Is Sexual Orientation Genetic? Yes and No, an Extensive Study Finds

Is there a gay gene? Is there a sexuality spectrum? A wide-ranging study reignites the debate

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Gif: Two boys holding hands in a tree house.
Credit: Rotem Teplow
Jonathan Jacobson
Jonathan Jacobson

The international group of scientists knew they were setting out to investigate an explosive subject: the hereditary basis of human same-sex behavior. Even so, the members of the prestigious Broad Institute in Cambridge, Massachusetts, may not have anticipated the magnitude of the public furor that erupted when they published their study, which identified several markers in certain genetic loci in the human genome related to “same-sex sexual experience.” The storm of reactions ranged from those who welcomed something seen as heralding significant progress in the field, to others who maintained that it would have been better if the scientists hadn’t published anything.

The research results were published in full in the journal Science, at the end of August. This was the most extensive study of its kind ever conducted (there were about a half a million subjects), in which use was made of the GWAS (genome-wide association studies) method to analyze genetic big data. The researchers discovered five genetic markers (frequent, minor changes in the DNA segments of certain chromosomes) that appeared repeatedly among individuals who reported having had same-sex sexual experiences. Slight and frequent genetic variations were identified in both women and men, two others in men only and one more only in women.

No less important in the study, entitled “Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior,” is the scientists’ claim that a large number of genetic markers, perhaps even thousands, might operate simultaneously together – although each in and of itself is of minuscule weight – and influence one’s same-sex orientation. Moreover, their study led the researchers to the conclusion that human genetics can explain up to 32 percent of same-sex sexual behavior.

What is at issue here, however, is not what the study contains but what it does not contain. As Melinda Mills, a sociology professor at Oxford, writes in the same issue of Science, there is no way that the researchers’ findings can be used as a tool to accurately predict same-sex behavior. Specifically, the fact that genetics can explain up to 32 percent of the fact that someone is gay or lesbian, does not mean that sexual identity is determined primarily by environmental factors – not to mention social ones. This story is far more complex and has not yet been fully deciphered. Mills’ views are shared by Andrea Ganna, one of the chief authors of the new study.

“What we basically do is statistical associations between having and not having these genetic markers and having or not having same-sex behavior,” Ganna told Haaretz in a phone interview. “Because we had this uniquely large study,” he continued, “which allowed us to have robust conclusions, and because we had the technology to measure the genetic markers of so many individuals, the time was right to confirm something that we expected: There is no one specific gay gene. Instead there are a lot of relatively common genetic markers, genetic mutations, that have a small effect on same-sex behavior.”

At the same time, adds Ganna, a geneticist at Harvard Medical School and at Finland’s Institute of Molecular Medicine, “Not everyone is interpreting the fact that there’s no single ‘gay gene’ in the right way.”

Ganna’s concern is shared by scientists around the world. They’re worried that the researchers’ findings will fuel prejudice and discrimination against the LGBTQ community, and even spark calls for genetic engineering and genetic diagnosis among its members. So serious are these apprehensions that some have wondered whether the study would not do more harm than good.

“As a queer person and a geneticist, I struggle to understand the motivations behind a genome-wide association study for non-heterosexual behavior,” Joseph Vitti, a postdoctoral researcher at the Broad Institute, wrote on its blog, adding, “I have yet to see a compelling argument that the potential benefits of this study outweigh its potential harms… [T]he results presented not only oversimplify the question of biological causality, but also threaten direct damage by perpetuating the stereotype of LGBTQIA+ people as imprudent, while also likening same-sex attraction to a medical or psychological disorder.”

Geneticist Andrea Ganna of the Broad Institute.Credit: Andrea Ganna

Moreover, a website called The American Conservative posted an article entitled “Not Born This Way After All?” which wondered, skeptically: If the study proves that homosexuality is related to the environment, above all, and not to heredity – why isn’t it right and proper, in scientific terms, to allow those who so desire to undergo treatment in order to reduce their same-sex desires, which have now been shown not to be genetic?

