Simon LeVay is a British-born neuroscientist who has served on the faculties of Harvard Medical School and the Salk Institute for Biological Studies. He has written ten previous books, including the textbook Human Sexuality, the New York Times best-seller, When Science Goes Wrongand Gay, Straight, and the Reason Why: The Science of Sexual Orientation. He made international headlines with his 1991 study showing that INAH3 region of the hypothalamus was different in gay and straight men. LeVay graduated from Harvard Medical School in 1974.

In a nutshell, what does the latest science say about sexual orientation?

The science really backs up the notion that being gay or lesbian, or being straight for that matter, is really a kind of central part of your nature.

You became world famous for your study showing that there were measurable brain differences between gay and straight people. Could you describe your research?

Well, let me tell you a little bit about the study. This was done, now, over 20 years ago. It was published in 1991. And it was an autopsy study. Taking brain tissue from men and women who had died. Some of them had died of AIDS, some of them had died of other diseases. And I was looking at the hypothalamus region of the brain that has a lot to do with our sex lives. It had been reported previously by scientists at UCLA that there was a region in the hypothalamus, a small group of cells that is typically larger in men than women. And what I wanted to see was whether this structure, which has the name INAH3, differs also in size between individuals of the same sex, but of different sexual orientation. I was able to show that, in fact, it is, in at least in regard to men. In heterosexual men this structure is larger than it is in gay men, at least in my study.

There were criticisms of your study, mainly by anti-gay organizations looking to explain away your results. Could you address those concerns?

There were many objections that people have brought to my study after it was published. For example, there was a lot of concern that this difference that I saw was not due to the men’s sexual orientation, but perhaps it was due to the diseases that they had died of.  Because all the gay men in my study had died of AIDS, but only about half of the heterosexuals had died of complications of AIDS. I am very confident that this was not the reason for the difference I saw. And, as an example of the reasons why I believe that, when I looked at the heterosexual men who died of AIDS there was no difference in the size of their structure as compared to the heterosexual men who died of other causes. So, there was no indication that just the fact of having AIDS had any effect on the size of this structure.

Now, that public study was published in 1991. Since then there has been one other study, follow-up study by a group at Columbia College of Physicians and Surgeons and in that study the researches found also that this structure, INAH3, is smaller in gay men than in straight men. The difference wasn’t as great as I found in my study. But it was in the same direction. So, I consider that, if you like, a confirmation of my results.

And also just to mention one other study that’s kind of relevant, is more recently a group at Oregon Health Sciences University, led by Charles Rosseli, found a very similar result in sheep. It might surprise you to know that you can find sheep who are essentially gay. Male sheep that will prefer to have sex or will only have same-sex partners. But that, in fact, is the case. And what Roselli and his group found was that, again, those male sheep who were sexually attracted to partners of the same sex had a smaller structure in this part of the hypothalamus than did the sheep who were heterosexual. So, I think those are two, if you like, follow-up studies that confirm my original finding and say “yes” there is some basic difference in the brain between individuals who are gay and those who are straight.

There have been many recent studies showing a strong biological link to sexual orientation. Talk about this cutting-edge research.

Generally these studies point to the idea that what’s going on differently between gay and straight people is something in their brain when first descending itself before birth. In other words, there are processes when the hypothalamus and parts of the brain are being created when the nerve cells are being born and start to develop and form connections before birth. It goes forward differently in fetuses that ultimately become gay adults and those that become straight adults. And the difference seems to be brought about, at least in-part, by sex hormones that are circulating in the blood of the fetus. These hormones come from the gonads, from the testes or ovaries of the fetus. They enter the brain and they influence how the brain develops in a sexual sense, whether it develops in a more masculine or more feminine direction. And there is more evidence now that this process really goes forward differently in, if you like, in gay and straight fetuses.

Differences in the development between gay and straight fetuses can sometimes be physically seen through biological markers. Could you elaborate on this?

Subtle anatomical differences or physiological differences that have been described recently, like finger length ratios, the physiological properties of the inner ear, finger print patterns, and so forth, they all seem to fit into the same basic picture. Which is they seem to be caused or result from differences in the way that sex hormones are interacting with the brain and the body during early life. So, high levels of testosterone seem to drive the brain and the body in a male typical direction. Low levels allow the brain and the body to develop in a more female typical direction. And its that sort of general process, if you like, that causes there to be some kind of link between these anatomical features like finger length ratios and a person’s sexual orientation. It’s as if it’s part of a big package of traits, all of which have some common developmental process behind them.

Can the influence of biology on sexual orientation be seen in animal studies?

There are experimental studies on animals. What’s been done in many species now is, you manipulate the hormone levels that a developing animal is exposed to, so you might add testosterone to a female fetus of a rat or some other species. Or, you might take away testosterone from a male by castrating it early in development. What is found in those cases is when the animal grows up it shows atypical sexual behavior. They actually associate more with same-sex partners than with opposite sex partners, for example. So, that’s one line of evidence to suggest that if something comparable is going on in humans it might have to do with differences in early sex hormone levels.

What is the role of genes on sexual orientation?

The evidence that genes play a role comes from comparing the similarity in sexual orientation between monozygotic, or so-called identical twins, and dizygotic, or so-called fraternal twins. And these identical twins share pretty much 100% of the same genes. Where as the fraternal twins, who are just like regular brothers and sisters, they share about half their genes, and to the extent a trait like sexual orientation is genetically influenced, you should see a higher agreement in sexual orientation between those pairs of twins who are identical compared with those who are fraternal. And that’s what you find. So, it’s certainly not the case that every pair of identical twins share the same sexual orientation, but they do so at a much higher rate than those twins who are fraternal. You can sort of go through the arithmetic from those numbers and derive a measure of what proportion of the total causation of that person’s sexual orientation is genetic.

Could you discuss the fraternal birth order effect?

The birth order effect is this finding that a man who has an older brother is slightly more likely to be gay than a man who doesn’t have any older brothers is an unexpected finding that came out in the last 10 years or so as a result of the research of these Canadian groups.

Now, no one knows exactly what is the reason for this birth order effect, but the supposition is that it is due to some interaction between a developing fetus and its mother. Such that if a mother has already carried one male fetus, then when she has a subsequent male fetus in her womb, there is some memory of that earlier pregnancy which interacts with the developing second, or later, male child, in such a way as to influence that child to be more likely to be gay when he grows up.

The exact mechanisms of that isn’t known, but that’s the supposition. I should say that the birth order effect is a pretty weak effect. You’d actually have to have about ten older brothers before you’d have even a fifty-fifty chance of being gay by the birth order effect alone. But, it does seem to be real and it is an intriguing finding that really needs to be explored.
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