A new study suggests many genes, each with small effects, may play a role in same-sex sexual behavior.

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Genetics may explain up to 25% of same-sex behavior, giant analysis reveals

People who have had same-sex partners are more likely to have one or more of certain DNA markers, according to the largest ever search for genes linked to sexual orientation. Even all the markers taken together, however, cannot predict whether a person is gay, bisexual, or straight. Instead, hundreds or thousands of genes, each with small effects, apparently influence sexual behavior.

The paper, published today in Science, builds on results presented by the same team at a 2018 meeting. The published study emphasizes that the genetic markers cannot be used to predict sexual behavior.

Still, the work is being hailed as the most solid evidence to date linking specific genetic markers to same-sex sexual behavior. “For the first time we can say without a reasonable doubt that some genes do influence the propensity to have same-sex partners,” says psychologist Michael Bailey of Northwestern University in Evanston, Illinois, who was not involved in the study. The results come with caveats, however, he and others say.

Studies of families and twins have long suggested same-sex behavior has a genetic component. Starting in the 1990s, scientists reported tentative evidence for genetic links to sexual orientation. In the past few years, huge data sets with DNA from hundreds of thousands of people have made possible much more powerful studies.

To explore the genetics behind sexual behavior, an international team co-led by geneticist Benjamin Neale of the Broad Institute in Cambridge, Massachusetts, used the UK Biobank, a long-term health study of 500,000 British people. The team worked with behavioral scientists and also consulted with lesbian, gay, bisexual, transgender, and queer (LGBTQ) advocacy groups.

Neale’s team examined DNA markers and data from surveys of sexual behavior filled out by nearly 409,000 UK Biobank participants and about 69,000 customers of 23andMe, the consumer testing service; all were of European ancestry. The UK Biobank survey asked: “Have you ever had sexual intercourse with someone of the same sex?”; the 23andMe survey featured a similar question. The team found five genetic markers significantly associated with answering yes to those queries. Two markers were shared by men and women, two were specific to men, and one was found only in women.

One of the genetic variants was near genes associated with male baldness, suggesting a tie to sex hormones such as testosterone, and another was in an area rich in smell genes, which have been linked to sexual attraction. When the researchers combined all the variants they measured across the entire genome, they estimate that genetics can explain between 8% and 25% of nonheterosexual behavior. The rest, they say, is explained by environmental influences, which could range from hormone exposure in the womb to social influences later in life.

But the five DNA markers they found explained less than 1% of this behavior, as did another analysis that included more markers with smaller effects. As with other behavioral traits such as personality, there is no single “gay gene,” says Broad team member Andrea Ganna. Instead, same-sex sexual behavior appears to be influenced by perhaps hundreds or thousands of genes, each with tiny effects.

As the researchers had reported last year, they also found people with these markers were more open to new experiences, more likely to use marijuana, and at higher risk for mental illnesses such as depression. LGBTQ people might be more susceptible to mental illness because of societal pressures, the researchers note.

Other researchers caution that the findings are limited by the fact that a person who had a single same-sex experience was counted as nonheterosexual. Having just one such encounter, for example, may reflect an openness to new experiences rather than sexual orientation, says Dean Hamer, a retired geneticist from the National Institutes of Health in Bethesda, Maryland. “These are fascinating findings, but it’s not really a gay gene study per se,” says Hamer, who in 1993 reported finding an area on the X chromosome that was more common in gay men; that region was not found in the new study. “I’m now much less excited about the possibility of getting good biological clues” to sexual orientation, he says.

Bailey wishes the UK Biobank had asked subjects which sex they feel more attracted to, not just about their behavior (as 23andMe did). “They didn’t have a particularly good measure of sexual orientation,” agrees evolutionary biologist William Rice of the University of California, Santa Barbara, who notes such a question would also capture gay or bisexual people who have not acted on their attractions. Still, he’s glad to see the study getting attention. “A big chunk of the population” is not exclusively heterosexual, he notes, and “they want to understand who they are and why they feel the way they do.”