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There may finally be an explanation for why men are often less verbally adept than women at expressing themselves. It's the testosterone.

Scientists have long known that language development is different between boys and girls. But in scanning the brains of 18 individuals before and after undergoing hormone treatment for female-to-male sex reassignment, Austrian and Dutch researchers found evidence of specific brain structure differences.

In particular, they found two areas in the left hemispheres of the transgender men that lost gray matter volume during high-testosterone treatment over a period of four weeks: Broca's area, which is involved in the production of language, and Wernicke's area, which processes language. All of which suggests, according to the study, which was presented this week at the European College of Neuropsychopharmacology Congress, why verbal abilities are often stronger in women than men.

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"In more general terms, these findings may suggest that a genuine difference between the brains of women and men is substantially attributable to the effects of circulating hormones," said one of the researchers at the conference, Rupert Lanzenberger from Vienna. "Moreover, the hormonal influence on human brain structure goes beyond the early developmental phase and is still present in adulthood."

Previous research has shown that higher testosterone is linked to smaller vocabulary in children and also that verbal fluency skills seemed to decrease after female-to-male sex reassignment testosterone treatment.

One of the surprises of the study was that while the gray matter decreased in the brains of the trans male subjects, their white matter increased. The researchers suggested that the white matter, which helps the flow of information pass from neuron to neuron, is a compensation for the deterioration in gray matter volume and that it appears more relevant to cognitive function.

"We think when it comes to certain language skills," said another of the study's scientists, Andreas Hahn, "the loss of gray matter outweighs the strengthened white matter connection."