Twins Study Uses Saliva Test to Predict Sexual Orientation With 70 Percent Accuracy

A study presented this week at a scientific meeting suggests that scientists could predict sexual orientation of males from its group with up to 70 percent accuracy using a saliva sample. Some scientists though urge caution in interpreting the results of this study at this point.

The research, which was presented at the 2015 annual meeting of the American Society of Human Genetics but has not yet been published in a peer-reviewed journal, is the "first example of a predictive model for sexual orientation based on molecular markers," Dr. Tuck Ngun, first author of the study, said in a statement.

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The study involved 37 pairs of identical twins where one twin was homosexual and the other was heterosexual. It also involved 10 sets of twins in which both were homosexual. Because the twins are known to have the same genetic code, the researchers believe they were able to identify nine different epigenetic markers in different regions of the genome that could be used to predict the person's sexual orientation.

Epigenetics refers to changes in gene expression (not the underlying DNA sequence itself). These changes to how a gene is expressed can be inherited or caused by environmental factors. This research specifically looked at patterns in DNA methylation, or "molecular modification to DNA that affects when and how strongly a gene is expressed," a news release about the research stated.

Ngun, a postdoctoral researcher at the David Geffen School of Medicine of the University of California, Los Angeles, said that the twins' DNA methylation patterns were "highly correlated" and provided a large data set, making it "difficult to identify differences between twins, determine which ones were relevant to sexual orientation and determine which of those could be used predictively."

The researchers developed an algorithm, dubbed FuzzyForest, and were able to narrow down the patterns they used to predict sexual orientation.

Science Magazine reported that why methylation patterns in twins are sometimes different is not known. But it offers this insight into possible reasons:

If [the hypothesis of William Rice, at UC, Santa Barbara] is right, their mothers' epi-marks might have been erased in one son, but not the other; or perhaps neither inherited any marks but one of them picked them up in the womb. In an earlier review, Ngun and Vilain cited evidence that methylation may be determined by subtle differences in the environment each fetus experiences during gestation, such as their exact locations within the womb and how much of the maternal blood supply each receives.

"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," Ngun said.

"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," he added.

Criticism of the study included the fact that it only involved twins and its small sample size. Here's more on that front from Reuters:

"The question as to whether that prediction is going to be useful outside of the small number of twins in the study is really unclear," said Dr. Christopher Gregg, a genetics professor at the University of Utah.

Others noted the small size of the population studied and stressed that such findings often fall apart when applied to larger groups of people.

"One thing you can clearly see is that the sample size is too small. They don't have enough power to make that claim," said Dr. Peng Jin, professor of human genetics from Emory University in Atlanta, who attended the meeting in Baltimore.

[...]

"Just because there is something different doesn't mean that's what's causing people to behave one way versus the other," [Gregg] said.

Further research, according to Ngum, includes figuring out how the DNA methylation in these nine regions are related to sexual orientation and also using the same algorithm on a more general population of men.

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