For many, the sex hormone testosterone is synonymous with everything manly, including aggressive behavior. But a new study finds this is not necessarily the case. Women given testosterone act nicer by making fairer offers during a bargaining game than those given a placebo, the researchers say.

However, the subjects' preconceived notions about how the hormone should make them act appear to influence their behavior. Those who simply believed they were given testosterone made less fair offers, regardless of which substance they actually received.

The findings add support to the idea that, rather than directly boosting aggression, testosterone might instead promote so-called status-seeking behavior, in which a person aims to raise or maintain their status in the social hierarchy.

If this is the case, testosterone might promote aggressive or kind behavior, depending on the circumstances.

"In certain environments, [such as prisons], it might be advantageous to be rather anti-social or even aggressive to maintain your status position," study author Christoph Eisenegger of the University of Zurich, Switzerland, told LiveScience. "And in other environments, maybe more peaceful environments, it perhaps rather pays off to be friendly rather than aggressive in terms of your status position."

Testosterone Equals Aggression?

Often thought of as the "male sex hormone" for its role in the development of male characteristics and reproductive organs, testosterone is also present in women, though in smaller amounts.

While animal studies have shown that testosterone is tied to an increase in aggression, the effect on humans is likely not so clear cut, experts say. Previous research that has found a link between testosterone and aggression in humans, such as studies of men in prison, has not shown a direct cause-effect.

In the new study, Eisenegger and his colleagues had 60 women play a bargaining game in which two people have to agree on how to divide a sum of money. One person decides how the money should be allocated, and the second either accepts or rejects the offer. If the offer is rejected, neither player gets any money. They tested women because researchers know more about how much testosterone is needed to see any effect and how long it takes to see that effect.

Low offers would be characteristic of aggressive, or anti-social, behavior, while an equal split would be seen as fair, and more pro-social.

Women given a dose of testosterone made significantly higher offers than those who received a placebo. The testosterone might have motivated pro-social behavior in this game since acting fairly would preserve their status by avoiding rejection of low offers the researchers say.

And while the women couldn't tell if they were given testosterone or a placebo, those who thought they received the hormone made lower offers than those who believed they were given placebo, suggesting that psychological factors also play an important role in how hormones affect behavior.

Implications

These findings could have clinical implications, the researchers say. For instance, if patients are given testosterone after surgical removal of the testicles due to cancer, they might believe it will make them more aggressive or risk-seeking, which could have a negative impact on their lives, Eisenegger said.

"By correcting the reputation of the hormone, we might also help some of the clinical populations," he said.

However, more research is needed to see if the same effect is found in men, and if the results hold true when the hormone is administered over long periods.

The study was published Jan. 21 in the journal Nature.