Sexual differentiation of the human hypothalamus: ontogeny of the sexually dimorphic nucleus of the preoptic area.

Sexual differentiation of the hypothalamus of the human brain is generally believed to take place around midpregnancy and thought to be related to the development of sexual orientation and gender identity. The present life span study on the human sexually dimorphic nucleus (SDN) of more than a hundred subjects revealed, however, that at the age of 2-4 years the SDN cell number reaches a peak value, and that only after this age sexual differentiation becomes manifest. Furthermore, the SDN cell number in homosexual men was not different from that of the male reference group, but significantly larger than the cell number in age-matched women.