Researchers have developed a test that they say can predict whether a man is homosexual or heterosexual. The findings were recently presented at the American Society of Human Genetics 2015 Annual Meeting in Baltimore, MD.

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The algorithm can predict whether a man is homosexual or heterosexual with up to 70% accuracy, according to researchers.

The test involves the use of an algorithm that can identify patterns of DNA methylation across nine regions of the human genome that might be related to sexual orientation.

DNA methylation is a molecular modification to DNA that controls gene expression. Such modifications are triggered by environmental factors, such as exposure to chemicals, stress, diet and exercise.

“To our knowledge, this is the first example of a predictive model for sexual orientation based on molecular markers,” says first author Dr. Tuck C. Ngun, of the University of California-Los Angeles.

Previous research has indicated that a person’s sexuality may be determined by the activity of certain genes, and this latest study – which involved identical twins – builds on such findings.

Despite identical twins possessing the same genetic sequence, only around 20% of them are both homosexual, which many researchers believe points to DNA alterations as a potential influencer for sexual orientation.

“It has always been a mystery why identical twins who share all their genes can vary in homosexuality,” Prof. Tim Spector, of King’s College London in the UK, told The Telegraph. “Epigenetic differences are one obvious reason and this study provides evidence for this.”

The team enrolled 37 pairs of identical male twins, in which one was homosexual and one was heterosexual, alongside another 10 pairs of identical male twins in which both were homosexual.

Studying identical twins enabled the researchers to control for genetic differences. As such, they were better able to pinpoint which DNA methylation patterns might be associated with sexual orientation, though this still proved difficult.

“A challenge was that because we studied twins, their DNA methylation patterns were highly correlated,” notes Dr. Ngun. “The high correlation and large data set made it difficult to identify differences between twins, determine which ones were relevant to sexual orientation, and determine which of those could be used predictively.”

To help determine which patterns could be linked to sexual orientation, the team created an algorithm called FuzzyForest to help them sift through more than 400,000 data points.

From this, the researchers identified DNA methylation patterns in nine small areas of the human genome that could predict whether each twin was heterosexual or homosexual with up to 70% accuracy.

“Previous studies had identified broader regions of chromosomes that were involved in sexual orientation, but we were able to define these areas down to the base pair level with our approach,” notes Dr. Ngun, adding:

“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.”

While the researchers say they are unable to explain how DNA methylation in the identified regions influences a man’s sexual orientation, this is something they plan to address with future research. At present, they are testing the accuracy of their algorithm in a more general population of men.

Earlier this year, Medical News Today reported on study published in PLOS ONE that found homosexual men are more likely to have female friends than heterosexual men.