Born gay or not

How do genes influence our sexuality? The question has long been fraught with controversy. An ambitious new study — the largest ever to analyze the genetics of same-sex sexual behavior — found that genetics does play a role, responsible for perhaps a third of the influence on whether someone has same-sex sex. The study of nearly half a million people, funded by the National Institutes of Health and other agencies, found differences in the genetic details of same-sex behavior in men and women. The research also suggests the genetics of same-sex sexual behavior shares some correlation with genes involved in some mental health issues and personality traits — although the authors said that overlap could simply reflect the stress of enduring societal prejudice. Even before its publication Thursday in the journal Science, the study has generated debate and concern, including within the renowned Broad Institute itself.

Those research findings have not been replicated. But it was never going to be that simple: decades of genetic research have shown that almost every human characteristic is a complex interplay of genes and environmental factors. A new study, published in Science this week, confirms that this is the case for human sexuality, too. The study, the largest ever into this difficult topic, was conducted by an international group of scientists working with 23andMe, a personal genomics firm.

The Pride parade in Brighton, UK. The findings, which are published on 29 August in Science and based on the genomes of nearly , people, shore up the results of earlier, smaller studies and confirm the suspicions of many scientists: while sexual preferences have a genetic component, no single gene has a large effect on sexual behaviours. But she cautions that the results may not be representative of the overall population — a limitation that the study authors acknowledge. The people who contribute their genetic and health information to those databases are predominantly of European ancestry and are on the older side. The study authors also point out that they followed convention for genetic analyses by dropping from their study people whose biological sex and self-identified gender did not match.

Few aspects of human biology are as complex—or politically fraught—as sexual orientation. Now, a new study claims to dispel the notion that a single gene or handful of genes make a person prone to same-sex behavior. The analysis, which examined the genomes of nearly half a million men and women, found that although genetics are certainly involved in who people choose to have sex with, there are no specific genetic predictors. Yet some researchers question whether the analysis, which looked at genes associated with sexual activity rather than attraction, can draw any real conclusions about sexual orientation.