How Genes Shape Our Sexual Orientation

Nearly 20 years ago, neuroscientist Simon LeVay helped pioneer the study of the science of sexual orientation. Observing the brains of gay and straight men and women, he discovered slight structural differences that seemed to occur on the basis of sexuality — with some brain structures of gay men resembling those of women more than those of straight men. “It got a lot of media attention back then,” said LeVay. Below, the author of Gay, Straight, and the Reason Why: The Science of Sexual Orientation, chats with Zócalo about where the field has gone, the biological differences between gay and straight men and women, how bisexuality differs among men and women, and what his research means for political and religious beliefs about homosexuality.

Q. Where has the study of sexual orientation gone since your study?

Gay, Straight, and the Reason Why, by Simon LeVayA. There are a number of different lines of evidence. One is the evidence that genes play a significant role. That is particularly true for men. We know from family studies and twin studies that genes account for about half the reason why a man has the sexual orientation he does. In women, it’s probably a bit lower – it’s maybe only a quarter of the reason, if you like, why a woman is heterosexual or a lesbian. There’s a significant effect in both sexes, though certainly not a complete effect. For example, you can have identical twins who share all the same genes but have different sexual orientations. It’s not the whole story, but it is significant. No one has gotten those genes in a bottle yet. No one has identified specific genes that contribute to a person’s being gay or straight. So that is something, hopefully, for the future.

There are also other biological factors that may not be under genetic control. One is hormone levels during fetal life, specifically the level of testosterone, which seems to be the key player in sexual differentiation before birth. High levels of testosterone, typically seen in male fetuses, tend to drive the development of the brain in a more masculine direction. Low levels of testosterone more typically seen in female fetuses allow the brain to follow its own endogenous, independent program of development, which tends to go in a more female direction. There is a lot of evidence now from animal experiments that those levels of hormones can influence the ultimate sexual preference of animals when they’re adults. So you can produce animals of a particular orientation by manipulating levels of testosterone during the early period of brain development.

Observations in humans suggest something similar is true. There are certain syndromes in which the levels of testosterone that a fetus experiences are not typical for that sex. There is, for instance, a syndrome called congenital adrenal hyperplasia, where female fetuses exposed to higher levels of testosterone than would usually be the case. Women who have this condition are more likely to experience same-sex attraction and engage in same-sex relationships than comparison groups of women, such as their unaffected sisters, who were brought up in a similar way but without the unusual hormonal experience.

Q. What are some of the physical ways these genes or conditions manifest?

Conditions like that are not common, so the question is, is that relevant to the great majority of gays and lesbians who don’t have an obvious medical condition? There does seem to be growing evidence that prenatal hormones really do influence the rest of us, too. One way people have looked at this is by looking for anatomical markers that say something about our early development. One is finger-length ratios. There has been quite a lot of media coverage of this. This basic difference seems to reflect differences in hormone exposure during early development. What has been found is that lesbians’ finger-length ratios are shifted in a more masculine direction.

It is important to emphasize that this is very much a statistical thing. You can’t diagnose sexual orientation by looking at different fingers. But if you look at large numbers of people, then these differences do emerge. The markers tell you something was different in early development between those women who eventually become lesbians and those who don’t. In some studies they’ve gone further and divided lesbians – to some extent, lesbians have called themselves “butch” or “femme,” those are the slang terms to describe lesbians who feel more masculine or more feminine. According to one study a few years ago, these finger length differences are particularly relevant to those who identify as “butch.” Butch lesbians may be under greater biological influence to become lesbians. That’s just one study, and it may turn out to be wrong, but that’s the direction we are moving, to look at finer distinctions rather than lumping all gay people together.

For men, the finger-length ratio doesn’t show any great difference, but other anatomical features, like arm lengths, do suggest the same sort of conclusion. These studies, as well as cognitive psychological studies, point to the fact that gay people are not just different because they are interested in same-sex partners. They also differ in other ways. Gay men tend to have better verbal abilities than heterosexual men. Lesbians tend to have better special abilities than heterosexual women. These are part of a package of traits that flow out of this process of sexual differentiation that happens very early in life, probably before birth. These lines of evidence collectively point rather strongly toward a biological interpretation of sexual orientation.

Q. Where does bisexuality fit into this picture – how do bisexuals differ from, say, straight people or gay people?

A. It’s very interesting because there is a major difference between the sexes. Bisexuality seems pretty uncommon in men. There are quite a few young men who go through a phase of calling themselves bisexual before identifying as gay. The number of men who throughout their lives feel they are equally or roughly equally attracted to both sexes is quite small, much smaller than the number of people who are gay.

For women, the opposite is true. There are many more bisexual women than there are lesbian women. Some researchers, depending on how you measure it, assert that almost all women are bisexual, based on studies of genital responses. You can measure a person’s physiological arousal to erotic images. With that kind of test, men fall very readily into two groups:. it is hard to find any men who will respond equally strongly to both men and women. Women, on the other hand, regardless of whether they identify as straight or bisexual or lesbian, respond about equally to all erotic imagery. At that level it is hard to define women as being lesbian or straight at all. We really don’t know why there is that basic sexual difference. I speculate toward the end of the book about the way inhibitory circuits work in the hypothalamus. There are inhibitory loops that set up a sort of winner-take-all circuit, whereby the synaptic mechanisms responsible for sexual orientation become very polarized during development. In women, this loop may be less active. But this is speculative, based on observations of animals.

