The Effects of Sexual Differentiation of the Human Brain on Gender Identity

From toy-preference at an early age to the likelihood of developing schizophrenia, sexual differentiation of the human brain plays a critical role. In their research paper [1], Ai-Min Bao and Dick F. Swaab explain the difference between sexual differentiation of the brain and sexual differentiation of the genitals, as well as investigate the effects of sexual differentiation of the human brain on gender identity, sexual orientation, and neuropsychiatric disorders.

As we have learned in Psych 212, testosterone exposure during early development and other sensitive periods, such as adolescence, is central to sexual differentiation of both genitalia and brain.

The surge of testosterone in the intrauterine period results in a masculine brain and reproductive organs, while its absence results in a feminine brain and genitals. However, sexual differentiation of the brain and sexual differentiation of the genitals are two distinct processes that can be influenced separately; sexual differentiation of genitals (weeks 6-12 of pregnancy) occurs before that of the brain. During adolescence, the brain circuits that have been sexually differentiated in the womb are activated by sex hormones.

The study found that structural differences between male and female brains, coupled with the fact that sexual organs develop before sexual differentiation of the brain, provide an explanation for gender identity disorders such as transsexuality. For instance, the researchers found that the interstitial nucleus of the anterior hypothalamus-1 (INAH1) is 2.5 times larger in men than it is in women, and it contains 2.2 times more cells than that of women. Similarly, INAH2 and INAH3 also exhibit larger volumes in men than in women. During early development, sexual differentiation of the brain develops in alignment with sexual organs in most cases, but reversals can arise; the study found that among male-to-female (MtF) transsexuals, a feminine INAH3 has been discovered, indicating that MtF persons have undergone sexual differentiation reversals during early development; developing male genitalia but brains with female characteristics. The only female-to-male (FtM) transsexual that the researchers studied also exhibits an INAH3 with male characteristics.

The study found that structural and functional brain differences are related to sexual orientation. For instance, a homosexual man has a smaller INAH3 and a suprachiasmatic nucleus that is twice as large as that of a heterosexual man. As we learned in Psych 212, the SCN is a region in the hypothalamus that serves as our biological clock. In addition to structural differences, the study found that the hypothalamus of homosexual men was less responsive to fluoxetine – a prescription drug used to treat depression – as that of heterosexual men, indicating different activities of the serotoninergic system.

The prevalence of psychiatric disorders varies significantly between men and women. For instance, 93% of anorexia nervosa patients are females, while 87% of REM sleep disorder cases were found among males. The study discovered that in addition to affecting the pervasiveness of a specific disorder, sex differences also affect the symptoms and severity of such disorder. For example, schizophrenia – a disorder that is 2.7 times more prevalent among men than women – manifests more aggressively among men, leading to more negative symptoms and a larger number of structural brain abnormalities, such as severe enlargement of the lateral ventricles.

I believe that by conducting primary research and consulting a wide range of corroborative references, the study did an excellent job of presenting the effects of brain sexual differentiation on gender identity, sexual orientation, and the risk of developing neuropsychiatric disorders. However, throughout the study, the authors allege that postnatal social environments play no role in gender identity and sexual orientation, disregarding numerous studies (such as [2] and [3]) that argue otherwise.

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