That, however, is a simplistic reading of the study’s findings. According to Michael Bailey, a professor of psychology at Northwestern University in Illinois, who was not involved in the study but has been conducting research on sexual orientation for 30 years, “It’s very important to understand that ‘environment’ does not simply refer to social surroundings, like what your parents teach you and what kids you know, trauma and so on… there’s also a biological environment that begins right after conception.”

Three years ago, Bailey and several colleagues published a survey of all the studies and professional literature in the field. “The best studies have shown that genes are probably important but not overwhelmingly important,” he tells Haaretz. “We estimated in our 2016 review… that 30 percent of the variation in sexual orientation is due to genetic variations.” It may be this finding that led him to conclude that “it is the biological environment that is mostly important.” Bailey is convinced that men are born with their sexual orientation and that it is not subsequently acquired at any stage. He notes that there are “several cases, I think there are seven throughout the professional literature, in which a baby boy was changed into a girl for medical reasons and was raised as a girl. When you follow these individuals through adulthood, you find that they are attracted to women and not to men.”

In Bailey’s view, the best example of how biological-environmental factors can influence sexual orientation is the “fraternal birth order effect.” The phenomenon, whose existence is “well established,” he says, shows that “the more older brothers a man has, the more likely he is to be homosexual.” In practice, every older biological brother increases the probability that the youngest brother will be gay by about 33 percent. Thus, if the probability that a man with no older brothers will be gay is 2 percent, one older brother will increase the probability to 2.6 percent, and a second, third and fourth brother to 3.5 percent, 4.6 percent and 6 percent, respectively. What’s not yet clear is the reason for this.

“In my mind,” Bailey suggests, “the best hypothesis as to why this happens is that a mother’s immune system becomes increasingly active and produces antibodies against male proteins over successive births.”

Fingers and hands

Behind this hypothesis is one of the most influential figures in the field, American-Canadian clinical psychologist and sexologist Ray Milton Blanchard. He was also among those who linked the fraternal birth order effect to another phenomenon of interest to scientists: the connection between being left-handed and having a same-sex orientation. The most extensive study in this regard was conducted in 2000, incorporating 20 different studies involving 7,000 gay male and female subjects and 16,000 heterosexual ones. It was found that gay men were 34 percent more likely to be left-handed. The situation was more extreme among lesbians: They were seen to have a 91 percent greater chance than straight women of writing with their left hand.

As a result, six years later, a research team led by Blanchard argued that the fraternal birth-order effect is relevant only among right-handed men. The reason is that, in any case, left-handed men who don’t have older brothers already have a greater likelihood of being gay than right-handed men with such siblings.

A person’s dominant hand turns out to be significant in another sense as well. An article published two years ago (about a study in which all the subjects had taken part in a gay pride parade in Toronto) found a connection between that hand and the gay person’s “role in bed”: that is, the proportion of left-handed gays who defined their sexual behavior as passive or versatile (i.e., sometimes passive, sometimes not) was significantly higher than among those who described themselves as actives – who clearly tended to be right-handed.

A gay pride parade in Toronto, Canada.Credit: Geoff Robins / AFP

In research conducted over the years on the subject of the connection between sexual orientation and other attributes of the body, the hand holds a place of honor. But while Blanchard developed his theory on the basis of the whole hand, sometimes a few fingers are also enough: two, to be exact. In his 1998 study, British biologist John Manning confirmed a relatively old hypothesis, first put forward in Germany almost 150 years ago. Its gist is that the proportion between the length of index and ring fingers is, typically, different in men and women. Manning found that this phenomenon was detectable as early as age 2, which led to the observation that its source lies in the differences in testosterone and estrogen levels that already exist in the womb – hereinafter: a biological-environmental factor.

