Blame it on the brain
A recent study published in the Proceedings of the National Academy of Sciences tries to isolate innate characteristics of the brains of homosexual men and women. Earlier studies have focused on parts of the brain which are more or less plastic–that is, they have to do with perception or reproduction, behaviors that may reinforce structural changes in the brain. Affinity between the brains gay men and straight women, then, can be explained as a result of similarities in their behavior.
In a study at the Stockholm Brain Institute, researchers tested paired groups of gays and straights (i.e., gay men versus straight women or HoM v HeW, gay women versus straight men or HoW v HeM) in two different ways, to try to isolate some reciprocal patterns in their brains which are more likely to be innate than learned. The results are interesting:
Cerebral and Cerebellar Asymmetry
One sex-dimorphic characteristic of the brain (something that varies between male and female) is the size of the hemispheres. Earlier studies have shown that there are differences between men and women in the connections between the hemispheres and thus some general differences in functions that are largely hemisphere-related. Of course, we have to be cautious about assigning functions to a specific hemisphere, but there have been some studies showing that gay women outperform straight women in things like mental rotation of three-dimensional objects, a task related to this kind of lateralization. They studied this volume through MRIs and the use of a computer program process something like the one here (from which I pulled the image above).
When the researchers looked at the hemispheric volumes, they saw a few patterns:
- HeM and HoW had larger right hemispheres and significant asymmetry
- HeW and HoM didn’t have significant asymmetry between hemispheres
Amygdala Connectivity
Studying this aspect of the brain required a PET (Positon Emission Tomography) scan, which means that it was looking at snapshots of blood flow. The researchers asked their subjects to simply breathe the lab’s air, which is unscented (to avoid activation of parts of the the brain focusing on smells). The idea is that there is a difference between how men and women use the amygdala in processing emotions, but these researchers were trying to find out if this is the case at a resting state. Why? This point is crucial to what they’re doing, so I’ll quote them:
Because measurements of the resting state functional connectivity are independent of user, perceptive, cognitive, or behavior-related tasks, they lend themselves to studies of more crude potential neurobiologial correlates to sex and sexual orientation.
As well, the amygdala is awash in hormones like estrogen and androgen, which are part of how the body makes humans into male and female members of the species. So we might expect it to be implicated in sex-differentiated tasks. What did the authors conclude?
- Connectivity between the hemispheres of HoW and HoM was nearly reciprocal in comparison to the HeM and HeW. That is, the ways that the connections between the hemispheres differed between straight men and straight women is close to how they differ between gay women and gay men.
- The conclusion is that the HoM have more “female” patterns and HoW more “male”, although the researchers say that the latter are “less pronounced”, making the opposition pairs not quite as close.
What to Conclude?
This is all interesting stuff, but what does it tell us, if anything, about sexual orientation? Can we conclude that because gay men and women have brains like straight women and men, respectively, that there’s a genetic basis for homosexuality? Does this mean that the difference in brains is not at all due to nurture, but is 100% nature?
The authors, like any good scientist, exercise restraint in making conclusions. They first summarize the results of their data, noting that there are methodological issues in making wide assumptions about populations based on a sample (for example, handedness is correlated with hemispheric asymmetry and gay men are more likely to be left-handed than straight men–so they chose all right-handed gay men to try to adjust for this). And, too, measuring parts of the brain can be tricky, although they explain how they tried to minimize vagueness.
Secondly, the pairs aren’t matched in their symmetry: HeW and HoM are more alike than HoW and HeM. So we have to be cautious in what we conclude about “homosexuals”, since lesbians and gay men aren’t simply mirror images of each other.
What of the origins of orientation? Well, in terms of hemispheric asymmetry, there is some evidence that even in the human fetus, it exists. So learned behavior may later impact it, but it’s improbable (or at least the authors believe) that it would account for it entirely.
How about the genetic card? The question in common parlance seems to be “is it genetic?” as if genes are all that make us male or female, and by inference, gay or straight. But exposure to androgens in the womb, in addition to genetic factors, are influential in the resultant fetus’ brain structure. And, back to the difference between lesbians and gay men, there seems to be more evidence for a genetic explanation for the latter than the former (see another study hitting the news today).
So, as with most other studies I’ve read, the conclusion seems to still be “multifactorial”, and no one study is going to untangle that knot. However, it at least gives us a hint that we’re looking in the right directions.
[Final Notes: First: I'm blogging on peer-reviewed research as a layperson, not as a scientist or neurologist. Please go to the original study and don't take my summarization as being infallible. Second: The ethical implications of these studies is a topic for a whole other post, but I'll just note that those who are concerned about "designer babies" eradicating gay people should take into heart by the fact that simply tweaking genes probably won't do the trick; although I'm sure the technology to influence other factors is on the way.]
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