Am I more likely to be homosexual if I have a homosexual sibling?

Submitted by rando999555 t3_11esrwq in askscience

Just had this thought today. It seems a lot of homosexuals know early on in life. Which makes me think it must be mostly genetic. But then, if it's mostly genetic, would that make me or my other siblings more likely to be homosexual or at least not fully straight if I have a sibling that is? It doesn't seem to work that way though in my family or in many others so it makes me wonder. Sexuality is complex though. I questioned my sexuality for a couple of years but outside of that I've felt very straight for the majority of my life.

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Sulfamide t1_jagr4hg wrote

To answer your question, for twins, yes it is a good predictor, but for non-twins siblings then no, no evidence of that.

Also, for a huge majority sexuality is actually quite simple. If you only have sexual feelings for the opposite gender (i.e. erections) then you are straight and will probably stay that way you whole life.

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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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SnooComics7744 t1_jaik7no wrote

The strongest data we have on the cause of male homosexuality is the fraternal birth order effect, which says that the likelihood of being gay increases with each additional older brother. This phenomenon can be interpreted as a genetic effect if the hypothetical gene(s) predispose women to greater fertility - more likely to have more children. That predisposition to have more children, coupled to a predisposition to launch an immune response to cellular 'male-ness' could explain the fraternal birth order effect.

This finding was supported by a 2000 paper by Williams et al, which found that finger length ratio in gay men, a physical characteristic set before birth, is related to their fraternal birth order.

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IllustriousArtist109 t1_jasqwk2 wrote

Yes, because you and your sibling were brought up in a culture that approves of or at least acknowledges the existence of homosexuality.

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I can't find a source, but the government of India (pop. > 1 billion) stated that there are no lesbians in India. No doubt many lesbian-attracted people stayed identified as straight as a result.

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[deleted] t1_jahhsz8 wrote

[removed]

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Sulfamide t1_jahmbut wrote

> Your sexuality can change/grow over your lifetime, like any other part of your identity.

Actually its variability with age is very small when measured objectively. Otherwise there would exist favorable outcomes to conversion therapy. Also non-straight sexualities are less common than heterosexuality, as straight people seem to constitute 70 to 83% of the world population

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[deleted] t1_jahu5yl wrote

I agree with what you are saying on all points, but from an evolutionary point of view there are many traits where the fitness advantage isn't as straightforward as "you make more babies". For example - the "gay uncle" hypothesis. Also, google "spandrel".

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BigJoe094 t1_jahn5mb wrote

I'm in no way trying to say it's genetic but if somehow it were in some cases gays wouldn't need to have babies for it to stay in the population. Those "genes" could be present in the siblings as well and passed on. Your genetic material isn't completely lost in a population if you have close kin.

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ThrowawayMcRib t1_jak4tmm wrote

Humans evolved as a tribe/group, rather than as individuals. What does this mean for homosexuality? It means that tribes/groups containing homosexual people survived, meaning, it can still be genetic. Aside from that, it's not uncommon for gay/bisexual people to have children of their own, especially today with things like surrogates or sperm donors.

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