Sexuality may emerge from a very complex 'genetic architecture.'
In 1993, geneticist Dean Hamer and colleagues first made the case for a heavy genetic component underlying human sexual orientation.
This and other studies indicating that genes likely play a key role in sexuality gave rise, in time, to the emergence of a controversial concept: the "gay gene," a single genetic player that could determine sexual orientation.
However, sexuality is not influenced by a single gene. Recent studies have suggested that different genetic loci — positions on a chromosome where different genes are located — are linked to sexuality and same-sex sexual behavior.
Now, a very large study that assessed the genetic information stored by two different databases — the UK Biobank and 23andMe — confirms that the genetic background of sexual orientation is not down to just one gene.
The first author of the study is Andrea Ganna, Ph.D., from the Broad Institute of MIT and Harvard in Cambridge, MA.
"The biological factors that contribute to sexual preference are largely unknown, but genetic influences are suggested by the observation that same-sex sexual behavior appears to run in families and is concordant more often in genetically identical (monozygotic) twin pairs than in fraternal twin pairs or siblings," the researchers write in their study paper, which now appears in the journal Science.
The researchers received funding from many state and academic institutions, including the Eunice Kennedy Shriver National Institute of Child Health and Human Development, which sponsored one of the researchers "specifically to investigate the genetics of sexual orientation."
Many of the scientists involved in this study are also members of the 23andMe Research Team, employees of 23andMe, and hold stock or stock options in 23andMe. This company has, in the past, expressed a particular interest in revealing more about the genetics of sexual orientation.
'No single genetic determinant'
One main question that the researchers aimed to answer was, "What genes are involved, and what biological processes do they affect?"
To find out, they conducted a genome wide association analysis on the data of over 408,000 people in the UK Biobank and over 68,000 people who provided genetic information through 23andMe.
The researchers only included in their analysis the data of people who had reported having had sex with someone of the same sex, with someone of the opposite sex, or both.
The researchers found five genetic variants "significantly associated" with same-sex sexual behavior — that is, the likelihood of having sex with someone of the same sex.
However, the researchers also noted that many more genetic loci likely underpin same-sex sexual behavior, and that the relationship between genetic factors is so complex that it would be impossible to pinpoint a predictive pattern.
"We identified genome wide significant loci associated with same-sex sexual behavior and found evidence of a broader contribution of common genetic variation," the researchers write.
They add, "We established that the underlying genetic architecture is highly complex; there is certainly no single genetic determinant."
"Rather, many loci with individually small effects, spread across the whole genome and partly overlapping in females and males, additively contribute to individual differences in predisposition to same-sex sexual behavior."
As for the genetic variants that seemed to be common in different cases of same-sex sexual behavior, some of them are also linked to biological pathways involved in sex hormone regulation, as well as the sense of smell.
However, how this information contributes to a better understanding of the genetic and biological mechanisms underlying sexual behavior remains to be seen.
A list of limitations
The research faces many limitations that the authors suggest future research projects should aim to address.
One such limitation is that the study did not include data from any individuals whose biological sex and gender identity did not match, no transgender individuals, and no intersex individuals. "This is an important limitation of our analysis," the authors write.
Another limitation was that they primarily took into account white populations from the United Kingdom, Sweden, and the United States. This means that the findings may not apply to other populations.
Also, as Ganna explained in an interview last year, the research looked at "a large spectrum of sexual experiences, that go from people who engage exclusively in same-sex behavior to those who might have experimented once or twice."
Simply because a person has had sex with someone of the same sex does not necessarily mean that they are not heterosexual.
Likewise, not having had same-sex intercourse does not mean that a person is not sexually attracted to people of the same sex.
Researchers urge against misusing findings
The researchers also note that on their own, the individual genetic factors that seem to contribute to same-sex sexual behavior appear to have a very small role in the matter.
These results may not come as a surprise to geneticists, but the investigators are wary of people and factions attempting to put a spin on the facts to suit their own agendas — particularly with the purpose of further marginalizing people in the LGBTQ+ community.
Assuming that genetic factors only play a minimal role in sexuality could provide erroneous support to dangerous — yet persistent — practices, such as conversion therapy. This, the Substance Abuse and Mental Health Services Administration warn, "perpetuates outdated views of gender roles and identities, [...] [putting] young people at risk of serious harm."
On the other hand, even showing that sexuality has a genetic basis can be harmful; historically, the eugenics movement has pushed the idea of preventing the birth of babies who might later express a certain sexual orientation.
The researchers behind the new study are well aware of these dangers, and they urge readers not to try and ply their findings to any agendas.
"Our findings provide insights into the biological underpinnings of same-sex sexual behavior but also underscore the importance of resisting simplistic conclusions — because the behavioral phenotypes are complex, because our genetic insights are rudimentary, and because there is a long history of misusing genetic results for social purposes."
Why it is necessary to determine exactly where same-sex sexual behaviors "reside" in the human genome, when we now know enough to be able to affirm that sexuality does have a strong genetic component, is a question that remains unanswered.
Going forward, it is vital to use this knowledge delicately and be mindful about how misinformation can affect people's well-being and healthcare.