Born Gay? It's True

It shouldn't come as a surprise that imaging studies of sexual orientation are increasingly revealing anatomic and functional differences between "straight" and "gay" brains. But demonstrating such changes doesn't answer the age-old question of how much our sexual preferences are innate and how much they are fueled by environmental exposure, cultural norms and conscious personal choices.
One way to distinguish the effects of nature from nurture would be to look at brain regions believed by neuro-anatomists to be fully formed at birth and impervious to subsequent environmental effects, both physical and psychological. Focusing on such brain regions, a research team at the Karolinska Institute in Stockholm, Sweden, headed by neuroscientist Ivanka Savic, obtained MRIs for 90 adult volunteers -- 25 straight men, 25 straight women, 20 gay men and 20 lesbians. Using the latest quantitative techniques for assessing cerebral symmetry and functional connections between various areas of brain, Savic was able to demonstrate highly statistically significant differences between straight and gay brains. Gay and lesbian brains more closely resembled the brains of straight volunteers of the opposite sex than the brains of heterosexual members of the same sex.

Although Savic admits that her study cannot distinguish between genetic or prenatal intrauterine environmental changes, such as relative differences in sex hormone levels, her studies do suggest that our sexual preferences are, at least in large part, determined by the time of birth.