Sexually dimorphic nucleus

The sexually dimorphic nucleus (SDN) is a cluster of cells in the preoptic area of the hypothalamus, an area of the human brain and that of other mammals, which is controversially considered by some studies to play a key role in the sexual differentiation of the brain. This cluster of cells contains about twice as many cells in adult men compared to adult women.

Role in sexual development
At birth, the sexually dimorphic nucleus contains around 20% of the cells found at 24-48 months of age; after that age, a decrease in cell count occurs in females, but not males, causing the physical sexual differentiation of the brain. Initially some studies considered this area of the brain a likely candidate for neurological changes leading to homosexuality, but studies such as Swaab et al found no statistically significant difference in cell count between test groups of heterosexual and homosexual male adults. However, studies by neuroscientist Simon LeVay have shown a statistically significant difference in other areas of the brain, such as in the third interstitial nucleus of the anterior hypothalamus (INAH-3), which was found to have a lower cell count in homosexual male adults compared to heterosexuals.