Are twins more likely to be gay

gay twins in a relationship

Matrilineal inheritance is also apparent in the children of same-sex couples; a male child conceived by sperm donation to one member of a lesbian partnership is more likely to be homosexual [30]. An page questionnaire on the "sexuality of twins" was filled out by one or both twins. Thirty-eight pairs of monozygotic twins (34 male pairs and 4 female pairs) were found to have a concordance rate of % for homosexual orientation.

Researchers estimate that just % 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.

Family studies indicate that homosexuality clusters in some families but not in others. Likewise, twin studies suggest that identical twins are more likely to have the same sexual. Beyond Kallmann and the first studies of homosexuality among twins, there have been many controversies, shortcomings, and inaccuracies.

The first major issue with most twin studies is the overestimation and underestimation of heritability (Segalowitz, ). For example, the assumption that MZ twins share % of genes is only partially accurate. 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. Identical twins sometimes have different orientations. For example, one may be straight while the other is gay. So why is that?

But this idea has been refuted scientifically. 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].

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. What might those environmental factors be? 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]. 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. 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.

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. Researchers estimate that just 0. 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.

Genome-wide association study of male sexual orientation. Scientific Reports, 7 1 ,