The genome-wide research on DNA from nearly half a million U.S. and U.K. adults identified five genetic variants not previously linked with gay or lesbian sexuality. The variants were more common in people who reported ever having had a same-sex sexual partner. That includes people whose partners were exclusively of the same sex and those who mostly reported heterosexual behavior.
The researchers said thousands more genetic variants likely are involved and interact with factors that aren't inherited, but that none of them cause the behavior nor can predict whether someone will be gay.
The research "provides the clearest glimpse yet into the genetic underpinnings of same-sex sexual behavior," said co-author Benjamin Neale, a psychiatric geneticist at the Broad Institute in Cambridge, Massachusetts.
"We also found that it's effectively impossible to predict an individual's sexual behavior from their genome. Genetics is less than half of this story for sexual behavior but it's still a very important contributing factor," Neale said.
The study was released Thursday by the journal Science. Results are based on genetic testing and survey responses.
Some of the genetic variants found were present in both men and women. Two in men were located near genes involved in male-pattern baldness and sense of smell, raising intriguing questions about how regulation of sex hormones and smell may influence same-sex behavior.
Importantly, most participants were asked about frequency of same-sex sexual behavior but not if they self-identified as gay or lesbian. Fewer than 5% of U.K. participants and about 19% of U.S. participants reported ever having a same-sex sexual experience.
The researchers acknowledged that limitation and emphasized that the study's focus was on behavior, not sexual identity or orientation. They also note that the study only involved people of European ancestry and can't answer whether similar results would be found in other groups.
Origins of same-sex behavior are uncertain. Some of the strongest evidence of a genetic link comes from studies in identical twins. Many scientists believe that social, cultural, family and other biological factors are also involved, while some religious groups and skeptics consider it a choice or behavior that can be changed.
A Science commentary notes that the five identified variants had such a weak effect on behavior that using the results "for prediction, intervention or a supposed 'cure' is wholly and unreservedly impossible."
"Future work should investigate how genetic predispositions are altered by environmental factors," University of Oxford sociologist Melinda Mills said in the commentary.
Other experts not involved in the study had varied reactions.
Dr. Kenneth Kendler a specialist in psychiatric genetics at Virginia Commonwealth University, called it "a very important paper that advances the study of the genetics of human sexual preference substantially. The results are broadly consistent with those obtained from the earlier technologies of twin and family studies suggesting that sexual orientation runs in families and is moderately heritable."
Former National Institutes of Health geneticist Dean Hamer said the study confirms "that sexuality is complex and there are a lot of genes involved," but it isn't really about gay people. "Having just a single same sex experience is completely different than actually being gay or lesbian," Hamer said. His research in the 1990s linked a marker on the X chromosome with male homosexuality. Some subsequent studies had similar results but the new one found no such link.
Doug Vanderlaan, a University of Toronto psychologist who studies sexual orientation, said the absence of information on sexual orientation is a drawback and makes it unclear what the identified genetic links might signify. They "might be links to other traits, like openness to experience," Vanderlaan said.
The study was a collaboration among scientists including psychologists, sociologists and statisticians from the United States, United Kingdom, Europe and Australia. They did entire human genome scanning, using blood samples from the U.K. Biobank and saliva samples from customers of the U.S.-based ancestry and biotech company 23andMe who had agreed to participate in research.