I N 1993 A region of the human genome called X q28 was linked to male homosexuality, and the controversial notion of a “gay gene” was born. 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. It used what is called a genome-wide association study ( GWAS ) on 408,995 individuals in the UK Biobank, a British health resource, and 68,527 American 23andMe users—all of whom remained anonymous and consented to the study.

A GWAS involves scanning a person’s DNA for tiny variations in the genetic code (simple changes in the A s, T s, G s or C s) that correlate with a given trait. The participants were divided on the basis of whether they answered yes or no to the question “Have you ever had sex with someone of the same sex?”—a woolly proxy for sexual orientation, even in the absence of little white lies. The figures the GWAS produced, therefore, relate only to a single act, not to whether someone identifies as gay.

The researchers found five genetic markers that were significantly associated with a reported homosexual act by one of the participants in the study. None of those markers was on the X or Y sex chromosomes and their total combined effect accounted for less than 1% of the variance. This is because the behaviour is the result of the aggregate effort of hundreds or thousands of genes, whose individual effects are infinitesimally weak. Totting up all the thousands of tested genetic variants accounted for between 8% and 25% of the variation in people’s self-reported homosexual acts. These variants also overlapped with other traits, such as a smoking and an openness to new experiences.

Interestingly, only about 60% of the genetic variants identified in the study were shared by both sexes. Most behaviours show more overlap between the sexes than this, intimating that male and female homosexuality, or at least sexual adventures, may be quite different. David Curtis of University College London notes that what overlap there is “suggests that there could be specific factors affecting same-sex attraction rather than simply being attracted to males versus being attracted to females.”

The riddles go on

Conscious of the tricky subject matter, the scientists are at pains to anticipate any misunderstandings or backlash. They collaborated with LGBT advocacy groups throughout the study.

Yet the research only scrapes the surface of the mysterious depths of human sexuality. Unravelling these riddles will be difficult and will inevitably beget misconception and controversy. But at least this study should add weight to the view that non-heterosexual behaviour is firmly within the normal, natural spectrum of human diversity and provide a firm foundation for future work.■