Researchers say they've come up with a formula to reveal someone's sexual orientation using DNA found in their saliva that is 67 percent accurate.
The study, presented Thursday to genetics experts meeting in Baltimore, traced the genetic changes in identical twins that when combined help determine whether someone is gay or straight. It's the study first to claim a method to detect sexuality, reported Reuters.
"To our knowledge, this is the first example of a predictive model for sexual orientation based on molecular markers," said Tuck Ngun, a researcher at the David Geffen School of Medicine of the University of California, Los Angeles, who led the study.
According to Gallup, about 3.8 percent of the adult U.S. population identifies as lesbian, gay, bisexual or transgender.
Of course, the idea of a genetic test for sexuality raises concerns that someone might try to alter these epigenetic modifications to change a baby's inherent orientation. Currently, there is no way to selectively change epigenetic patterns on DNA, although the technology is being developed.
These concerns over the potential misuse of the test led the study's lead researcher to quit the project entirely, and many scientists have expressed caution over the results, including Ngun himself.
He, too, worries the test has the potential to be used and abused. "I'm gay," he told New Scientist, "and I've always wondered why I am the way I am. But once you have this information, you can't control how it's used or disseminated."
From the moment a child is conceived and all through a person's life, genetic changes occur, and they can be handed down from generation to generation. These are called epigenetic changes. The underlying code remains unaltered, but how a gene is expressed -- how it works -- that's what changes.
At an American Society of Human Genetics meeting in Baltimore, Ngun stated that he studied epigenetic changes called methylation in 47 pairs of male twins. Identical twins have the same underlying DNA, but the epigenetic changes can make big differences in what happens to them later in life.
In 37 of the twin pairs, one brother was homosexual and the other wasn't. In 10 pairs, both brothers were gay.
Dr. Margaret McCarthy, who studies the developing brain at the University of Maryland School of Medicine, said in a statement that this study provides more evidence we were indeed "born this way:"
"Developing male fetuses produce very high quantities of testosterone during the second trimester and this directs psychosexual development along masculine lines, a component of which is preference for females as sexual partners.
"This study provides a major step forward in our understanding of how the brain can be affected by factors outside of the genome. It is also possible that the experience of being a homosexual or a heterosexual has itself impacted the epigenetic profile. But regardless of when, or even how, these epigenetic changes occur, their findings demonstrate a biological basis to partner preference."
Other experts said Ngun may be going too far in saying he can predict someone's sexual orientation by looking at his or her genes, given that his study group was very small.
Since the associations have not yet been tested in a completely independent study population, the results should be considered no more than suggestive, caution experts. There needs to be verification before any firm conclusions can be drawn, according to Johnjoe McFadden, a molecular geneticist at the University of Surrey, U.K.
Ngun is optimistic his findings may lead to bigger discoveries:
"Sexual attraction is such a fundamental part of life, but it's not something we know a lot about at the genetic and molecular level. I hope that this research helps us understand ourselves better and why we are the way we are."