TUESDAY, Jan. 31 (HealthDay News) -- Testosterone makes people more self-centered and less cooperative, a finding that may explain why group decisions can be affected by dominant individuals, researchers report.

Their study included 34 females who had never met. The women were divided into 17 pairs and asked to complete a series of tasks designed to assess their levels of cooperation. The tests were conducted on two separate days. On one day both women received a testosterone supplement. On the other day, they were given a placebo.

As expected, cooperation helped the pairs perform much better on the tasks than when individuals worked alone. Cooperation was normal when the women received the placebo, but was much less common after the women received the testosterone supplement, the investigators found.

Increased levels of testosterone were associated with the women behaving egocentrically and deciding in favor of their own selection over their partner's, said the researchers at the Wellcome Trust Center for Neuroimaging at University College London in England.

The findings were published in the Jan. 31 issue of the journal Proceedings of the Royal Society B.

"When we are making decisions in groups, we tread a fine line between cooperation and self-interest: too much cooperation and we may never get our way, but if we are too self-orientated, we are likely to ignore people who have real insight," study author Dr. Nick Wright said in a center news release.

"Our behavior seems to be moderated by our hormones -- we already know that oxytocin can make us more cooperative, but if this were the only hormone acting on our decision-making in groups, this would make our decisions very skewed," Wright explained.

"We have shown that in fact testosterone also affects our decisions, by making us more egotistical. Most of the time, this allows us to seek the best solution to a problem, but sometimes, too much testosterone can help blind us to other people's views," Wright noted. "This can be very significant when we are talking about a dominant individual trying to assert his or her opinion in, say, a jury."

Testosterone is naturally secreted in both men and women. Previous research suggests that testosterone influences a number of social behaviors. For example, female prisoners with higher levels of testosterone have been found to be more anti-social and more aggressive.

More information

The U.S. National Library of Medicine has more about hormones.