By Justin Lehmiller

A growing amount of research suggests that sexual orientation has a genetic basis. In fact, scientists recently identified two specific genes that appear to differ between gay and straight men [1].

If sexual orientation is indeed genetically determined, it would be tempting to assume that identical twins would always have the same orientation, right? If they have the exact same genes and our genes control our sexuality, this would seem like a pretty logical conclusion. As it turns out, however, it’s not accurate.

Identical twins sometimes have different orientations. For example, one may be straight while the other is gay. So why is that?

In cases like this, some might argue that perhaps both twins are actually gay, but one just hasn’t come out yet. In other words, maybe there isn’t a true discrepancy. This idea has been refuted scientifically, though.

In a study where scientists looked at the sexual arousal patterns of identical twins with different sexualities—specifically, where one was gay and the other was straight—they found that gay twins demonstrated more genital arousal in response to same-sex images, whereas straight twins demonstrated more arousal in response to opposite-sex images [2].

This tells us that, while identical twins have the same genes, they don’t necessarily have the same sexual attractions.

So if it’s not genes, then what? One possibility is that rather than sexual orientation being genetic, perhaps it’s epigenetic. The field of epigenetics tells us that our genes interact with our environment, and that the environment is capable of turning specific genes on or off. In theory, this means two people could carry “gay genes,” but both of them wouldn’t necessarily be gay depending on certain environmental factors.

What might those environmental factors be? Some researchers point to the hormones we’re exposed to in the womb. Perhaps one twin is being exposed to different levels of a given hormone or has a different response to that hormone than the other, and this is ultimately what contributes to later differences in sexual orientation. This is plausible because identical twins sometimes develop with different placentas, and those placentas might not transfer the same level of hormones to each fetus.  

One recent study provides some preliminary support for this idea [3]. Researchers looked at the ratio of the length of the index (or “pointer”) finger relative to the ring finger in 32 pairs of identical twins who differed in their sexual orientation. They compared the lengths of these two fingers because they are affected by early exposure to testosterone and several studies have shown that the ratio of these two fingers differs according to one’s sexual orientation, at least for women (the results for men have been somewhat mixed) [4].

Specifically, what earlier studies found is that, for heterosexual women, their index finger tends to be about the same length as their ring finger. By contrast, non-heterosexual women tend to have an index finger that is shorter than their ring finger. Why? This is thought to stem from higher exposure to testosterone in the womb. Men—who are also exposed to more prenatal testosterone—tend to have finger length patterns similar to non-heterosexual women in that the index finger is typically somewhat shorter than the ring finger.

Before we go on, let’s be clear about one thing: these finger length findings reflect average differences and, as always, there’s individual variability. In other words, you can’t necessarily tell a person’s sexual orientation just by looking at their hands.

Returning to the new study, researchers replicated the previous sexual orientation findings for women. Specifically, it turned out that the non-heterosexual twins showed a bigger difference in finger lengths on average than did their heterosexual co-twins, but only on the left hand. This is consistent with the idea that some twins might have had different hormone exposure in the womb.

Comparisons between male twins were not statistically significant.

Of course, there are several caveats in order here. For one thing, researchers only found effects for women and, further, they were limited to one hand. For another, the sample was small. However, it’s important to point out that recruiting twins with different sexualities is extraordinarily difficult. Researchers estimate that just 0.012% of the population consists of a gay or bisexual person who happens to have an identical twin [2]. This means that locating relevant participants and, further, getting both them and their twins to take part in a study is quite challenging.

Limitations aside, more research in this area would be informative because it has the potential to help us better understand the origins of sexual orientation. Although we must await the results of future research, one conclusion that seems clear right now is that sexual orientation is not purely genetic. Genes certainly seem to be part of the story, but there appear to be other factors involved, and prenatal hormone exposure just might be one of them.

----------------------------------------

Dr. Justin Lehmiller is an award winning educator and a prolific researcher and scholar. In addition to publishing articles in some of the leading journals on sex and relationships, he has written two textbooks and produces the popular blog Sex & Psychology. Dr. Lehmiller’s research addresses topics including casual sex, sexual fantasy, sexual health, and friends with benefits. His latest book is Tell Me What You Want: The Science of Sexual Desire and How It Can Help You Improve Your Sex Life. Follow him on Twitter @JustinLehmiller or facebook.com/psychologyofsex.