Q. Are there any factors after birth that can have an impact on sexual orientation?

A. We don’t know of factors later in life that influence our sexual orientation. There may be such factors, but evidence points rather strongly to early phases of life. Certainly in animals, you can manipulate sexual orientation with hormones in the early phases. But you can take an adult rat and mess with its hormones all you like, but you’ll never change its sexual orientation. Still, we can’t rule out that something might happen during childhood or puberty that might be relevant.

There is a lot more fluidity of sexual orientation in women than men. Women may change how they describe themselves over their life spans, and they may become aware of same-sex attraction later in life, often in a way that is a complete surprise to them. Men may also come out as gay later in life, but it’s generally not a surprise. It’s a matter of being open about something they knew all their lives. There’s something different going on in women. They can acquire a new sexual orientation later in life. They may say it was a certain woman they met, or molestation or assault they suffered. I don’t want to rule out the possibility of these causal factors, but still, you have to remember that genes operate throughout life. The fact that I’m bald, for example – I’m bald because bald genes run in my family. But I had a full head of hair for three decades before that trait showed itself. The same could be true for psychological traits and sexual orientation. As geneticists say, we become more and more the victims of our genes the older we get. In childhood, we see nongenetic effects because parental influence is so strong. After we branch out on our own, we show our true colors, if you like. I wouldn’t rule out the idea that genes are influencing a sexual trait just because it doesn’t emerge until you’re 30 or 40 or 50.

Q. Genes have been shown to be remarkably similar for people of different races. How “different” are gay people from straight people?

A. There have always been stereotypes about gay people, mostly around their gender nonconformist attitude, the ‘effeminate’ gay men or ‘mannish’ lesbians. These are the descriptions you hear. That has been around a long time and it has not been helpful in terms of acceptance of gay people, particularly the idea that gay men are unmasculine. When you think about, for example, all of the gay teens who have taken their lives recently, a lot of it may have to do with their coming out as gay, but there is also clearly the notion that it is their unmasculinity that people pick up on. Often times, people see this long before these kids are old enough to have a sexual orientation. “Pre-gay” children are already notably different. That gets picked up on and kids are victimized. There are stereotypes, and there is a kernel of truth to them. Sometimes it is subtle, a slight difference in verbal or spatial ability, but sometimes it’s striking and flamboyant. In that case, you may be able to recognize gay people by using ‘gaydar’ – that is, by picking up on unconscious behavioral traits that are gender-atypical.

There are so many studies looking at so many different traits where gay people tend to be shifted in the direction of the female sex. It’s hard to say how that compares to a racial difference. There are so many conceptual differences between race and sexual orientation, but I do think there is a lot more to being gay than simply who you’re sexually attracted to. Science really supports that. Being gay is part of a package of traits that are gender variant or gender nonconformist that varies a lot from one person to another. It’s what makes gay people interesting. It’s what allows them to make unique contributions to culture. It makes them worthwhile, as having something specific to offer, rather than just saying, “Be attracted to whoever you want, I don’t care.”

Q. What have been the political implications of your research, particularly in terms of discrimination against gay people?

A. Generally it’s quite positive. I speak from my perspective as a gay man who thinks gay people should be welcomed in the world and protected from discrimination and allowed to marry. There are people, particularly some Christian conservatives, who tend to see homosexuality as a chosen lifestyle. It’s the idea that there is really nothing more to homosexuality than straight people saying, “I think I’ll try that gay thing this weekend.” Having that point of view allows people to not like gay people, or not want to give them fair protection. I think this is why most of the opposition I’ve gotten is from conservative Christians, who say my research is wrong or biased. And of course not all churches take that point of view at all, I’m not labeling Christians generally as anti-gay.

There has been, in some quarters, a sense that this kind of research might be dangerous. Some have feared it might bring us back to a time when people thought of homosexuality as some kind of disorder, and that we need to get into the hypothalamus and replace cells and check genes of fetuses so you can abort the ones that were likely to be gay. I think we need to work together to create a world where people don’t feel it necessary to take that view. This is part of the whole trend in human biology – that we will get this power over our own lives and the lives of our children that preceding generations never had. What we do with that power is going to be a major dilemma in this century.

Q. Where do genes that influence sexual orientation fit into evolution?

A. The short answer is, we don’t know. The long answer is in my book. So here is a short version of the long answer. If we assume there are genes influencing sexual orientation, it is very likely that those genes are reducing the reproductive success of gay people. After all, both gay men and lesbians have fewer offspring by virtue of being gay. There are plenty of genes that have the effect of reducing people’s reproductive success. And when I say reproductive success, I don’t mean it’s great to have tons of children – it’s just the technical term for how many children you have.

The usual interpretation is that such genes are kept in circulation because they have effects that also increase the reproductive success of other people who have the genes. To give a more specific example, there may be a gene that, for example, makes a man more likely to be gay. But if that gene is present in that man’s sister, it might make her more attracted to men than she otherwise might have been. This might be a gene for being attracted to men operating in both sexes. Female relatives of gay men might have more children than they otherwise would. There is evidence to support that – a couple of Italian studies that show that female relatives of gay men do have more offspring, even making up for the offspring that the gay men did not have. On balance it was a net benefit to the gene because of the extra offspring.

There are a number of models like that, which would say that the genes are kept going in the whole population because they influence the reproductive success of the relatives of gay people. I know that kind of model, in a sense, portrays gay people as the losers because the genes had a “bad” effect on them and a “good” effect on their sisters, let’s say. But what’s good or bad for a gene is not what’s good or bad for a person. We don’t judge our goodness or success or happiness based on genes in our cells. It’s much more complicated than that. So anyway, there are models that explain how ‘gay genes’ might persist in the population, but we won’t really know until the genes have been identified and studied.

*Photo courtesy Kevin Wong.


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