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Testosterone does not induce aggression

Posted on 08/12/2009

Michael Naef is co-author of the study

New scientific evidence refutes the preconception that testosterone causes aggressive, egocentric, and risky behaviour. A study conducted by Royal Holloway, University of London and the University of Zurich, involving more than 120 experimental subjects, has shown that the sexual hormone can actually encourage fair behaviour.

For decades, popular scientific literature, art and the media have been associating testosterone – arguably the best-known sexual hormone - with the role of promoting aggression. Research appeared to confirm this – the castration of male rodents evidently led to a reduction in combativeness among the animals. Prejudice has grown over decades, dictating that testosterone causes aggressive behaviour.

However, the conclusions that testosterone produces the same effects in humans have now been proven to be false by the study – published today (8 December) in ‘Nature’ – conducted by economist Michael Naef of Royal Holloway, neuroscientist Christoph Eisenegger, and economist Ernst Fehr, both from the University of Zurich.

For the study, some 120 test subjects took part in a behavioural experiment where the distribution of a real amount of money was decided. The rules allowed both fair and unfair offers. The negotiating partner could subsequently accept or decline the offer. The fairer the offer, the less probable a refusal by the negotiating partner. If no agreement was reached, neither party earned anything.

Before the game the test subjects were administered either a dose of 0.5 mg testosterone or a corresponding placebo. “If one were to believe common opinion, we would expect subjects who received testosterone to adopt aggressive, egocentric, and risky strategies – regardless of the possibly negative consequences on the negotiation process,” says Dr Eisenegger.

The study’s results, however, contradict this view sharply. Test subjects with an artificially enhanced testosterone level generally made better, fairer offers than those who received placebos, thus reducing the risk of a rejection of their offer. The findings suggest that the hormone increases sensitivity of social status. For animal species with relatively simple social systems, an increased awareness of status may express itself in aggressiveness. “In the socially complex human environment, pro-social behaviour, not aggression, secures status,” surmises Michael Naef. "It is probably the interplay between testosterone and the socially differentiated environment of humans, not testosterone itself, that causes fair or aggressive behaviour.”

Moreover the study shows that the popular wisdom that the hormone causes aggression is apparently deeply entrenched: those test subjects who believed they had received the testosterone compound and not the placebo stood out with their conspicuously unfair offers. It is possible that these persons exploited the popular wisdom to legitimate their unfair actions.

Michael comments, “It appears that it is not testosterone itself that induces aggressiveness, but rather the myth surrounding the hormone. In a society where qualities and manners of behaviour are increasingly traced to biological causes and thereby partly legitimated, this should make us sit up and take notice.”

To view the article in 'Nature', visit: http://www.nature.com/nature/journal/vnfv/ncurrent/abs/nature08711.html


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