Manning did not emphasize the element of sexual orientation in the two books and over 60 articles he wrote on this subject, but in the two decades that have elapsed since his study, more than 1,400 papers have been written on the ratio between the length of the second and fourth fingers (known as 2D:4D) and the connection between it and the level of risk of contracting certain diseases, as well as personality traits, cognitive and athletic abilities – and sexual orientation.

One such study, published in 2010, maintained that straight and lesbian women are differentiated by the ratio between the length of the index and ring fingers, with lesbians tending to show a more “masculine” ratio – i.e., closer to the average difference between the length of the fingers, among men. However, no such differences were found between gay and straight men.

Last year a team of scientists led by a British psychologist measured the fingers of 18 pairs of identical female twins, one lesbian, the other straight. Overall, differences in proportion were documented only in the lesbians and only in their left hand, and were comparable to the situation among men. This fact, the team concluded, could indicate a heightened exposure to testosterone in the womb – but their study was based on a very small sample and drew much criticism. The critics charged that the conclusion was based on an overly simple means of measurement: of the way only two variables impacted each other. And, they added to bolster their argument, findings of studies involving those fingers have not been replicated in scientific experiments.

The field of “gay science” has been on a roll in recent years, but has a far longer history. Its modern phase dates to the early 1990s, when scientists began to publish increasing numbers of studies arguing that sexual orientation has a biological component. A leading scientist in this field is British-American neurobiologist Simon LeVay, who in 1990 performed autopsies on the bodies of 41 people: 19 gay men, 16 straight men and nine women. He discovered that the brain cells known as INAH-3 among the deceased gay men were relatively small, and closer in size to those of women than to heterosexual males.

“In 1991,” LeVay told Haaretz in a phone conversation, “I published a study that got a lot of media attention, related to my observation that there was a region inside the hypothalamus that was different in size between men and women, and also between gay and straight men… My additional finding was the difference in size between gay and straight men in this region inside the hypothalamus that is involved in the regulation of sexual behavior.”

Adds LeVay, “My general feeling is that there are certainly strong biological influences on people’s sexual orientation, but we can’t say everything is genetic.”

In the spirit of the period, and in light of the AIDS epidemic at the time, LeVay tried to be as cautious as possible about his conclusions. “It’s important to stress what I didn’t find,” he said in an interview to Discover magazine, in 1994. “I did not prove that homosexuality is genetic, or find a genetic cause for being gay. I didn’t show that gay men are ‘born that way,’ [which is] the most common mistake people make in interpreting my work.”

Three decades after publishing his study, he still thinks media coverage is doing an injustice to research – even if it’s not his. “I’ve seen some headlines saying, basically, that this study [i.e., that of Ganna and his associates] shows it’s not genetic, or that are no gay genes, or something like that; and, of course, it’s not what the study shows at all.”

Truly gay

In recent decades, scientific research (on men and women alike) in this realm has relied on an additional field: molecular genetics. The pioneer is geneticist Dean Hamer, who in 1993 conducted the first study of its kind.

“We noticed that being gay, for males, tended to pass down through the mother’s side of the family,” he told Haaretz. “And that is characteristic in genetics of something on the X chromosome – because males get their X chromosomes from their moms… That led us to look in families where there were gay brothers, to see if they shared anything on the X chromosome.”

And thus, recalls Hamer, he and his team discovered Xq28: a genetic marker that plays a part in determining whether a person will be heterosexual or gay. He emphasizes that this is “a factor, it’s not the factor and actually, overall, it’s not even the most important factor.” He adds, “What’s good about genetic studies, is that you know that whatever you find is a causal factor, because – of course – people are born with their genes, and it’s not something that changes over time.”

LeVay, he explains, “is looking directly at the brain, and we’re looking at what we think is building the brain and genes.” Yet, “it’s very difficult to know whether one was born with a brain like that, or whether that brain developed that way because of your behavior – the causality is rather unknown.”

