Raised levels of testosterone can make women less trusting of men, research has shown.
Scientists found that women were not so easily taken in by a stranger's face after receiving a dose of the hormone.
The findings suggest that one role of testosterone may be to aid survival in the "social jungle".
Although normally thought of as the "male hormone", testosterone fuels the sex drive of both men and women.
The new research indicates that it may also make trusting souls less gullible.
In this way, testosterone may compliment the so-called "cuddle" hormone oxytocin, which appears to increase trusting behaviour.
Scientists led by Dr Jack van Honk, from Utrecht University in the Netherlands, administered under-the-tongue doses of both testosterone and a "dummy" placebo to 24 young women with an average age of 20.
After taking each one the women were asked to rate the trustworthiness of strangers' faces in a series of photographs.
Each picture was assigned a score from minus 100 (very untrustworthy) to plus 100 (very trustworthy),
Women who appeared the most trusting after receiving the "dummy" placebo reduced their scores by an average of 10 points when their testosterone was boosted.