When testosterone was given, the hormone disrupted the two women’s ability to work together and come to a mutual decision, Wright said. Often one woman stood by her answer and entered it into the computer, the researchers said. This was “egocentric” behavior, focused on the self.

“Egocentricity bias is the degree to which people over-weight their own opinion. If you are more egocentric, you are more likely to think you are right,” Wright said. “These women were more likely to say they were right when they were on testosterone than when they were on placebo.”

Other effects of higher testosterone levels in females have been found (even within the normal hormone range), including antisocial behavior in female prisoners, aggressiveness, and decreased ability to trust.