By studying the DNA of more than 470,000 people in the United States and United Kingdom, scientists have determined that there is no single "gay gene." Instead, human sexual behavior is determined by a constellation of genetic and environmental factors.
Previous research has indicated that same-sex preferences are determined -- at least in part -- by a person's genes, and the idea of a single "gay gene" was born in 1993 when researchers identified a correlation between gay male sexuality and a genetic marker called Xq28.
As society embraced the concept of a gay gene, some took comfort in the idea that some people are "born gay," while others worried this might create the false impression that homosexuality is an aberrant pathology.
But the new study, published today in the journal Science, finds that same-sex sexual preferences cannot be attributed to a single gene -- or even a small handful of genes. Further, the study finds that even with advanced techniques available today, it would be impossible to predict whether a child or an embryo will grow up to prefer same-sex partners.
Evaluating more than 470,000 genomes, the study is at least a hundredfold larger than any prior study on the topic, according to lead author Andrea Ganna, Ph.D., a research fellow at the Broad Institute of MIT and Harvard.
Drawing from two large databases of genetic information -- the UK Biobank and 23andMe -- researchers performed a genome-wide association study, which is a type of analysis that scans the complete genetic codes of thousands of people, hunting for similarities among people who share certain traits.
In this case, the trait the researchers were looking for was same-sex sexual behavior. Ultimately, they found five locations on the human genome that were common enough to be "significantly" associated with same-sex behaviors. However, there were also thousands of other genetic variants that appeared to be involved.
When they tallied up all these common genetic markers, they represented somewhere between 8% and 25% of the individual differences in same-sex behavior. Further, the researchers found that some of the genes that appear to play a role in same-sex behavior partially overlap with genes affiliated with other traits, including certain risk-taking behaviors and psychiatric conditions.
However, those associations should be interpreted cautiously, researchers warned. They emphasized that the five genetic variants they found -- while interesting -- represent nothing more than clues for future study. They are not definitive "gay genes." Combined, they represent less than 1% of the genetic variation thought to be associated to same-sex sexual behavior.
"Genetics is less than half the story for sexual behavior, but it's still a contributing factor," said co-author Benjamin Neale, Ph.D., director of genetics in the Stanley Center for Psychiatric Research at the Broad Institute of MIT and Harvard.
"I wouldn't see this as the final answer," he said, speaking during a press briefing. "What we've clarified is that there's a lot of diversity out there."
Homosexuality "is not like eye color," said Neale. Instead, he said, it's more like height -- a trait determined by a complex interplay between many hundreds of gene variants, and later influenced by non-genetic circumstances in the womb, early childhood, and adolescence.
"This is a very, very personal topic for me -- both as a scientist and as a gay man," said study co-author Fah Sathirapongsasuti, Ph.D., senior computational biologist at 23andMe, speaking during the same press briefing.
Sathirapongsasuti said that, as a gay teenager, he began Googling the possible causes of homosexuality and stumbled across the theory of a single "gay gene" -- on the marker Xq28 -- which is passed down from mothers on the X chromosome.
That online discovery meant that -- at least in his teenage years -- Sathirapongsasuti "blamed" his mother for passing on genes that determined his sexual preferences.
"Luckily, this study disproves that," Sathirapongsasuti said. "A sense of blame is the wrong thing to put on because it's such a complex interplay between upbringing, environment, and genetics."
The LGBTQ advocacy group GLAAD applauded the study.
"This new study provides even more evidence that that being gay or lesbian is a natural part of human life," said GLAAD chief programs officer Zeke Stokes in prepared remarks. "The identities of LGBTQ people are not up for debate."
Beyond the primary results of their study, the researchers questioned the utility of the Kinsey scale, a scoring system that places people on a continuum that ranges from heterosexual to bisexual to homosexual. Survey responses from the study found that people's sexual preferences are complex and can't really be mapped on a linear scale.
According to Neale, the Kinsey scale may be "an oversimplification of the diversity of sexual behavior in humans." He added that the study reinforces the current movement of expanding acronyms in the LGBTQIA+ spectrum.
"This is a natural and normal part of variation in our species, and that should support the position that we shouldn't try to develop gay cures," he said.