Understanding Sexual Orientation

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Sexual orientation has been linked to genetic factors.

Not all identical twins have the same sexual orientation, though.

Epigenetics can help explain why two people can have the same genes, but different sexualities.

Differences in prenatal hormone exposure may be a key to understanding this phenomenon.

A growing amount of research suggests that sexual orientation has a genetic basis. If sexuality is genetically determined, this means identical twins should always have the same orientation, right? While that might seem like a logical conclusion, it's not true.

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

Same Genes, Different Attractions

In cases like this, it might be tempting to assume that both twins are actually gay, but one just hasn’t come out yet. In other words, maybe there isn’t a true discrepancy. However, science doesn't support this interpretation.

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.

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

Genetics Versus Epigenetics

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 genes linked to same-sex attraction, but both 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 was exposed to different levels of a given hormone or had a different response to that hormone than the other, and this contributed 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.

A recent study provided preliminary support for this idea. 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, especially for women. (The results for men have been more inconsistent.)

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. 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 cannot necessarily tell a person’s sexual orientation just by looking at their hands.

So what did these researchers find? Although comparisons between male twins were not statistically significant, they replicated the previous sexual orientation findings for women. Specifically, 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.

Understanding Sexual Orientation

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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. Also, the sample was quite 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. This means that locating relevant participants and, further, getting both them and their twins to take part in a study is 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, these results suggest that sexual orientation isn't a simple function of possessing a specific set of genes. While genes seem to be part of the story, other factors appear to be involved, and prenatal hormone exposure is likely one of them.

Sanders, A. R., Beecham, G. W., Guo, S., Dawood, K., Rieger, G., Badner, J. A., ... & Gejman, P. V. (2017). Genome-wide association study of male sexual orientation. Scientific Reports, 7(1), 16950.

Watts, T. M., Holmes, L., Raines, J., Orbell, S., & Rieger, G. (2018). Sexual arousal patterns of identical twins with discordant sexual orientations. Scientific Reports, 8(1), 14970.

Watts, T. M., Holmes, L., Raines, J., Orbell, S., & Rieger, G. (2018). Finger Length Ratios of Identical Twins with Discordant Sexual Orientations. Archives of Sexual Behavior, 47(8), 2435-2444.

Grimbos, T., Dawood, K., Burriss, R. P., Zucker, K. J., & Puts, D. A. (2010). Sexual orientation and the second to fourth finger length ratio: a meta-analysis in men and women. Behavioral Neuroscience, 124(2), 278.

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