Is there a biological basis for sexual orientation?
So, I’ve been studying the genetics of sexual orientation and using twin studies for twenty-five years or so. I think that male sexual orientation is inborn. And I think that we have pretty good reason to think that it is. I mean very inborn, resistant to change. There is no evidence that it changes.
Your twin studies have shown a genetic link to sexual orientation. Tell us about your work.
So, in 1990, shortly after I got at Northwestern, I began a twin study that I expected to have more accurate results from. We advertised throughout the country for gay twins. And why is that better? In part we tried to publicize it widely and also I think that homosexuality was a lot less stigmatized in 1990 than it was in the 50’s. And in the results of that study for both men and women were consistent with moderate genetic influence. So, for example, for the men 52% of the identical twins who were gay had gay identical twins. Compared with 22% of the fraternal twins. So, the fact that it was only 52% and not 100% shows it can’t be completely genetic. But the fact that it was 52 for identical versus 22 for fraternal is consistent with some moderate genetic influence.
Your work also addressed gender non-conformity in twins.
With respect to gender non-conformity, here’s what we found in twins. Among identical twins they were very similar if they were both gay. So, if they were both gay, if one of them was very feminine as a boy the other was very feminine as a boy. If one of them was typically masculine as a boy, the other one was typically masculine as a boy. Now, this similarity broke down completely in the pairs with different sexual orientations. A very feminine gay man did not necessarily have a very feminine straight man, brother that is.
What should people know about the influence of genes?
Much genetic influence can be thought of as something that pushes one a certain way, but doesn’t push them all the way there. And this is true for all kinds of things, including diseases and personality and Intelligence and so on. And I think the same is true for sexual orientation. We don’t think of genes as determining these things and they can’t be unless heritability or the index of genetic causation is 100%. And for none of the things that I mentioned is heritability 100%.
So-called “ex-gay” activists like to claim that homosexuality is a learned behavior that can be fixed. What is your view?
People will often get confused in their terminology. They ask for example is homosexuality genetic or learned? Well, genetic is not the opposite of learned. I think inborn is the opposite of learned. A trait can be completely inborn without being completely genetic. And I think male sexual orientation is a case in hand.
I think that we can reject immediately the idea that male homosexuality is caused by having a distant father or an overbearing mother. We have lots of evidence to the contrary.
So, in your view, is sexual orientation is inborn?
In men sexual orientation is completely inborn. The reason why I believe that comes from cases of boys who due to some accident or medical condition are turned into girls early in life and followed into adulthood. These cases are very rare. When these cases are followed into adulthood you want to know who are they attracted to?
If it’s nurture, then because they are raised as girls they should be attracted to men. If it is nature because they were born males, they should be attracted to women. And it is to women they are attracted in every single published case. There are about five cases in the literature like this. I think that if you can’t make a male attracted to other males by cutting off his penis and rearing him as a girl, then its impossible that sexual orientation is learned in men.
It seems like we are only really scratching the surface of understanding sexual orientation.
In terms of the specific causes of sexual orientation we know hardly anything. In terms of the big question like, “is sexual orientation innate or learned?” I think that at least for men we know it is innate. For women, the picture is a little less clear. There are some similarities to the male case. So, for example, in both men and women we have the same developmental predictors. Both gay men and lesbians tend to be cross-gendered a bit as children. But, also people think that female sexuality is more malleable, perhaps more responsive to environmental inputs, and social causation, then male sexual orientation is. So, I wouldn’t be surprised if women’s’ sexual orientation does respond a bit to their experiences. I don’t think male sexual orientation does at all.
Scientists like to use the word environment. Anti-gay activists can easily distort this. Could you set the record straight on the meaning of this word in a scientific context?
Now, something to make very clear is the nature of environment. When I talk about environment as a behavioral geneticist, all I mean is something that is not genetic. I don’t mean it has to be social, or what your mother did to you, or what your father did to you. Or that experience that you had early on. It could also be whatever happened to you in the womb. Or, illnesses that you might have had. Things you might have eaten. Even kind of random processes that we don’t understand very well that seems to affect development.