New genetic links to same-sex sexuality found in largest study of its kind

The largest study of its kind finds genes contribute to same-sex sexual behaviour, but environment also plays a role.

Sexual attraction isn’t defined by one gene alone, but is instead the result of the complex interplay between multiple regions of the genome and hard to pin environmental factors. Picture:

Same-sex sexual behaviour has genetic underpinnings but no single gene is associated with it, according to a broad study of more than 470,000 people.

An international team of researchers found five genetic markers linked to whether someone has ever engaged in sexual behaviour with a person of the same sex, according to the paper published in the journal Science.

But the markers, which can be found across a range of genes, can’t be used to predict a person’s sexual orientation, researchers say, as sexual orientation and behaviour, like most human traits, are influenced by an array of genetic and environmental factors.

“Complicated behaviour, such as same-sex sexual behaviour, is a compilation of genetics and the social environment. It’s not just one or the other,” said Melinda Mills, a professor of sociology and demography at the University of Oxford who wasn’t involved in the work. “They’re starting the conversation in a very strong, empirical way.”

The study involved people who self-reported whether they had ever had sex with someone of the same sex, not whether they considered themselves gay, lesbian, bisexual or another orientation.

“Behaviour versus identity are different components and different pieces of how people think about their sexual behaviour,” said Benjamin Neale, a geneticist and member of the Broad Institute of MIT and Harvard University who is one of the study’s senior authors. “That is a really critical aspect to sort of appreciating and understanding how we think about these kind of issues.”

Past research found that biology and genetics play a role in sexual orientation and behaviour, though those studies involved relatively small groups and largely focused on men.

In the 1990s, researchers studied 40 pairs of gay brothers and found a link on the X chromosome in a region called Xq28, which was subsequently dubbed the “gay gene.” Studies attempting to replicate the research have had mixed results.

In recent years, more advanced research tools and a wealth of genetic data have made it possible to compare hundreds of thousands of genomes to better explore the relationship between genetics and human traits and behaviour.

In the new report, a team of researchers conducted a genome-wide association study using genomic data from 408,995 people in the UK Biobank and 68,527 participants from the genetic-testing company 23andMe.

Participants reported whether they had ever had sex with someone of the same sex, and the researchers analysed the data to determine if there were any genetic variants — single differences in the genetic code, like an A rather than a C — associated with the behaviour.

Taking into account the entire contribution of genetic markers that may play a role, the researchers estimated that genetic variation could account for up to 8 per cent to 25 per cent of same-sex sexual behaviour in the population studied. But when the researchers pooled all of the identified markers to create a score for an individual person, the genetic variation explained less than 1 per cent, making it practically impossible to predict a person’s sexual orientation or behaviour based on their genome.

“It does solidify the fact that genes matter,” said Michael Bailey, a psychology professor at Northwestern University who wasn’t involved in the work. “But it also provides more support for something that we all knew, pretty much — that genes can’t be close to the entire story.” Experts also noted that only two of the five genetic markers were significant in both sexes: Two variants were significant in just males, and one variant in just females. The genetic correlation between the sexes for any given trait is typically much higher, experts say.

The two male-associated markers also are linked to other traits, the researchers noted, including male-pattern baldness and sense of smell. The overall meaning or mechanisms of these linkages, however, are unclear.

Culture and societal factors frequently influence same-sex sexual behaviour, experts say. Most of the participants were white and from either the UK or the US, where stigma has waned relatively in recent years.

In anticipation of publishing the study, the researchers created a website called Genetics of Sexual Behaviour to help communicate their results. They worked with several advocacy and alliance groups, such as Sense about Science and GLAAD, before the study’s publication and received feedback on the science and about how to best communicate the findings. The authors then rewrote sections of the paper to highlight the paper’s focus on behaviour rather than a person’s identity or orientation.

“This new study provides even more evidence that being gay or lesbian is a natural part of human life, a conclusion that has been drawn by researchers and scientists time and again,” said Zeke Stokes, the GLAAD chief programs officer. “This new research also reconfirms the long established understanding that there is no conclusive degree to which nature or nurture influence how a gay or lesbian person behaves.”

“It was inevitable that this [study] was going to be done,” said Dr Neale, one of the study’s senior co-authors. “I thought it was important that we do it in a way with multiple scientific expertises and advocacy groups because that actually enriches the science and hopefully improves public communication of the work.”

The Wall Street Journal