At the same time, Hamer adds, “That doesn’t mean there aren’t specific pathways, because there has to be some sort of a pathway in the brain that controls sexual orientation. We know, for example, that the reason you become a male or a female is very simple: If you have a certain gene on the Y chromosome, you will produce male hormones, and if you have those you make a penis and scrotum and you become male.” Accordingly, “There’s probably some pathway in the brain that does same thing for sexual orientation, but we’re not going to discover it from genetics… The answer will probably emerge from some sort of very sophisticated brain and developmental studies.”

IllustrationCredit: Rotem Teplow

For 35 years, Hamer accumulated experience as a scientist at the National Institutes of Health in Bethesda, Maryland. That period is behind him. He doffed the white coat and now lives in Hawaii, where he makes films. But even if he’s no longer occupied with research, it still occupies him.

Hamer: “Back in the 1990s, I, along with all the scientists involved, believed that if we did good genetic studies we’d find the important genes. For example, we’ll find a gene that is responsible for the production of testosterone, and if its functioning was low, it would be possible to say that this is the cause of homosexuality in a particular person…. But it turns out that it doesn’t work that way. For every mental trait that has been studied… everything you can imagine in the brain, for every single trait, there’s a [vast number of] genes” – not to mention a host of complex societal and environmental factors.

For his part, Hamer has much praise for the Broad Institute study: “The new GWAS study is really important, because for the very first time they used a huge sample and they mapped every inch of the genome. And this has never been done before. All the other studies were much smaller, or used many fewer genetic markers.” But he also demurs: “What’s very important is to look at what they actually analyzed. They didn’t analyze people who were gay or lesbian, but anyone who had one single same-sex experience, which is quite different... They were measuring something more like openness to sexual experimentation.”

As Hamer sees it, “If you look for those five markers, or even just the three strongest markers, they are not necessarily found in people who actually identify as gay or lesbian. If you take people who are gay, like me, and look for those markers – they’re not significantly there.”

Hamer thinks that the whole field is lagging behind because of insufficient research, owing to the stigmas that plague the subject. “I don’t think sexuality is any more complicated than many other areas of human personality and individual differences,” he observes, noting, “We formally established that male sexuality is something that is deeply ingrained in people, it’s not any sort of choice really. It starts really early in life, and it has a major biological component to it. But, how it works? What the biological component is? We’re completely unaware and don’t know anything, and we barely know more than we did 25 years ago, or in the 1940s, when Kinsey did his work, to be honest.”

Hamer was referring to biologist Alfred Kinsey, who in 1948 stunned the American public with his book, “Sexual Behavior in the Human Male,” which addressed previously taboo subjects, and challenged the traditional beliefs and existing knowledge about human sexuality. Kinsey had conducted a survey of men, which found that 37 percent of his subjects said they had undergone a homosexual experience of some kind, and 10 percent said they had been exclusively gay for three years of their adult life – a statistic which to this day is generally said to represent the proportion of people engaging in same-sex behavior.

At the same time, subsequent studies reveal that the percentage of people who define themselves as “exclusively” homosexual is far lower, though it fluctuates from one article to the next. For example, a 2011 survey of nine different studies on the subject revealed that approximately 3.5 percent of Americans identify themselves as gays, lesbians or bisexuals. A poll involving 1,000 Jewish Israelis in 2012 found that 11.3 percent of the male respondents and 15.2 percent of the female ones said they felt an attraction to members of the same sex. However, only 8.2 percent of the men categorized themselves as gay or bisexual, while 4.8 percent of the women said they were lesbian or bisexual.

For his part, Ganna, of the Broad Institute, understands some of the criticism of his research. “What we studied is not related directly to the biology, but to extended environmental factors related to it. It’s not about our sample size – once you have a lot of individuals, you can capture very small effects. But are these directly influencing same-sex behavior, or other things related to this topic? As a medical example, think about a study that looks for associations between genetic markers and lung cancer. In that example, what we found are genetic variants regarding how much you smoke, which is related to lung cancer.”

