Oxytocin Increases In-Group Empathy – and Aggression Toward Out-Groups

us vs them

Lately there has been a ton of press about the potential of oxytocin (the “cuddle hormone”) to increase empathy and kindness.

As an example of the oxytocin makes people more kind idea, here is a 2007 article from PLoS ONE, “Oxytocin Increases Generosity in Humans“:


Human beings routinely help strangers at costs to themselves. Sometimes the help offered is generous—offering more than the other expects. The proximate mechanisms supporting generosity are not well-understood, but several lines of research suggest a role for empathy. In this study, participants were infused with 40 IU oxytocin (OT) or placebo and engaged in a blinded, one-shot decision on how to split a sum of money with a stranger that could be rejected. Those on OT were 80% more generous than those given a placebo. OT had no effect on a unilateral monetary transfer task dissociating generosity from altruism. OT and altruism together predicted almost half the interpersonal variation in generosity. Notably, OT had twofold larger impact on generosity compared to altruism. This indicates that generosity is associated with both altruism as well as an emotional identification with another person.

You can read the whole article online.

Now we know that such results are limited to one’s in-group, and several studies suggest that oxytocin may increase aggressiveness toward out-group members, e.g. this article from Science Daily:

Neurobiological Cause of Intergroup Conflict: ‘Bonding Hormone’ Drives Aggression Towards Competing Out-Groups.

ScienceDaily (June 14, 2010) — Researchers at the University of Amsterdam provide first-time evidence for a neurobiological cause of intergroup conflict. They show that oxytocin, a neuropeptide produced in the brain that functions as hormone and neurotransmitter, leads humans to self-sacrifice to benefit their own group and to show aggression against threatening out-groups. This finding qualifies the wide-spread belief that oxytocin promotes general trust and benevolence.

Results were published in the journal Science.

An important qualification of this research is that oxytocin, commonly referred to as the “bonding hormone,” functions as a cause of defensive aggression — aggression oriented towards neutralizing a threatening out-group. When the competing out-group was not considered a threat, oxytocin only triggered altruism towards one’s own group. This finding provides a neurobiological explanation for the fact that conflicts between groups escalates when other groups are seen as threatening. When such threat is low, for example because there are (physical) barriers between the group territories, conflict escalation is less likely.

The evolution of altruism in intergroup conflict

The research team at the University of Amsterdam, directed by Dr. Carsten de Dreu, wondered why oxytocin would promote altruistic behavior. Whereas classic economic theory has difficulty accounting for altruism, an evolutionary perspective suggests that altruism functions to strengthen one’s own group, from which the individual benefits in the long run. Because aggression towards competing out-groups helps one’s own group to become relatively stronger, aggression is an indirect form of altruistic, loyal behavior towards one’s own group.

Charles Darwin already observed that groups whose members are altruistic towards the own group have a greater likelihood to prosper, to survive, and spread. The researchers reasoned that if this is true, neurobiological mechanisms should have evolved that sustain altruism towards the own group, and aggression towards competing other groups. The discovery that oxytocin promotes altruism towards the own group, and aggression towards threatening out-groups, supports this evolutionary perspective.


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