Feeling a bit full of yourself lately? It could be because of testosterone.

A new study from the University College of London finds elevated levels of male hormones makes people overvalue themselves at the expense of cooperation — and it doesn’t just happen among men.

Over a period of two days spaced a week apart, Dr. Nick Wright and his colleagues at the Wellcome Trust Centre for Neuroimaging at the University College London (UCL) did a series of experiments using seventeen pairs of female volunteers who had previously never met.

On one of the days, both volunteers in each pair were given a testosterone supplement, and on the other day, they were given a placebo.

During the experiment, both women sat in the same room and watched the exact same thing on separate screens. In each trial they were shown two images, one of which contained a high contrast target, and then tasked with deciding individually which image contained the target.

If their individual choices agreed, they received feedback and moved on to the next trial. However, if they disagreed, they were asked to collaborate and have discussions with their partner to arrive at a joint decision. One of the pair then input the decision.

As expected, cooperation enabled the group to perform much better than the individuals alone when they’d received only the placebo. But, when given a testosterone supplement, the benefit of cooperation went down dramatically. In fact, higher levels of testosterone were associated with individuals behaving egocentrically and deciding in favor of their own selection over their partner’s.

“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,” explains Dr Wright in a press release.

“We have shown that in fact testosterone also affects our decisions, by making us more egotistical,” he continued. “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. This can be very significant when we are talking about a dominant individual trying to assert his or her opinion in, say, a jury.”

Commenting on the findings, Dr. John Williams, Head of Neuroscience and Mental Health at the Trust, said, “Cooperating with others has obvious advantages for sharing skills and experience, but we know it doesn’t always work, particularly if one alpha male or alpha female dominates the decision-making. This result helps us understand at a hormonal level the factors that can disrupt our attempts to work together.”

The study results were published in the journal Proceedings of the Royal Society B.

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