What is it about?
Stress is known to affect social behavior, but how exactly it does so is still unclear. In the past, its effects were contradictory: stress has been linked to either more aggressive reactions (fight-or-flight), or affiliative, prosocial efforts. In our study, we found that both can happen at the same time. In a lab experiment, we used two substances that activate key parts of the body’s stress response: cortisol (the main stress hormone) and noradrenaline (the key arousal transmitter in the brain). Participants then played a group-based decision-making game in which they teamed up with other participants, forming an in-group, and played against other teams of players, the rival out-groups. In this game, participants could choose how to divide money: keep it for themselves, financially support their own group, or both support their own group and harm their rival group. Participants with increased cortisol action were more cooperative toward their own group, while those with increased noradrenaline action were more hostile toward rival groups. This suggests that under stress, people may simultaneously seek connection with their peers, and act aggressively against adversaries, depending on the bodily stress response and the specific social situation.
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Why is it important?
Our findings may help clarify the complex ways stress affects social behavior. Instead of assuming that stress leads to either aggression or cooperation, we show that both tendencies can emerge simultaneously, driven by different biological systems. This challenges long-standing theories and suggests that stress-related behavior is more flexible than previously thought. Most importantly, we show that the dual-effect of stress (in-group support and, simultaneously, out-group aggression) amplifies an “us-versus-them” mentality through distinct, neurobiologically separable systems. This helps explain how stress contributes to rising polarization in society.
Perspectives
We often talk about stress as either making people lash out or come together—but it’s rarely that simple. We hope this research sparks further studies that move beyond either-or models and ask when, how, and why stress brings out our more cooperative or competitive sides. Understanding this may shed light on dynamics we see in everyday polarization, and perhaps even in large-scale intergroup tensions. In a world marked by rising division, it feels especially timely to understand how stress can both unite and divide us.
Luca Marie Lüpken
Heinrich-Heine-Universitat Dusseldorf
Read the Original
This page is a summary of: Dissociable glucocorticoid and noradrenergic effects on parochial cooperation and competition in intergroup conflict, Proceedings of the National Academy of Sciences, July 2025, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2502257122.
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