The study, published in Science, used data from the UK Biobank and 23andMe, and found some genetic variants associated with same-sex relationships.
But genetic factors accounted for, at most, 25% of same-sex behaviour.
Advocacy group GLAAD said the study confirmed "no conclusive degree to which nature or nurture influenced how a gay or lesbian person behaves."
The researchers scanned the genomes - the entire genetic make-up - of 409,000 people signed up to the UK Biobank project, and 68,500 registered with the genetics company 23andMe.
Participants were also asked whether they had same-sex partners exclusively, or as well as opposite-sex partners.
The Harvard and MIT researchers concluded genetics could account for between 8-25% of same-sex behaviour across the population, when the whole genome is considered.
Five specific genetic variants were found to be particularly associated with same-sex behaviour, including one linked to the biological pathway for smell, and others to those for sex hormones.
But together they only accounted for under 1% of same-sex behaviour.
'Impossible to predict'
Ben Neale, an associate professor in the Analytic and Translational Genetics Unit at Massachusetts General Hospital, who worked on the study, said: "Genetics is less than half of this story for sexual behaviour, but it's still a very important contributing factor.
"There is no single gay gene, and a genetic test for if you're going to have a same-sex relationship is not going to work.
"It's effectively impossible to predict an individual's sexual behaviour from their genome."
Fah Sathirapongsasuti, senior scientist at 23andMe, added; "This is a natural and normal part of the variation in our species and that should also support precisely the position that we shouldn't try and develop gay 'curism'. That's not in anyone's interest."
David Curtis, honorary professor at the UCL Genetics Institute, University College London, said: "This study clearly shows that there is no such thing as a 'gay gene'.
"There is no genetic variant in the population which has any substantial effect on sexual orientation.
"Rather, what we see is that there are very large numbers of variants which have extremely modest associations.
"Even if homosexuality is not genetically determined, as this study shows, that does not mean that it is not in some way an innate and indispensable part of an individual's personality."
Zeke Stokes, from the LGBT media advocacy organisation GLAAD, said: "This new research re-confirms the long-established understanding that there is no conclusive degree to which nature or nurture influence how a gay or lesbian person behaves."
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