Page last updated at 00:38 GMT, Wednesday, 9 December 2009
Testosterone link to aggression 'all in the mind'
Hormones may dictate only a small part of our attitude
Giving women more of the male hormone testosterone can turn them into fairer and more amiable game players, according to tests.
A single dose of testosterone was enough to have this effect, European scientists found, but only if the woman was oblivious to the treatment.
If she realised she had received the hormone and not a dummy drug, she turned to greed and selfishness.
The work in Nature magazine suggests the mind can win over hormones.
Testosterone induces anti-social behaviour in humans, but only because of our own prejudices about its effect rather than its biological activity, suggest the authors.
They believe the same is true in men, although they only studied women.
Power of suggestion
For the study, they asked more than 120 women to pair up and play an "ultimatum" bargaining game with real money at stake.
In the game, one of the pair is the "proposer" and is tasked with suggesting to the other player - the responder - how to split the money between them.
The responder can then only accept or reject the offer.
This puts hormones in their place. Hormones provide a basic backdrop, but changes in levels will do little to behaviour compared to personality, culture and society
Endocrinologist Professor Ashley Grossman
If they reject it, neither of the pair gets any of the cash.
The researchers gave the proposers either a dummy pill or one containing testosterone, but did not tell the women which pill they had been given.
Once they had played the game, the proposers were asked to say which pill they thought they had taken.
Those who received testosterone behaved more fairly, had fewer bargaining conflicts and were better at social interactions.
However, women who thought that they had received testosterone, whether or not they actually did, behaved more unfairly than those who thought that they had received placebo, again whether or not they actually did.
The researchers, led by Ernst Fehr of the University of Zurich, Switzerland, said the results suggested a case of "mind over matter" with the brain overriding body chemistry.
"Whereas other animals may be predominantly under the influence of biological factors such as hormones, biology seems to exert less control over human behaviour," they said.
UK endocrinologist Professor Ashley Grossman said: "This puts hormones in their place.
"Hormones provide a basic backdrop, but changes in levels will do little to behaviour compared to personality, culture and society."
Published online 8 December 2009 | Nature | doi:10.1038/news.2009.1131
Testosterone link to aggression may be all in the mind
A dose of the hormone makes human game-players behave more fairly.
Angry girlHigher levels of testosterone do not necessarily lead to more aggressive behaviour.iStockphoto
The popular idea that testosterone always makes people more aggressive has been debunked by researchers. A team based in Switzerland has shown that the hormone can make people behave more fairly in an effort to defend their social status.
Ernst Fehr, an experimental economist at the University of Zurich, and his colleagues used the 'ultimatum bargaining' game to test how testosterone would affect behaviour in a group of 121 women. Counter-intuitively, women who were given testosterone bargained more fairly.
But the idea that testosterone causes aggression in humans, as it clearly does in rodents, is so firmly ingrained in the human psyche that women who believed they had been given testosterone ― whether or not they had ― bargained much less fairly.
Women, not men, were tested because they have less variable 'baseline' blood testosterone levels.
The study is published in Nature1. "It is a folk hypothesis that testosterone causes aggression," says Fehr. "But human society is more complex than this."
Several studies in humans have shown positive correlations between high blood testosterone levels and confrontational behaviour. But it has been hard to determine experimentally whether the aggression is caused by testosterone or is instead a consequence of a challenge to a person's social status.
The ultimatum game makes it possible to distinguish between these possibilities.
“It is a folk hypothesis that testosterone causes aggression. But human society is more complex than this.”
In the game, two individuals must agree on the division of a sum of money. The proposer suggests a particular splitting of the sum and the responder must accept or reject the offer. The proposal is an ultimatum ― the responder may not make a counter-offer. If the responder accepts the proposal, the money is duly allocated. If the responder rejects the proposal, neither the proposer nor the responder gets any money.
Responders normally reject very low offers as unfair ― they would rather receive no money than see their partner carry off a disproportionate amount of cash.
Some proposers offer a 50-50 split because they are motivated by fairness, although most push to keep a bit more for themselves ― but not so much more that they risk rejection and ending up with nothing.
Fehr's team reasoned that if testosterone caused aggression, it would cause proposers to make low offers. If, however, it promoted social-status-seeking behaviour, proposers would make higher offers to avoid the social affront of having their offers rejected.
The women were given either 0.5 mg testosterone or a placebo four hours before playing the ultimatum game for the sum of 10 money units. Before they played, they were asked to say whether they believed they had been given testosterone or placebo.
Women who received testosterone made significantly higher offers than those who received placebo ― an average of 3.9 money units compared with the placebo group's average offer of 3.4 money units.
"In the socially complex human environment, pro-social behaviour, not aggression, secures status," says Michael Naef, an experimental economist at the Royal Holloway, University of London, who is a co-author on the paper.
The study has an additional, equally important message: those who believed they received testosterone, whether they had or not, made much lower offers ― as low as 2 money units in some cases, or even nothing. "We think their belief that they had received testosterone, and that testosterone promotes aggression, gave them an up-front excuse to act more aggressively," says Fehr.
Responders remained as likely to reject a shabby offer when they were treated with testosterone as when they received placebo, showing that the hormone was not promoting altruistic behaviour.
Adam Goodie, a psychologist at the University of Georgia in Athens who works on decision-making, says: "The paper is a major blow to the popular wisdom that testosterone simply makes you more aggressive and less cooperative ― the true picture is not nearly as negative."
"And it takes the field of neuroeconomics an important step further by showing that not only does biology affect economic behaviour ― but so does belief," he adds.
The powerful impact of belief is a good lesson for neuroeconomists, adds Fehr: "Belief should always be controlled for in neuroeconomics studies, but often it is not."
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