In a study published in PNAS this week two researchers report that the brains of heterosexual men and homosexual women are slightly asymmetric, with the right hemisphere larger than the left. Ivanka Savic and Per Lindström also found that the brains of gay men and straight women are not slightly asymmetric.

Here come the headlines…

Gay men and straight women share brain detail – Reuters

Scans see ‘gay brain differences’ – BBC

Study Says Brains of Gay Men and Women Are Similar – Scientific American

Study: Gay Men, Straight Women Share Brain Characteristics – Fox News

Gay men and straight women have similar brains, study says – LA Times

Interestingly no-one seems to have gone with a ‘Straight men and gay women have similar brains’ headline. So I’ve done it.

In their study Savic and Lindström used PET scans of the brains of 90 people. They looked at the connections of the left and right amygdalae. Crucially, the study says “The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities.”

Why these differences arise isn’t clear as the study, in the paper’s words, “does not allow narrowing of potential explanations”.

However, the paper does note:

It nevertheless contributes to the ongoing discussion about sexual orientation by showing that homosexual men and women differed from the same-sex controls and showed features of the opposite sex in two mutually independent cerebral variables, which, in contrast to those studied previously, were not related to sexual attraction.

The observations cannot be easily attributed to perception or behavior. Whether they may relate to processes laid down during the fetal or postnatal development is an open question

So while this shows that sexual orientation is probably not a choice, it doesn’t show whether it’s a nature or a nurture thing.

The paper should appear here soon.

Image: courtesy of National Academy of Sciences, PNAS (copyright 2008).