Sexual dimorphism of the human brain: myth and reality.

Many neuroanatomical sex differences have been identified in both animals and humans, which may form the neural bases for sex-specific behavior and reproductive as well as non-reproductive functions. The present essay gives a brief review of the findings on sex differences in the human brain. Our observations on the human hypothalamus revealed that the shape of the suprachiasmatic nucleus (SCN)--a structure involved in the regulation of circadian rhythms and reproductive cycles--is elongated in females and more spherical in males. In addition, an extremely large SCN was observed in the brains of homosexual men who died from AIDS. Both the volume of the SCN and the number of vasopressin neurons were about twice as large as in a male reference group. In contrast to the SCN, in which only shape differences were found in relation to gender, the volume and cell number of the sexually dimorphic nucleus of the preoptic area (SDN-POA) showed a marked sexual dimorphism. The mean volume of the SDN-POA was 2.2 times larger in males than in females and contained about twice as many cells. The function of this sexually dimorphic area in humans is not known, but presumably it is involved in the control of male sexual behavior. The fact that no differences in either volume or cell number were observed between the SDN-POAs of homo- and heterosexual men indicates a selectivity of the SCN in this respect and contradicts the view that male homosexuals have a 'female' hypothalamus.