Genes play a role in—but cannot alone predict—same-sex sexual behaviors, according to a study published today (August 29) in Science. Using genetic data from nearly half a million participants who consented to be surveyed about their sexual experiences, the authors find that at most, genetics accounts for 8–25 percent of the variation in sexual behaviors and only some of the genes involved are shared between men and women.
“The strength of the paper is that it used a very large dataset,” says Jacqueline Vink, a behavioral geneticist at Radbound University who was not part of the study but has worked with some of the researchers before. The methods allowed the researchers to “find novel genes associated with same-sex sexual behavior and learn more about possible biological pathways.”
Joel Gelernter, a psychiatrist and geneticist at the Yale School of Medicine who was not involved in the study, agrees. “This study included the largest sample to date for this kind of trait, and meticulously careful analyses,” he says. “There is a high level of support here that the genetics of same-sex sexual behavior is similar to that of other complex traits,” in that many genes are involved, each of which only has a very minor effect on its own.
Previous studies of families had suggested that about one-third of the variation in sexual behaviors could be explained by genetics. Others have attempted to study the genetic underpinnings of these behaviors but have only been able to analyze data from limited numbers of participants.
The researchers gathered genetic information from people who had submitted DNA to the UK Biobank and 23andMe and asked them to answer questions about their sexual experiences and to what degree they were with partners of the same sex or other sex.
“One of the top requests that we got from our customers as a topic to study is sexual orientation/sexual behavior,” says 23andMe’s Fah Sathirapongsasuti in a press conference.
The authors found two genes that were significantly associated with having engaged in same-sex sexual behaviors. Then, when the team separated the data by the individual’s sex, they found two more genes associated with same-sex sexual behaviors in men and one gene associated with the behaviors in women. These differences identified in men and women suggest that some of the variation in behavior may be related to hormonal influences, the authors say. One of the genes, for example, is tied to balding in men, which is affected by hormone levels.
The five genes each explained less than 1 percent of the variation in whether or not an individual reported participating in same-sex behaviors. When they included all sequences in the genome associated with same-sex sex, the researchers estimated that genes account for a maximum of 8–25 percent of the variation in the population’s behaviors, suggesting that much of what drives sexual activity is beyond genetics.
“Genetics is less than half of this story for sexual behavior but it’s still a very important contributing factor,” Ben Neale, a behavioral geneticist at the Broad Institute and a senior author on the study, said during the press conference. Still, the genetic associations he and his colleagues observed could not predict the likelihood that an individual would report having sex with partners of the same sex.
Younger participants reported more same-sex behaviors than older ones, which the researchers chalk up to cultural influence.
The study challenges—at least genetically—the popular “Kinsey Scale” theory, which suggests that heterosexuality and homosexuality exist along a single continuum, and that the more an individual is attracted to one sex, the less he or she is attracted to the other. There was no genetic signature that corresponded to particular proportions of same-sex to other-sex encounters. For example, there was no association that differentiated someone who reported only engaging in same-sex behavior from someone who only sometimes engaged in same-sex sexual behaviors.
“Overall, our findings suggest that the most popular [scales of heterosexual versus homosexual behavior] are based on a misconception of the underlying structure of sexual orientation and may need to be rethought,” the authors write in their study.
Although this analysis is the largest yet to probe these genes, its power is still limited by the fact that the sample of 477,522 participants only included 26,827 who had engaged in same-sex sexual behaviors. Gelernter says researchers will still need even larger samples to understand the specific genes and pathways involved. “This is a huge increase in the state of our understanding of the genetics of same-sex sexual behavior,” he says.
A. Ganna et al., “Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior,” Science, doi:10.1126/science.aat7693, 2019.
Emma Yasinski is a Florida-based freelance reporter. Follow her on Twitter @EmmaYas24.