"Gay gene part n": Biology != Genetics

By Razib Khan | November 8, 2007 6:10 am

Genetics Has A Role In Determining Sexual Orientation In Men, Further Evidence:

In other research, Witelson and research associate Debra Kigar, had found that left-handers have a larger region of the posterior corpus callosum — the thick band of nerve fibres connecting the two hemispheres of the brain — than right handers.
This raised the hypothesis for the current study — whether the anatomy of the brain of the sub-group of right-handed homosexual men is similar to that of left-handers.
They found that the posterior part of the corpus callosum is larger in homosexual than heterosexual men.
The size of the corpus callosum is largely inherited suggesting a genetic factor in sexual orientation, said Witelson “Our results do not mean that heredity is destiny but they do indicate that environment is not the only player in the field,” she said.


The size of the corpus callosum may very well be argely inherited in the general population, but one should be cautious about extrapolating the heritability to an aytpical sample such as this. Genes which result in the normal human variation in the corpus callosum size may still have bearing on the trait variance among homosexual males. In fact, genetic variation on the loci responsible for the range in the normal population may also explain within group variation among homosexuals, who might be scaled differently because of the effect of another variable.
Now, I’m not waving my hands and making the neurological features disappear. This is a real biological finding, but too often genetics has become so sexy that this subfield is recruited to “explain” any novel resut. Don’t get me wrong, the concordance numbers from the twin data does suggest that something is going on which tracks genetic relationships, but the quest for the silver-bullet “gay gene” has come up with enough strikes that we should be a bit cautious about zeroing in in genetics instead of looking more broadly for other biological explanations.

CATEGORIZED UNDER: Genetics
  • http://www.chimericfire.com Nathaniel

    That’s fascinating. Of course we all know that the corpus callosum is larger in women and now we’ve found that it’s also larger in gay men… and left handed people. That’s very interesting indeed. Is this also suggesting that south paws are more likely to be homosexual?
    In order for this to hold any water, we would have to examine a vast number of brains to see what the percentages of individuals with enlarged corpus callosums as apposed to normal ones and what those percentages are for each demographic: gay, straight, left handed, right handed.
    I idea is kind of wild though. Although I agree, every other attempt to find the “gay gene” has ended with those involved saying “sorry, that’s not it, just kidding.” So yeah, caution is certainly well placed.

  • http://www.60secondscience.com christopher

    Dollars to donuts its a maternal effect. I mean, having older brothers makes you more likely to be gay. And female mice who are in between brothers in the womb have masculinized behavior when they’re grown. Isn’t it obvious by now?
    This would mean, of course, that it could be a) not genetic but b) still determined by biology.

  • diana

    Gay gene, no. But gay biologically? Methinks yes.
    There comes a point where too much evidence builds up and you just have to admit to the truth, which was obvious all along anyway: the effeminate little boy who plays with dolls has an identity that was fixed at birth, and he’s going grow up to be gay.
    Science fiction speculation: in 100 years, will there be a pill for him to take that will afford the choice to be straight?

  • Pender

    It’s possible that gayness is a product of both genetics and prenatal environment, diana. We haven’t found any evidence of the gay gene, but on the other hand, gayness is still so unpredictable (even accounting for the effect of older brothers) that it seems premature to dismiss the possibility.

  • Ulf

    Hmm, I thought that the idea of a larger corpus callosum in females was well and thoroughly refuted by now… Or rather, that the refutation had penetrated far enough to clense us from that particular piece of “mythology”.
    The modern source of that idea seems to be a 1982 Science article, that was later countered by many papers showing that the original claim was mistaken and didn’t take the overall size difference into account. (Something I guessed when this was mentioned when I took a basic course on neuroanatomy, many years ago by now, but couldn’t prove back then…)
    But this little “factoid” seems to fit so well into some socio-cultural paradigm of a) there IS a sexual dimorphism and b) women are “better at communication and empathy”* that is spawns endless repetition…
    That there seem to be a somewhat larger splenium in some sub-populations – left handed and/or musicians – is not that groundbreaking. First of all, that is a lot more probable to generate a need for more cross-hemispherical communication.
    And second, one interesting thing often overlooked, none of them seem to imply that strong inheritance. It would be much more plausible that the relative size difference was en effect of progressive development due to the increased need. (At least, even if musicality made be influenced by heriditary traits, the choise to become a musician isn’t. And the size-factor is liked primarily to occupation – not aptitude.) That fact would also effectivly counter the argument that any similar sexually dimorphic size difference (if there was one) was heriditary.
    *) Because as everybody knows form follows function, so that someone who is good at communication and interpersonal skills MUST have brain parts that are also very good at communication…

  • diana

    Ulf,
    My understanding is that you are right, the 1982 paper was challenged, but that recent studies seem to back it up.
    But the article didn’t take a side on the “corpus collosum larger in women” issue. How is it relevant to this study?
    Pender,
    Regarding prenatal development – I include this under the category “biological”. In any case, sexual orientation is pretty well fixed at birth.

  • Caledonian

    the effeminate little boy who plays with dolls has an identity that was fixed at birth, and he’s going grow up to be gay.

    This is something I’ve wondered about for a long time: are male homosexuals significantly more likely to be effeminate, not significantly more likely to be effeminate, or is there a subset of gay men that are likely to be effeminate?
    I see no reason why sexual orientation should necessarily be associated with ‘feminine’ behavior preferences.

  • diana

    “are male homosexuals significantly more likely to be effeminate,”
    Yes. Study after study has shown that gender atypical boys are likely to grow up to be gay, and that the majority of gay men were gender atypical as boys. Straight men were essentially gender typical: rough and tumble play, team sports, yada yada. Do I have to run through the list? Go to South Africa, go to Montana, go to China…boys will be boys.
    “I see no reason why sexual orientation should necessarily be associated with ‘feminine’ behavior preferences.”
    No? What planet do you live on?
    Whether or not the corpus callosum of gay men is more “feminine” than not (Ulf is right, there is a question), there’s already a variety of studies piling up that gay men’s brains tends towards the unmasculine in varying ways, and that gay men have abilities in certain areas that are (surprise surprise) more feminine. Here’s one: http://www.newscientist.com/article/dn7069.html
    There are many others. I don’t have time to look them all up.

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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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