Dan Gorodezky

Humans might not be the only primates motivated by spite, according to recent research at Yale.

The study, published in Evolution and Human Behavior, looks at capuchin monkeys’ tendency to punish a peer when faced with an unfair distribution of resources. The monkeys were presented with a scenario in which they shared food, and researchers were able to observe their behavior under varying conditions of inequity and social pressure to gain insight into the evolutionary origins of punishment.

“This study provides some of the first evidence that psychological motivations like spite may not be as unique to humans as previously thought,” said psychology professor Laurie Santos, a study co-author, in an email to the News.

Humans punish others who violate social norms, and previous studies have observed that chimpanzees behave similarly, Santos said. This study was designed to identify the same punishment behavior in capuchin monkeys, a more distant relative of humans, she added.

Capuchin monkeys are more cooperative, but also a lot more averse to inequity, said Kristin Leimgruber, a post-doctoral fellow at Harvard University and study co-author. In previous studies, they have demonstrated a willingness to help a partner get food as long as it was equally shared, said Sarah Brosnan, a Georgia State University professor who has conducted research with Leimgruber in the past, but was not involved with this study.

“Given these social differences, we thought it would be interesting to see what capuchins do in this paradigm and how it differs from chimpanzees,” Leimgruber said.

The setup in this experiment was nearly identical to a setup previously used with chimpanzees, Leimgruber said. Food was placed on a wooden table with a collapsible extension controlled by a rope. If the rope was pulled, anything on top of the collapsed section would fall into an inaccessible container below the table. Certain iterations of the experiment only involved one monkey, while others centered on the interactions of two monkeys located at opposite ends of the table: the subject — who had the option to collapse the table and remove the shared food source — and its partner.

Researchers tested two conditions in which the subject could react to inequitable distribution of the food. In the first condition, which tested the subject’s reaction to unintentional inequity, the experimenter moved the food source closer to the partner. In the other condition, intended to simulate intentional theft, the partner had access to a rope with which he could pull the food closer to him. Researchers measured how often the subject would collapse the table under each of these conditions.

The results of the experiments showed that the subjects chose to collapse the table more often when there was an inequitable distribution of food, but they did so at the same frequency regardless of whether the imbalance was intentional. Leimgruber said they were surprised by how often the subjects chose to punish their partner.

Santos said the finding was surprising because it showed that capuchins were spiteful and were willing to hurt others at cost to themselves. Other researchers have suggested that spite reinforces cooperation because it disincentivizes unfairness, Leimgruber said.

“This definitely causes us to reconsider spiteful behavior in a different way,” Leimgruber said. “It might be more broadly tied to cooperative behavior. It might not be exclusively human.”

Another factor that varied was the distance between the two monkeys and the rest of their social group. The presence of an audience of peers is thought to influence the decision to punish, but the study found that the subjects collapsed the table at the same rate with or without an audience.

Brosnan lauded the study for using a number of controls that distinguished between whether the subject’s decision to collapse the table was due to general frustration or a targeted punishment. She added that it was interesting that the results differed from the results of the chimpanzee experiment, in which the chimpanzees demonstrated sensitivity to intention as well as inequity.

“It will be interesting to see what future work uncovers about the role of punishment in maintaining social relationships and cooperation and how this varies across different primate species,” Brosnan said in an email.

Capuchin monkeys are native to South America.