A new study claims to have located nine parts of the male genetic code that indicate whether a person is heterosexual or homosexual.
However, the study's authors also say that there is some evidence that environment plays a role in a man's sexual orientation by altering the activity of certain genes.
The study's lead researcher, Dr. Tuck Ngun of the University of California-Los Angeles (UCLA) told the annual meeting of the American Society of Human Genetics Thursday that his team studied the genetic material for 47 pairs of male identical twins. In the course of the research, Ngun said his team identified nine so-called "epigenetic marks." The term refers to areas of the epigenome, a set of chemical marks that lie between human genes and turns them on or off in response to certain stimuli.
Ngun's team claim that by studying molecular data from the epigenetic marks, they were able to guess whether a twin pair was heterosexual or homosexual with 70% accuracy.
"To our knowledge, this is the first example of a predictive model for sexual orientation based on molecular markers," Ngun told the conference, according to Sky News. "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."
Of the 47 sets of identical twins studied, 37 were pairs in which one twin was homosexual and the other was heterosexual. In the other ten pairs, both sets of twins were homosexual. Because identical twins have the same DNA, the presence of "split" twin pairs indicate that sexual orientation is not down to DNA.
The study does not indicate how or why men take on the epigenomic marks that distinguish him as homosexual. Sky News reported that British attendees at the Baltimore conference greeted the study with skepticism.
Professor Tim Spector, from King's College London, a leading expert on twin studies and genetics, said: "It has always been a mystery why identical twins who share all their genes can vary in homosexuality.
"Epigenetic differences are one obvious reason and this study provides evidence for this. However the small study needs replicating before any talk of prediction is realistic."
Darren Griffin, professor of genetics at the University of Kent, said: "To claim a 70% predictive value of something as complex as homosexuality is bold indeed. I wait with bated breath for a full peer-reviewed article.
"While there is strong evidence in general for a biological basis for homosexuality my personal impression has always been one of a multiple contributory factors, including life experiences."