Dr. Eric Vilain, M.D. is a Professor of Human Genetics, Pediatrics and Urology at UCLA, and the Director of the Institute for Society and Genetics. He is also Chief of Medical Genetics, and an attending physician in the Department of Pediatrics. His laboratory explores the genetics of sexual development and sex differences, focusing on the molecular mechanisms of gonad development, as well as on the genetic determinants of brain sexual differentiation. An internationally renowned expert in the field of gender-based biology, he has identified a large number of mutations in sex-determining genes, developed animal models with atypical sexual development, and identified novel mechanisms of sex differences in the brain. Dr. Vilain received his Ph.D. from the Pasteur Institute and his MD from Faculté de Médecine Necker – Enfants Malades

Pseudo-scientific groups, such as NARTH, claim that bad parenting causes homosexuality and they largely dismiss biological factors. What is your view?

I don’t know of published scientists who come out and say that there is no evidence for any biological influence on sexual orientation. I have not met any. So, I think we’re in the realm here of there is a lot of evidence for biological influence. And still a lot to do in understanding how it works.

How long have you been studying the science of sexual orientation?

I started about now seven years ago to look into just regular gay men and lesbian women and just asked the question, “well how is it that this small percentage of the population is attracted to same-sex partners. What is going on in their brain? Is it something that is genetic, is it something that goes by hormones,” and I started recruiting subjects, in gay prides or other events and started asking them for access to their DNA and then just test scientific hypotheses. Is there any evidence that there could be a biological cause?

Is there evidence of biological underpinnings for sexual orientation?

There are parts of the brain that are more activated or less activated between men and women. But strikingly again, gay men have in certain regions of the brain, patterns of activation that looks more like the pattern of activation that is seen in straight women. And it is certainly different from the pattern of activation that you can see in straight men. So, what it suggests is that there is a biological underpinning. There is something that has changed the brain, probably during development of gay men and lesbian women that makes their brain look differently anatomically, but also it functions differently.

What has research with intersex children shown?

Based on research done with intersex children who are female genetically, but who were exposed very early on, as fetuses in the womb of their mother, they were exposed to very high doses of male hormones, testosterone. When these children are being born and followed up, they identify as women, but they do have on average two things happening. One is what we call their gender role is more masculinized. That just means that their interests in toys, occupations, manner of dress, is more masculine than typical. But also there is an increased percentage of these girls who have been exposed to very high doses of testosterone that in fantasy and sometimes in actual sex act are homosexual or lesbian women.

The more older brothers one has, the more likely he is to be gay. What is the cause of this phenomenon?

There is this idea that maybe at each male pregnancy the mother is producing some kind of an immune response. Think of it as a response to vaccination shots. There is this male fetus in her body and the female body of the mother sort of reacts to it. And at each new boy the mother has, maybe the immune reaction becomes stronger. And, maybe it interferes with the masculinization of brain development. That is a hypothesis. So we don’t know the mechanism. But, we do know that older brothers do influence the likelihood of being gay.

Do twin studies show that there is a genetic component to homosexuality?

Identical twins are much more likely to have a co-twin, the brother, to be gay if they are gay, compared to fraternal twins who only share half of their genetic material. And this difference between the rate of concordance between identical twins and fraternal twins really clearly demonstrates a genetic influence on sexual orientation.

The twin studies do not show that it is 100% genetic. They just demonstrate that there is a genetic influence. There could be other influences on sexual orientation. Some of them might be environmental. For instance, the older brother effect is an environmental influence. That’s what has happened in the past, in the same womb seems to influence the sexual orientation of future pregnancies. So, it is certainly not an all or nothing phenomenon.

Anti-gay organizations like to say that twin studies prove that homosexuality is not genetic, because if it were there would be a 100% concordance rate. How would you respond?

All sorts of traits that are clearly genetic, do not show 100% of concordance. I’ll give you examples, if people look at obesity. Or autism, looking at twins. There is undeniably, no one will even challenge that there is a genetic cause for both obesity and autism. We don’t know how it works very well. But the twin studies certainly are not at 100%, so what those studies just show — very clearly they demonstrate a genetic effect. But biology is complex, so it is going to be more than just one factor.

Groups opposed to gay rights like to claim that homosexuality is caused by nurture instead of nature. They often blame sexual abuse or poor parenting. What is your view?

For the purposes of sexual orientation, every time researchers have looked at the broad external environment — the macro environment — so for instance family structure. Is it true that single mothers are more likely to raise gay kids? That would be an influence from the environment. Well, you can actually test this hypothesis, and it’s been done by teams at the Kinsey Institute. And there is absolutely no evidence that this kind of environment, the family structure, influences sexual orientation. The environment can be past sexual experience whether pleasant or unpleasant, and again, this has been studied and there no evidence that that is the case. Child abuse. You can ask gay men or lesbian women if they have been abused sexually. And if you do that and do that in a control group of straight men and women, you actually find that there is no evidence that these kind of environment changes you, or is associated with different sexual orientations.
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