It sounds like something out of the satirical journal Annals of Improbable Research: a team of Swedish neuroscientists scanned people's brains as they smelled a testosterone derivative found in men's sweat and an estrogen-like compound found in women's urine.

In heterosexual men, a part of the hypothalamus (the seat of physical drives) responded to the female compound but not the male one; in heterosexual women and homosexual men, it was the other way around.