Women given testosterone while undergoing a sex change start to think more like men after the treatment, research suggests.
Their brains undergo structural changes and shrink in areas that play a key role in language, scientists found.
The discovery reinforces the idea that "men are from Mars and women from Venus" because of the way their brains are wired.
Women are known to have better verbal and multi-tasking skills than men, while men are believed to have a superior spatial ability.
Scientists put 18 female-to-male transsexuals through magnetic resonance imaging (MRI) brain scans before and after four weeks of testosterone treatment.
They found that exposure to the male hormone reduced the volume of grey matter - nerve cells - in two key regions of the brain linked to language processing, Broca's and Wenicke's areas.
At the same time, connections between these two regions became stronger.
Researcher Professor Rupert Lanzenberger, from the University of Vienna, said: "What we see is a real quantitative difference in brain structure after prolonged exposure to testosterone. This would have been impossible to understand without looking at a transsexual population.
"In more general terms, these findings may suggest that the genuine difference between the brains of women and men is substantially attributable to the effects of circulating sex hormones. Moreover, the hormonal influence on human brain structure goes beyond early developmental phases and is still present in adulthood."
The findings were presented at the annual meeting of the European College of Neuropsychopharmacology (ECNP) in Amsterdam, the Netherlands.
Commenting on behalf of the ECNP communications committee, Dr Kamilla Miskowiak, from Copenhagen University Hospital, said: "It is well known that language development differs between girls and boys and that this is related to gender-related differences in brain maturation.
"However, this intriguing neuroimaging study of transsexuals before and after their female-to-male gender reassignment suggests that even adult men and women differ in brain structure within regions involved in language and speech.
"In particular, female-to-male gender reassignment resulted in local brain matter decrease within language processing regions, which may explain why verbal abilities are often stronger in women."