The idea that a single “gay gene” exists has been disproven by scientists, who found that homosexual behavior is instead influenced by a wide group of genetic variants, each of which has a small, cumulative effect.

The study, published in the journal Science, was carried out by an international team of researchers. They compared the situation to factors determining a person’s height, in which multiple genetic and environmental factors play roles.

 “This study highlights both the importance of genetics as well as the complexity of the genetics, but genetics is not the whole story,” said Dr. Benjamin Neale, co-author of the study from the Broad Institute in the US.

The team looked at data from about 500.000 individuals. About 4% of men and nearly 3% of women said they had ever had a same-sex sexual experience. They did not focus on identity or orientation and did not include transgender individuals.

Then, by looking at the sexual behavior and relatedness of individuals, they estimated that about a third of the variation in same-sex behavior is explained by genetics. That clashes with previous twin studies that put the figure at about 30% to 50%.

“I hope that the science can be used to educate people a little bit more about how natural and normal same-sex behavior is,” said Neale. “It’s written into our genes and it’s part of our environment. This is part of our species and it’s part of who we are.”

The team looked at which genetic variants might be behind the link. They found five genetic variants that showed a clear link to same-sex sexual behavior — two in both men and women, two found only in men and one found only in women. The team believes one of these, found only in men, might be involved in sex hormone regulation.

These five genetic variants explain less than 1% of the variation in same-sex behavior among participants – suggesting many other variants are involved, each playing a very small role. it is not possible to use genetic information to predict whether an individual will have same-sex partners, they concluded.

The authors said their findings call into question the idea that sexuality exists on a single scale.

 “There seem to be genes associated with opposite-sex attraction and other genes associated with same-sex attraction, and these are not related,” Dr. Brendan Zietsch, co-author of the research, said. “These results suggest we shouldn’t be measuring sexual preference on a single continuum from straight to gay, but rather two separate dimensions: attraction to the same sex and attraction to the opposite sex.”

Limitations and questioning

The study has limitations, including that it is based mainly on people of European ancestry, while the age range of participants does not fully reflect that of the wider population. It also relied on self-reported behavior.

The idea that genetics might play a role in same-sex attraction was propelled into the spotlight in 1993 when Dean Hamer, a scientist at the US National Cancer Institute, and his team found links between DNA markers on the X chromosome and male sexual orientation.

Subsequent research has thrown up mixed results, although recent studies have supported the theory that genetics plays a role in sexual orientation.

Qazi Rahman, a leading authority on sexual orientation research from King’s College London, welcomed the study but said that the databases involved only captured information from a small percentage of people who were invited to participate. That, he said, means that the genetic variants found in the latest research might reflect another trait particular to those who chose to respond.

The study has generated debate and concern, including within the renowned Broad Institute itself. Several scientists who are part of the L.G.B.T.Q. community there said they were worried the findings could give ammunition to people who seek to use science to bolster biases and discrimination against gay people.

“I deeply disagree about publishing this,” said Steven Reilly, a geneticist and postdoctoral researcher who is on the steering committee of the institute’s L.G.B.T.Q. affinity group, Out@Broad. “It seems like something that could easily be misconstrued,” he said