Trusting women become more skeptical when they are given doses of the sex hormone testosterone, a new study suggests.
In the study, these "socially naïve" women rated pictures of faces as less trustworthy after they were given testosterone compared with when they received a placebo. However, testosterone did not appear to have an effect on those who were naturally less trusting, the researchers say.
Testosterone could serve as a balance to oxytocin, a hormone that has been implicated in human social bonding and trust, the researchers figure.
The study also adds support to the idea that testosterone influences human behavior, not necessarily by increasing aggression, but by motivating people to raise their status in the social hierarchy or become more socially dominant.
Testosterone might boost social watchfulness, making those who are most trusting a little more vigilant and better prepared for competition over rank and resources, the researchers say.
"To be more successful in competition you have to be sharp ... you have to be also socially sharp," researcher Jack van Honk of the University of Cape Town, South Africa told LiveScience. "And to be socially sharp it's not smart to trust people you don't know," he said.
Van Honk and his colleagues administered either testosterone or a placebo to 24 women with an average age of 20. Two rounds of the experiment were conducted so that all women ended up getting both substances, though three days apart.
Only women were used in the study because the physical effects of testosterone are better studied in women than in men. (While testosterone is often thought of as the "male sex hormone" for its role in the development of male characteristics and reproductive organs, it is present in women, in smaller amounts.)
The women were asked to rate 150 unfamiliar faces on a scale of minus 100, or very untrustworthy, to 100, or very trustworthy.
The researchers divided the women into two groups ("high trusting" and "low trusting") based on how they rated faces after they had received the placebo, which was considered their baseline trust level under normal circumstances.
Women in the "high trusting" group rated the faces on average 10 points lower than baseline after they had received testosterone.
The subjects' mood and natural levels of testosterone did not appear to affect the results. Also, there was no evidence that the women knew which day they had received testosterone.
Testosterone might increase production of a hormone called vasopressin, which is known from animal studies to increase aggression and territorial behavior, van Honk said.
So while testosterone might act on the same part of the brain in animals and in humans, the effects on behavior could be different, van Honk said. Being mindful of those you don't know, or competitors seeking to increase their own status, is an important part of rising through the ranks, he said.
And being less trusting could have benefits in economic or bargaining situations. Indeed, previous studies have linked high testosterone levels in traders with above average profits in the stock market.
The results are published this week in the journal Proceedings of the National Academy of Sciences.