One of the lessons, and one of the most interesting points arising from the study has to do, says Ganna, with the mode of measurement that had been in use since 1948, when Kinsey’s scale ranked individuals as being between 0 (totally heterosexual) and 6 (totally homosexual).

Ganna: “Basically, the tendency is to locate individuals on a continuum. You can supposedly be anywhere between 100 percent heterosexual to 100 percent homosexual, which implies that the more you’re homosexual, the less you’re heterosexual, and vice versa. We show that this assumption actually doesn’t hold water: When we look at the genetic data, it’s not that straightforward, there’s no simple continuum of sexuality.”

So, actually, you are refuting the Kinsey scale?

Ganna: “That’s exactly one of our conclusions. What we’re now doing is, rather than asking people to put themselves on a scale somewhere between being exclusively heterosexual or exclusively homosexual, we ask them how much they’re attracted to men and women. You could be attracted to either of them, very attracted to both of them – or to one more than the other. And that information will be crossmatched with genetic markers.”

In the final analysis, he adds, “We showed that this is just another natural human variation. Sexual orientation, similar to many other behavioral traits, is complicated and is composed of different factors. The interesting thing is how genetics and environment work together. If you think about how much more prevalent same-sex behavior has become lately, people engage in it more than in the past. And that’s clearly not because our genetics are changing. It’s because of the environment, because society is becoming more open and laws are changing.”

Alfred Kinsey.Credit: Keystone Features / Getty Images

Further research should focus on the relationship between environmental factors and genetics, Ganna says, and on how they interact. “It’s somewhat misleading to think of nature and nurture as separate aspects; they both contribute. So, it would be wrong to say that you can use only DNA to predict if someone will engage in same-sex behavior, but you also can’t say it’s simply a [matter of] choice.”

In summary, he says, “I think that the more people who will understand that there are genetic and environmental components to sexual behavior, the better – and this is a message that goes beyond just sexuality.”

Choice and lifestyle

However, the relationship between science and the environment, and particularly the people living in it, is a complicated one. “The subject definitely should be studied, but the social aspect of it is problematic,” says LeVay, the neurobiologist. “I am gay myself, and I feel strongly that gay people should be valued and accepted into society, regardless of what caused their sexual orientation. I don’t think it’s vital for gay liberation to prove that ‘gay people can’t help but be gay’ – there are plenty of other reasons [for accepting them], including basic human rights.”

At the same time, he adds, “this issue is socially relevant, because of traditional notions that see same-sex relations as a choice, a ‘lifestyle’ or sinful behavior.”

In recent years, “there have been many studies showing that people’s attitudes toward homosexuality are closely tied to their beliefs about what makes people gay,” says LeVay, citing a survey that showed there was a high probability that people who think homosexuality is a choice will object to a gay person being their children’s teacher – which in a way might make sense, he adds: “If you think being gay is something infectious, socially contagious, and you didn’t want your kid to be gay, then you wouldn’t want their teacher to be gay ... It follows that demonstrating that biological factors are involved, helps counter those ideas. Still, I’m a bit ambivalent about the use of this type of research as some sort of a political weapon in the struggle for gay rights.”

The Broad Institute study contains a reminder of the problems and stigmas that still exist with regard to the LGBTQ community. One of the parameters it considers are genetic correlations between genes that are ascribed to homosexuality, and certain psychological problems.

Bailey, the psychologist: “One thing that was perceived as controversial, was to look for – and find – a genetic overlap between homosexual sex genes and genes associated with depression. It’s not the same as saying all people who engage in homosexual sex are depressed for genetic reasons, but it’s also not something that can be easily ignored. There are assumptions that the higher rates of depression among gay men and lesbians is due to the way they are mistreated by society, but the evidence for that is not so overwhelming. There is also the fact, for example, that you have as high a rate of depression among homosexual men in the Netherlands, which is very tolerant, as you have in some less tolerant places, like the United States.”

