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flyingkiwisaurus t1_jaih192 wrote

I hope a true subject matter expert will chime in but as I understand it there is a strong suspicion that sexuality has a lot to do with epigenetics.

As another post pointed out, there is good data that if one male identical twin is gay there's a higher than typical chance (20-50%) that the other twin will also be gay.

If there we a "gay gene" we would expect all identical twins to have the same sexuality, so this is strong evidence against a "simple" genetic cause for sexuality.

But the fact that if one identical male twin is gay there's an increased chance that the second twin will be gay suggests genetics are involved somehow.

So the suspicion is that sexuality is linked to epigenetics, basically you can think of epigenetics as the dials and switches which regulate how strongly a gene is "turned on". These switches and dials can be affected by environmental factors, which in the case of sexuality means environmental factors inside the womb during fetal developmemt.

Given identical twins share the same womb at the same time they are exposed to similar environmental factors so it's thought they are more likely to have those dials and switches set in a similar way than non identical twins.

As I understand it (again I hope a real expert will chime in) epigenetics is currently the leading hypothesis for the cause of sexuality, but it's still a bit of a mystery.

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SnooComics7744 t1_jaiqpi4 wrote

I think of myself as an expert on the biological basis of homosexuality, and concur with the statement above. I published several papers on the topic, including the 1999 Williams Nature paper that I cited below, before my professional interests turned exclusively to neurobiology.

As I noted below, the fraternal birth order effect is the most well-established finding we currently have on the cause of male homosexuality, and it is amenable to an epigenetic intepretation. For example, circumstantial evidence suggests that increasing parity immunizes or innoculates mothers against HY antigens, which *could* underlie the development of male homosexuality. Such a mechanism would probably involve immune cells and cytokines from the mother passing through the placenta and influencing epigenetic marks on the developing fetus' genome. That, in turn, could influence brain and bodily development.

EDIT: Note that the fraternal birth order finding implies that something occurs in utero to affect the psychosexual development of the male fetus in a way that heightens the likelihood that the boy will be gay. We do not know what that something is.

It is considered well-established in this field that prenatal androgen levels sexually differentiate the brain and the body in a male-like direction. Abundant evidence supports this general idea. And since sexual attraction is a sexually dimorphic trait (most men are attracted to women and only women, and vice versa), its reasonable to suppose that something about prenatal androgen could be involved in male homosexuality.

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IllustriousArtist109 t1_jasqlfh wrote

Wasn't the fraternal birth order effect based on an underpowered, p-hacked study? Or have there since been better ones?

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