Department of Clinical Neuroscience, Karolinska Institute, Stockholm, Sweden.
Prog Brain Res. 2010;186:41-62. doi: 10.1016/B978-0-444-53630-3.00004-X.
It is believed that during 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. According to this concept, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation should be 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. Data on genetic and hormone independent influence on gender identity are presently divergent and do not provide convincing information about the underlying etiology. To what extent fetal programming may determine sexual orientation is also a matter of discussion. A number of studies show patterns of sex atypical cerebral dimorphism in homosexual subjects. Although the crucial question, namely how such complex functions as sexual orientation and identity are processed in the brain remains unanswered, emerging data point at a key role of specific neuronal circuits involving the hypothalamus.
人们认为，在子宫内，胎儿的大脑通过睾丸激素对发育中的神经细胞的直接作用向男性方向发育，或者通过缺乏这种激素激增向女性方向发育。根据这一概念，我们的性别认同（属于男性或女性性别的信念）和性取向应该在我们还在子宫内时就被编入我们的大脑结构中。然而，由于生殖器的性别分化发生在怀孕的头两个月，而大脑的性别分化始于怀孕的后半期，这两个过程可以独立受到影响，这可能导致性别认同障碍。这也意味着，在出生时出现性别模糊的情况下，生殖器的男性化程度可能无法反映大脑的男性化程度。没有证据表明出生后的社会环境对性别认同或性取向有影响。目前，关于遗传和激素独立影响性别认同的数据存在分歧，无法提供有关潜在病因的令人信服的信息。胎儿编程在多大程度上决定性取向也是一个讨论的问题。许多研究表明，同性恋者的大脑存在性非典型二态性模式。尽管关键问题，即性取向和身份等复杂功能如何在大脑中处理，仍然没有答案，但新出现的数据表明，涉及下丘脑的特定神经元回路起着关键作用。
