Oxytocin, the 'hugging hormone' causes aggression toward competing groups

Altruism within a group has its flip side: aggression towards other groups.

Altruism within a group has its flip side: aggression towards other groups.

Researchers from the University of Amsterdam (UvA) and Leiden University are the first to reveal a neurobiological cause of conflicts between groups. They posit that oxytocin, a neuropeptide produced in the brain that functions as a hormone and neurotransmitter, causes people to demonstrate preferential treatment towards people in their own group and aggressive behaviour towards members of other competing groups.

With their results, the researchers are able to further refine the currently held assumption that oxytocin makes people more pleasant and more altruistic.  Their findings were published last week in the journal Science.

Altruism only applies within groups

An important modification that came out of the research was that oxytocin, commonly known as the 'hugging hornmone', causes defensive aggression. The aggression was focused on neutralising a threatening group; if the competing group did not constitute a threat, oxytocin only led to altruism towards the subject's own group. These findings provide a neurobiological explanation for the fact that conflicts between groups escalate when people regard other groups as a threat. Where this is not the case, for example when there is a physical barrier between the territory of the two groups, the likelihood of violent conflicts is less.

Altruism difficult to understand from classical economic perspective

The researchers, supervised by Professor Carsten de Dreu from the UvA and Professor Eric van Dijk from Leiden University, questioned why oxytocin promotes altruistic behaviour. From the viewpoint of classical economic theory, altruism is difficult to understand. However, an evolutionary perspective suggests that people exhibit altruistic behaviour in order to make their own group more effective and stronger, which in the long term also benefits the individual. Seen in this light, aggressive behaviour towards competing groups is an indirect form of loyal, altruistic behaviour towards one's own group, which becomes stronger as competing groups become weaker.


Charles Darwin notes that groups where the members are altruistic towards their own group and aggressive towards other groups have a greater chance of survival than groups where there is a lack of such altruism. The researchers reasoned that if this evolutionary perspective is correct, there should be neurobiological mechanisms that direct this altruism and aggression simultaneously. The discovery that oxytocin promotes both altruism to one's own group and aggression towards competing groups supports this theory. 

Leiden's contribution

Eric van Dijk is  member of the Social and Organisational Psychology group at Leiden University. He is involved in the research into oxytocin because his group is working on experiments based on economic games, to arrive at a better understanding of social dilemmas and conflict situations. In the research into the influence of oxytocin, use was also made of such games, where some of the volunteers were administered with artificial oxytocin. 

Publication details

C.K.W. de Dreu, L.L. Greer, S. Shalvi, M.J.J. Handgraaf, G.A. van Kleef, M. Baas, F.S. ten Velden, E. van Dijk and S.W.W. Feith: The neuropeptide oxytocin regulates parochial altruism in intergroup conflict among humans. Science (Vol. 328, no. 5984, p. 1343).

Complete article

14 June 2010/HP
Source: University of Amsterdam

Last Modified: 17-06-2010