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Why do violent conflicts between groups persist – even when all sides suffer as a result? Researchers from psychology and medicine at Heinrich Heine University Düsseldorf (HHU) have now examined the dual effect of physiological stress messengers on social behaviour in intergroup conflicts. In an article in the scientific journal Proceedings of the National Academy of Sciences (PNAS), they report that stress can simultaneously trigger cooperativeness between individuals within a group and aggression toward strangers.
How do different components of the neurohormonal stress response affect the behaviour of individuals in different group constellations? A team of psychologists headed by Professor Dr Tobias Kalenscher from the “Comparative Psychology” research team at HHU has examined how the two neurophysiological messengers noradrenaline and cortisol – which are released in the brain in the event of acute stress – affect the social behaviour of study participants. The research was supported by Professor Dr Alfons Schnitzler, Director of the Institute of Clinical Neuroscience and Medical Psychology at University Hospital Düsseldorf.
In the psychopharmacological study, the participants were given either the drug hydrocortisone (which imitates the activity of the stress hormone cortisol) or Yohimbin (which increases the effect of the excitatory neurotransmitter noradrenaline), both together or a placebo. They were then split into groups and played economic games against other groups. The games involved real money – the participants were allowed to take their payoff home with them.
Luca Marie Lüpken and Damon Dashti, the lead authors of the study: “We wanted to see how the different neurochemicals influence the behaviour of the participants. Cortisol did in fact promote cooperative behaviour toward the participants’ own group. By contrast, noradrenaline increased hostile behaviour toward out-groups, even when this involved financial costs for the participants.”
The findings thus present a complex picture. Although both neurochemicals are essential components of the neurophysiological stress response, the participants responded very differently, depending on which substance they were given. Professor Kalenscher, head of the study: “Stress does not make you aggressive or cooperative in general. Depending on which neurochemical pathway prevails in the physiological stress response and who you are dealing with – friend or foe – one of the two behavioural patterns can dominate.”
According to the authors, the findings can help to understand how stress promotes an “us versus them” mentality and which neurobiological mechanisms are involved. They thus offer a neurobiological explanation for the increasing polarisation in the world.
Background: the game scenario
Each participant plays a total of three rounds, one against each group of opponents (“out-group”). They are assigned to a group of friends (“in-group”). In each new round, the experimenter gives each participant €10, which they can distribute freely across various choice alternatives.
The study participant can choose to
- keep the money themselves,
- invest some of it so that, while they personally lose money, the members of their own group (the in-group) benefit financially,
- or invest some of it so that, while they personally lose money, the members of the in-group benefit financially and, at the same time, the members of the out-group lose money.
Accordingly, the out-group can only lose money when the members of the participant’s own team receive money at the same time, i.e. when the participant acts cooperatively within their own group.
Author: Dr. Arne Claussen
Prof. Dr. Tobias Kalenscher
Dashti D, Lüpken LM, Seidisarouei D, Forbes PAG, Schnitzler A, Kalenscher T. Dissociable Glucocorticoid and Noradrenergic Effects on Parochial Cooperation and Competition in Intergroup Conflict. PNAS 2025, Vol. .122 (29) e2502257122.
DOI: 10.1073/pnas.2502257122
Under the influence of neurochemicals, people can behave cooperatively within a group at the same ti ...
Copyright: AI-generated image: HHU/Paul Schwaderer/Midjourney)
Schematic diagram of the game scenario.
Copyright: Fig.: HHU/Tobias Kalenscher
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Under the influence of neurochemicals, people can behave cooperatively within a group at the same ti ...
Copyright: AI-generated image: HHU/Paul Schwaderer/Midjourney)
Schematic diagram of the game scenario.
Copyright: Fig.: HHU/Tobias Kalenscher
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