There is no genetic signature that predicts how an individual will behave sexually, new research suggests.
In a landmark study that examined the genomes of almost half a million people, findings indicate that same-sex sexual behavior is influenced by a complex mix of genetic and environmental influences, similar to what's seen for most other human traits, investigators note.
"There is no gay gene that determines whether someone has same-sex partners," lead author Andrea Ganna, PhD, Center for Genomic Medicine, Massachusetts General Hospital and Harvard Medical School, Boston, said during a telebriefing with reporters.
The study was published online yesterday inScience.
Complexity of Human Sexuality
Ganna noted that twin and family studies have shown that same-sex sexual behavior is partly genetically influenced. But these studies were "small and underpowered" and the results were "mostly not reputable." The new study is roughly 100-times bigger than previous studies.
The researchers analyzed survey responses on same-sex sexual behavior and performed genome-wide association studies (GWAS) using data from 477,522 people from the UK Biobank and 23andMe.
They discovered five genetic variants were significantly associated with ever having same-sex sexual behavior. Two of the loci were found in both sexes, two in males, and one in females.
"These variants are common in the population and have very small effects," said Ganna.
When considered together in a combined score, "they explain considerably less than 1% of the variance in the self-reported same-sex sexual behavior," said Ganna.
Using a different analysis technique, genetics could account for an upper limit of 8% to 25% of same-sex sexual behavior of the population, the researchers calculate. Therefore, there is no genome signature that predicts a person's sexual preference, they conclude.
It's also noteworthy, they say, that there was no evidence that sexual orientation is associated with variants on the chromosome X, as has been reported previously.
"Same-sex sexual behavior is, in fact, very polygenic, meaning there are a lot of variance that contribute to these traits, and this is very similar to many other behavioral traits," Ganna said at the briefing.
Despite these small effects, the genetic variants could hint at some biological pathways that may be involved in same-sex sexual behavior. One variant, for instance, was located in a stretch of DNA that houses several genes related to the sense of smell. "We know that smell has a strong tie to sexual attraction, but its links to sexual behaviors are not clear," said Ganna.
Natural Part of Human Life
With this study, "we see how large-scale GWAS studies can give insights into the biological underpinnings of behavior, but at the same time we are warned that behavior phenotypes are complex and we cannot draw simplistic conclusions," Valda Vinson, PhD, research editor forScience, said at the briefing.
Also on the call with reporters, Ben Neale, PhD, of the analytic and translational genetics unit at Massachusetts General Hospital, noted that the Kinsey Scale, "which really places individuals on a continuum from basically exclusively opposite sex partners to exclusively same sex partners, is really an oversimplification of the diversity of sexual behavior in humans."
Commenting in an accompanying editorial, Melinda Mills, PhD, sociology professor at the University of Oxford, United Kingdom, writes that there is "an inclination to reduce sexuality to genetic determinism or to resent this reduction."
"Attributing same-sex orientation to genetics could enhance civil rights or reduce stigma. Conversely, there are fears it provides a tool for intervention or 'cure,' " Mills writes.
"Same-sex orientation has been classified as pathological and illegal and remains criminalized in more than 70 countries, some with the death penalty. Because [the researchers] found that the genetic loci they isolated predict less than 1% of same-sex behavior of individuals, using these results for prediction, intervention, or a supposed 'cure' is wholly and unreservedly impossible," Mills concludes.
The 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," Zeke Stokes, chief programs officer for the Gay & Lesbian Alliance Against Defamation, said in a statement.
"The identities of LGBTQ people are not up for debate. 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," Stokes added.
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