“A common danger unites even the bitterest enemies.” – Aristotle

Popular belief holds that nothing unites like a common enemy, but is this really true? Politicians seem to think so, often using divisive rhetoric and fearmongering to unite their political party against a common enemy. Recent research supports the premise that between-group conflict may be a “necessary evil” to promote and maintain the high levels of cooperation characteristic of human groups. Furthermore, parochial altruism—heightened ingroup favoritism in the face of conflict—may have biological underpinnings deeply rooted in our evolution.

Specifically, oxytocin and testosterone, hormones linked to prosociality and competition, respectively, moderate the positive feelings we experience toward our groupmates and the hostility we feel toward others. Of course, all social animals have rich social lives and elaborate social capacities. They should, like humans, also benefit from a high degree of group cohesion and cooperation when encountering a common threat.

How common is conflict as a driver of cooperation? And do other social animals experience ingroup favoritism and outgroup hostility? This Diverse Intelligences project, directed by Sarah Brosnan, aims to answer these questions to examine how competition influences cooperation, and the mechanisms that mediate cooperation during conflict in capuchin monkeys.

Brosnan’s team presents two novel advances over previous work. First, past studies have been constrained to laboratory settings, where between-group competition is nonexistent. In contrast, Brosnan’s team proposes a controlled cooperative paradigm in the wild, where competition is a near-constant stressor, allowing for, rather than controlling against, a rich array of social interactions. Second, while researchers cannot directly ask how animals feel, their ability to monitor hormone levels and endogenously manipulate oxytocin—again in the wild—allows the team to answer these questions in new ways. This provides valuable evidence on how capuchins experience the world and how it relates to human experience.