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A huge new study finds a faint hint of genetic variation that may be linked to same-sex sexual behavior. The study broadly reinforces the observation that both biology and a person's environment influence sexuality. But the results reveal very little about that biology, as NPR's science correspondent Richard Harris reports.
RICHARD HARRIS, BYLINE: The study involves almost half a million middle-aged people from Britain who volunteered to donate blood samples and answer questionnaires for a project called the UK Biobank. Scientists paired that with information from tens of thousands of white Americans who volunteered to answer sex-related questions for the genetics testing company 23andMe. Benjamin Neale at the Broad Institute says the study's focus isn't on a person's sexual identity or desires.
BENJAMIN NEALE: What we really focus on is behavior.
HARRIS: The scientists compared genetic variance in people who say they had at least one sex partner of the same sex to those who said they had no such encounters. The study, published in Science magazine, looked through millions of genetic variants to see if any significant differences appeared
NEALE: And when we do that, we, you know, identify five variants that have some influence.
HARRIS: Just five out of millions of variants analyzed.
NEALE: The first thing that's most important is that they represent very, very, very small effects, Together, the five variants account for much less than 1% of the variability in the traits that we're looking at.
HARRIS: So they explain virtually nothing about the behavior, and they reveal little, if anything, about the biology that might underlie those genetic variants. Using another technique, the authors say genes could still influence 8% to 25% of the behavior they studied. But even in a sample of a half a million people, it was impossible to tease out anything about those genetic variants. One obvious conclusion from these results is nobody's going to come up with a blood test to predict these sexual behaviors.
NEALE: Individual-level prediction is effectively an impossibility.
MELINDA MILLS: It doesn't explain a lot, but it's at least a first step.
HARRIS: Melinda Mills wasn't involved with the study, but she wrote a commentary about it. The Oxford University sociologist says, for one thing, it further undercuts a previously published report of a so-called gay gene on the X chromosome. And since they didn't find a gradient of sexual behavior, it undermines Alfred Kinsey's decades-old scale, which ranked people on a spectrum of sexuality, from exclusively heterosexual to exclusively homosexual.
Mills was intrigued to see that of the five variants that the study did find, two appeared in both men and women, two were only in men and one was only in women.
MILLS: It looks like there's something different driving women and men. And I think that just means that, you know, there's a lot more to be examined in terms of women's sexuality. And that's really been under-researched.
HARRIS: Studies like this that seek to link genetic patterns to behaviors or disease usually explain far more than the fraction of a percent of the variance that was the case for this study. That leaves Cecile Janssens, a professor of epidemiology at Emory University, puzzled why this study was even published.
CECILE JANSSENS: I don't think that they found anything that is worth reporting.
HARRIS: She notes that findings like this should be replicated in different samples. But in this case, only three of the five genetic variants showed up in a separate sample.
JANSSENS: So two of them didn't replicate at all.
HARRIS: And the broader findings don't apply to other populations - different races, places with different cultural norms or different age groups. Study co-author Benjamin Neale acknowledges that the study only really tells you about what's happening within the particular population they studied.
NEALE: I do think that we've done a good job, but there's no absolute guarantee that it'll turn up elsewhere.
HARRIS: Perhaps an even larger and more diverse study would shed more light on these questions. But pulling together a study like that would be an enormous challenge.
Richard Harris, NPR News.
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