The hypothalamus regulates what we call the 4 F’s: fighting, fleeing, feeding, and mating. In the first half of the 1990’s, scientist Simon LeVay found a nucleus in the hypothalamus that has a statistically significant difference between sexes, the INAH3. Later on, he found that the difference has more to do with sexual orientation than sexual identity, since the INAH3 in homosexual men was smaller than in their heterosexual counterparts, more similar to the size women have. In following studies, it was shown that homosexual women have a larger INAH3, much like that of heterosexual men. This shows that sexual orientation is a gradient more than it is a binary, and it would explain the wide range of sexual orientations we see today.

It is worth noting that LeVay’s study was highly controversial because, aside from investigating such a hot topic, the post-mortem examinations on homosexual men were men who died from AIDS since they were the only ones prompted to disclose their sexuality. There is a possibility of AIDS being a confounding variable. Subsequent studies found the same tendency, but with less drastic results.