Ganna, for his part, tries to soften that criticism: “Even if we see genetic overlap, or correlation, it is not set in stone that we’ve found a biological mechanism that causes depression and same-sex behavior,” he says. “There are many explanations for why this one genetic marker is associated with both things. But finding these correlations help us study human traits in general.”

In the meantime, there is a price to be paid for conducting research in this realm, which all those involved must be aware of. Reminders of this abound, and are almost routine. In some cases what’s at stake is not even a groundbreaking study or one of tremendous scientific importance. In 2017, for example, two researchers from Stanford published an article stating that “gay men are predicted to have smaller jaws and chins, slimmer eyebrows, longer noses, and larger foreheads; the opposite should be true for lesbians.” In the next stage, they created a facial-recognition program with the aid of more than 14,000 images taken from a singles site of straights and LGBTQs. The program was able to distinguish between gays and lesbians and heterosexuals with an accuracy of 81 percent for men and 71 percent for women, in contrast to an average rate of successful human guesses of 61 percent and 54 percent, respectively. Even though the program achieved relatively impressive results, the study as such drew widespread criticism – not unusual for researchers engaged in such studies.

The Stanford “gays identification” program may be an extreme example, in this respect, but it’s also a byproduct of the considerable surge in studies in this field, a trend that began in the early 1990s. Together with the scientific community, media interest in the subject of same-sex orientation and its causes has contributed substantially to transmitting messages and shaping public opinion.

In the United States, this can be seen in a series of polls conducted by Gallup, Inc. The first one, conducted in 1977, found that only 13 percent of the respondents believed that homosexuality is an innate tendency, while 56 percent attributed it to environmental factors. This approach remained largely constant until the period between 1989 and 1996, when the rate of those supporting the innate thesis leaped from 19 percent to 31 percent; by 2001, it stood at 40 percent. Almost a decade and a half later, the annual poll produced, for the first time, a larger proportion who agreed with the innate argument. The latest survey, from the end of last year, showed this trend continuing: More than half of the American public believes that gay people are born with their sexual orientation, whereas only 30 percent attribute it to environmental factors (10 percent said both factors play a part, 4 percent cited other factors and 6 percent said they weren’t sure).

Changes in the perceptions of the origins of sexual orientation are having a pronounced effect on the struggle LGBTQ individuals are waging for equal rights. The latest Gallup poll shows that an absolutely majority (88 percent) of those who believe that homosexuality is an innate trait also support legitimizing same-sex marriages. In contrast, most of those who see this orientation as being environmentally driven (61 percent) are against.

“When it comes to public opinion, which is very important, the ‘born this way’ idea has been really resonant and has had a very positive impact on society,” Hamer maintains. “Public opinion polls asked people whether they think [gays] were born this way or not, and we know that believing that homosexuality is innate correlates with having positive feelings toward gay rights. Overall, it’s been important in educating the public about who we are, as gay people.”

Such messages are reaching Israel as well. A poll conducted by the Dialog Institute for Haaretz at the end of 2013 found that 70 percent of those questioned favored full rights for same-sex couples, while 64 percent specifically backed their right to surrogacy. However, two polls conducted in the wake of the surrogacy law protest in July 2018 presented slightly lower numbers: About 57 percent of respondents expressed support for the right of same-sex male couples to surrogacy.

These polls did not ask Israelis whether they believe the origin of same-sex orientation is innate or environmental. If you ask Bailey, though, that doesn’t really matter.

“I’ve gone to great lengths to try to persuade people not to base equal rights for gay people on the causal hypothesis,” he says. “It’s a terrible idea to say gay people should have equal rights because they were born that way. It’s terrible in part because some criminals might be born that way, and you don’t want to them to have the same rights. Being gay doesn’t harm anybody, other than people who are close-minded and easily offended. Preventing people from expressing their homosexuality is quite destructive for them. That’s true whether gay people are born that way or not.”

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