The study of genetic variations in a sample of almost half a million people reveals the complexity of human sexuality
In 1993, a study published in the journal Science caused a stir in the scientific community by postulating a link between male homosexuality and a specific segment of DNA. The research was based on the genetic analysis of 40 pairs of homosexual twin brothers. The researchers found that 33 of these pairs (or 64%) had the same polymorphic markers – that is, the same variations – in the final part of their X chromosome, specifically in the genetic marker Xq28. The study seemed to scientifically establish the concept of the ‘gay gene’.
Since that paper, the idea that male sexual orientation can be genetically determined has been the subject of controversy both in the media and in the scientific community. Dozens of investigations have approached the matter with mixed conclusions. The latest of those investigations is, in fact, the largest study ever conducted on the influence of genetics on sexual behavior. And his conclusion is, this time yes, resounding: there is no gene for homosexuality.
«We can safely say that there is not a single genetic determinant, not a single gene for same-sex behavior or sexual orientation,» the scientists conclude.
BREAKING THE MYTH OF THE ‘GEN GAY’
In the first part of the study, the researchers analyzed information from 477,522 people. On one hand, they were asked to answer a questionnaire that included questions about sexual identity, attraction, sexual experience, and fantasies. 26,827 people reported having sex with people of the same sex. They then compared that data to millions of DNA markers from their respective genomes, collected through the UK Biobank project.
In the second part of the study, the team looked at genetic variants that might show some significant connection to homosexuality. They found five genetic markers that offered a clear link to homosexual behavior. However, they found that each marker alone had a very small effect on sexual behavior (less than 1% adding the five markers). They were not predictive factors.
«What we found is that there is no one “gay gene” – instead, there are many, many genes that influence a person’s likelihood of having had same-sex partners”, explains geneticist Brendan Zietsch, director of the Center for Psychology and Evolution at the University of Queensland.
«There is not a single gay gene, but rather many, many genes that influence the likelihood of a person having same-sex partners.»
As with other human traits, sexual behavior is explained as a complex sum of multiple factors. Genetic variants play a role. But Zietsch and his colleagues believe that environmental factors play a bigger role, in the same way that they have a decisive influence, for example, on people’s height.
«In this case, the word environmental only means that they are not genetic influences», qualifies Zietsch. “It doesn’t have to be anything related to education or culture. It could be non-genetic biological effects or the prenatal environment in the womb. Our study does not shed light on these influences ”.
In their analysis of the huge body of data they have worked with, the researchers have found that there is a statistical correlation between genetic predisposition to homosexual behavior and some personality traits. What are those traits? Things like openness to new experiences, a recurring feeling of loneliness, and a tendency to risk behaviors. Also predisposition to mental health problems, which are disproportionately common among the LGBTQ community. “One possibility is that the stigma associated with homosexuality causes or exacerbates mental health problems. This could create a genetic correlation, ”says Zeitsch.
Beyond their refutation of the existence of a ‘gay gene’, the authors of the study explain that their findings call into question whether sexuality exists on a single scale.
«The results suggest that we should not measure sexual preference on a single continuum from heterosexual to homosexual, but rather on two separate dimensions: same-sex attraction and opposite-sex attraction.»
The study has limitations, as recognized by its own authors. Among them, the fact that all the information came from personal testimonies that could be biased. Or that the sample was based solely on people of European descent (and mostly white). That is why the experts involved encourage new research with even larger population samples to better understand the interaction between the environment and genes.
«We hope that our results will help rethink the way sexual behavior is measured and that it is done in collaboration with the LGBTIQ + community and stakeholders,» says Andrea Ganna, lead author of the study. «Diverse sexual behavior is a natural part of the general human variation,» she adds.