Specifically, what earlier studies found is that, for heterosexual women, their index finger tends to be about the same length as their ring finger. 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). 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. One recent study provides some preliminary support for this idea. 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. 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. What might those environmental factors be? Some researchers point to the hormones we’re exposed to in the womb. In theory, this means two people could carry “gay genes,” but both of them wouldn’t necessarily be gay depending on certain environmental factors. 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. So if it’s not genes, then what? One possibility is that rather than sexual orientation being genetic, perhaps it’s epigenetic. This tells us that, while identical twins have the same genes, they don’t necessarily have the same sexual attractions. 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 idea has been refuted scientifically, though.
In other words, maybe there isn’t a true discrepancy. In cases like this, some might argue that perhaps both twins are actually gay, but one just hasn’t come out yet. For example, one may be straight while the other is gay. Identical twins sometimes have different orientations.
As it turns out, however, it’s not accurate. 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. In fact, scientists recently identified two specific genes that appear to differ between gay and straight men. A growing amount of research suggests that sexual orientation has a genetic basis.