The role of sex steroids in sexual differentiation of the human brain
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Evidence shows that testosterone and estrogens play a major role in the sexual differentiation of the brain and the subsequent behavioral and cognitive differences between men and women. The brain is thought to develop in the male direction due to organizing effects of both testosterone and estrogens during prenatal development and increases in testosterone levels during early neonatal development and puberty. Female brain development is considered to occur due to the absence of sex steroid action during prenatal development and subsequent surges of estrogens during the neonatal period and puberty. Interactions between sex steroids and the brain during specific organizational periods in early life are hypothesized to form the basis of some core aspects of male and female differentiation, i.e. gender identity, sexual orientation and sexual behavior. Indeed, disorders of sex steroids, such as congenital adrenal hyperplasia and complete androgen insensitivity syndrome, show that distortion of sex steroid action early in human life permanently affects later behavior. Changes in sex steroid levels during puberty and their activational effects are closely related to sex differences as well. The higher testosterone levels in men compared to women are thought to result in sex differences in aggression, risk-taking and visuospatial abilities, with men showing higher levels of aggression and risk-taking and increased visuospatial performance. The higher estrogen levels, specifically the higher estradiol levels, in women compared to men are thought to be responsible for the better verbal episodic memory in women. However, many of the exact neurobiological mechanisms by which these sex steroids affect the described functions, remain to be determined by future research.