A new study offers a glimpse into the genetics of same-sex attraction

What are the Darwinian underpinnings of same-sex attraction? And why do same-sex relations persist, generation after generation, if these individuals are less prone to procreate?

A massive genetic study aims to unravel these basic biological questions. It also touches on the question of whether it’s worthwhile, or even ethical, to study the genetics of sexual attraction in the first place.

Surveying the genes of nearly 500,000 men and women, researchers found four variants that were linked to people who had self-reported same-sex encounters. When those variants showed up in heterosexual men, those men tended to have a larger number of lifetime sexual partners and — and, though researchers didn’t say who did the judging — to be more physically attractive.

“This is a very important advance in the field,” said J. Michael Bailey, a professor of psychology at Northwestern University who has conducted several twin studies studying homosexuality. “But it won’t be a huge advance until some of the genetic variants predisposing one to homosexual activity are better understood.”

The paper is under review for publication in Science, but hasn’t yet been released. Andrea Ganna, a geneticist at the Broad Institute, shared an overview of the data at last week’s American Society of Human Genetics meeting in San Diego.

Researchers hastened to add that this is not a study of sexual orientation. Rather, it’s a study of non-heterosexual behavior, including the behavior of people who have had same-sex encounters but don’t identify as gay.

“There is no single ‘gay gene,’” Ganna said. “Sexuality cannot be reduced to a single trait. Rather, non-heterosexuality is in part influenced by many tiny genetic effects, and environmental components.”

There have been a number of smaller studies that examine the genetics of homosexuality and non-heterosexuality, but none comes close to the scope of the new study.

The data were drawn from two sources: the DNA of about 69,000 participants was from the consumer genetics company 23andMe and the DNA of just over 400,000 participants was from the British government’s UK Biobank.

Scientists should be able to try to replicate the findings, Bailey said, because the sample size is huge. And, unlike prior studies of twins — of which he’s contributed more than anyone else — volunteer bias simply is not an issue.

“People don’t know their DNA,” Bailey said.

The researchers identified four regions in the genome that influence a person’s choice in sexual partner. Two were observed in men and women, and two were seen in men alone. The DNA identified could account for only 8 percent to 12 percent of the genetics behind non-heterosexual behavior.

One of the variants was linked to the olfactory receptor — which is fitting, Ganna said, because smell has been linked to attraction. Another of the four variants was linked to male-pattern baldness, which could indicate that hormone regulation plays a role in choosing a sexual partner.

The reporting of non-heterosexual behavior is “drastically changing over time,” Ganna said. For instance, in 1967, homosexuality was decriminalized in Britain, Ganna said — which may have increased non-heterosexual behavior. Men are more likely to have same-sex encounters than women, and were more likely to have exclusively homosexual relations.

The genetics of sexuality has stirred up debate for decades.

“This is one of the most central questions in biology,” said Dean Hamer, the geneticist who identified the first potential “gay gene” in 1993. “It’s also one of the most central questions from a social perspective that consumes so much of our time.”

Hamer found a correlation between the genetic marker Xq28 and gay male sexuality. The scientific community was largely fascinated by his initial discovery, and the gay community largely embraced it — proof, at last, that being gay was an immutable, inherited trait, like eye color.

But he faced dissent from “all the anti-gay people, who were furious because their entire argument was that people choose to be gay — and should be punished for it,” he recalled.

Even some members of the gay community were leery of probing into the genetics of their sexuality, back then: “They were very nervous that the information would be misused, and might even force people to have abortions,” Hamer said.

That concern persists.

“In my view, the search for which gene is involved in the functional genomics of sexual orientation is not science, but scientific voyeurism,” said Andrea S. Camperio-Ciani, a professor of psychobiology at the University of Padua, near Venice.

Camperio-Ciani has studied the evolutionary basis of homosexuality, positing in a 2004 paper that female relatives of gay men produced more offspring — and figures that offers a sort of Darwinian rationale for same-sex attraction.

He found that this new study, however, stems from “a lot of useless curiosity,” and worries that pinpointing such genes could lead to the “identification, eradication, or preventative avoidance” of people who engage in same-sex behavior. (Ganna, of course, emphasized during his ASHG talk that these results would not be used for predictive purposes.)

“I’m sorry, but I do not approve of this as good science,” Camperio-Ciani said.

Ganna said the study’s authors wanted to remain as sensitive as possible to the LGBTQ community, conducting two workshops to talk through what the science means — and doesn’t mean — and how it might be communicated to the public.

“I think it’s wonderful the research is being pursued,” Hamer said. “I think it’s much more important to know the scientific truth, than be in the dark. It’s ignorance that’s always hurt gay people.”

An earlier version of this story incorrectly stated that Dean Hamer’s 1993 research had not been replicated.