human past by studying their behavior, cognition, neuroanatomy, genes and reproduction in a noninvasive manor. Another goal is to educate the public about apes and to help guarantee their continued existence in the wild.