The INAH 3 is the third interstitial nucleus of the anterior hypothalamus. Part of the sexually dimorphic nucleus, the INAH 3 is significantly larger in males than in females irrespective of age. Homologues of the INAH 3 have been observed taking a direct role in sexual behavior in rhesus monkeys, sheep, and rats.
Research
A scientific paper authored by Simon LeVay and published in the journal Science suggests that the region is an important biological substrate with regards to sexual orientation. This article reported the INAH 3 to be smaller on average in homosexual men than in heterosexual men, and in fact has approximately the same size in homosexual men as in heterosexual women. This study was criticized as having various methodological flaws.
A study by Dick Swaab similarly found the INAH 3 to be smaller in male-to-female transsexual people than in control males, and similar to control females.