On Sept. 11, 2008, Salon.com reported on a new Swedish study that strongly correlates adult sexual orientation with brain differences not subject to significant post-natal plasticity. Money quote:
As the accuracy and resolution of brain imaging improve, we can expect virtually all behavior to be shown to be associated with demonstrable brain changes. 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.
In their study, reported in the June 16, 2008, issue of the Proceedings of the National Academy of Sciences, Savic said, “This is the most robust measure so far of cerebral differences between homosexual and heterosexual subjects.” 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.