The fluffy-faced monkeys judge the social interactions of others and hold biases against individuals behaving poorly, new research shows.

In a pair of studies, researchers investigated how capuchin monkeys in captivity reacted to different third-party social interactions. In one study, capuchins watched two actors engage in reciprocity exchanges, in which one actor handed over several balls to another, who then either reciprocated or selfishly kept all the balls. The second study involved a similar setup, but this time one actor helped or refused to help another actor who was struggling to open a container.

After each scene, the monkeys chose a treat from one of the actors — they consistently avoided treats from actors who refused to reciprocate or help. Capuchins in the wild may keep tabs on group members to figure out whom to avoid interacting with on a specific day, researchers said.

"The research implies capuchin monkeys are judging other individuals even when they aren't involved in the action, something that humans do all the time," said Sarah Brosnan, an ethnologist at Georgia State University, who wasn't involved in the new research. "It suggests the behavior may be deeply rooted in the primate family tree."