Jumping Into the Salon on Cerebral Asymmetry and Sexual Orientation

In an article from salon.com, neurologist and novelist Dr. Robert A. Burton interviews¹ fellow neurologist and "sometimes controversial gay activist"² Dr. Jerome Goldstein about a recent study on gay brains and straight brains. The primary finding of the Savic and Lindström (2008) paper was that on measures of cerebral asymmetry and amygdalar connectivity, gay men were similar to straight women, and gay women were similar to straight men. The authors attributed these differences to alterations in sex hormones during fetal or postnatal development. They ruled out genetic factors, because...

...the current view is that they may play a role in male homosexuality, but they seem to be insignificant for female homosexuality. Genetic factors, therefore, appear less probable as the major common denominator for all group differences observed here.

They also ruled out environmental factors after brushing off the caveats:

Cerebral maturation continues after puberty, especially in boys, providing a substrate for effects of social/environmental factors. However, to attribute such effects to the present results would require a detailed comprehension of how specific environmental factors relate to the four groups investigated, and how they affect various cerebral circuits. In the light of currently available information this can only be speculative.

The article in Salon takes a strongly biological stance on homosexuality, as we'll see in a moment.

Born that gay

Do recent neurological studies prove once and for all that homosexuality is biological?

by Robert Burton

Sept. 12, 2008 | 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.

The problem with this approach is that functional connectivity of the human amygdala does not seem to be fully formed at birth. Instead, it appears to be rather plastic and influenced by things like personality dimensions and whether one is looking at faces with angry or neutral expressions (Passamonti et al, 2008). Furthermore, one can review the PTSD literature to see how life experiences can alter amygdala connectivity.

Thus, some observers were skeptical:

While the results were striking, they would be more convincing if the authors had matched the groups for IQ, education and measures of depression and anxiety, said Suzanne Corkin, a professor of behavioral neuroscience at the Massachusetts Institute of Technology... Also, the authors are "overly dismissive'' of the potential role of environmental influences, Corkin said.

"In short, I would be reluctant to draw strong conclusions about heterosexual versus homosexual brain structure and connectivity from this single experiment,'' Corkin said. She wasn't involved in the study.

Back to Salon. Goldstein gives an overview of previous studies on the biological basis of homosexuality before examining the latest findings.

You've seen the studies. How impressive are the differences?

There are obvious-to-the-naked-eye differences in cerebral symmetry and in the functional connections in various portions of the limbic system, including the differing degrees of connectivity between amygdala and other brain regions critical for emotional responsiveness. It's as though you can actually see the brain changes that most gays have always suspected; and, believe me, it's a great relief to realize that these findings are clearly present at birth and aren't anyone's "fault." They simply are [present] in the same way that one has blue eyes or red hair. No more and no less.

In fact, there is absolutely no evidence that these differences were present at birth. In order to prove that, one would need to do a prospective neuroimaging study in newborn infants, then wait until puberty or young adulthood to assess sexual orientation.

As with all functional-brain-imaging studies, there is the very real problem of interpretation. Is it possible that the Savic studies are less than conclusive?

I've read some of the critiques. But to me, the statistical significance of her studies is beyond question. As to absolute proof, only time will tell.

See Jumping Into "The Fray" on Cerebral Asymmetry and Sexual Orientation for details on some of the methodological shortcomings.



Illustration by Fausto Fernós [slightly edited from the original]


Footnotes

¹ For a recent interview with Burton, see Five minutes with Robert Burton by Dr. Vaughan Bell of Mind Hacks.

² Burton described Goldstein that way in the article. The first question of the interview was "Jerry, you've been an outspoken gay activist for 40-plus years. Why the sudden interest in the biology of sexual orientation?" Goldstein answers by recounting some painful personal experiences of guilt and prejudice in his youth.


Reference

Savic I, Lindström P. (2008). PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects. PNAS 105: 9403-9408.