• Dick F Swaab and Alicia Garcia-Falgueras.
• The Netherlands Institutes for Neuroscience, Amsterdam, The Netherlands. d.f.swaab@nin.knaw.nl
• Funct Neurol. 2009 Jan 1; 24 (1): 17-28.

AbstractDuring the intrauterine period the fetal brain develops in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed into our brain structures when we are still in the womb. However, since sexual differentiation of the genitals takes place in the first two months of pregnancy and sexual differentiation of the brain starts in the second half of pregnancy, these two processes can be influenced independently, which may result in transsexuality. This also means that in the event of ambiguous sex at birth, the degree of masculinization of the genitals may not reflect the degree of masculinization of the brain. There is no proof that social environment after birth has an effect on gender identity or sexual orientation.

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