There was some noise at ASHG about someone (Tuck Ngun) claiming that he’d found evidence supporting that idea (published a couple of years ago) that homosexuality might be caused by epigenetic leakage. Which I doubted. Everyone seems to have disbelieved Ngun’s claim, mainly because the study had no statistical power. He looked at discordant identical twins (which I think could be very informative if done right, although low N) and tried to see if some subset of the epigenetic state of thousands of loci might predict homosexuality. Multiple hypothesis testing, uncorrected, bad.
Ngun’s work is probably worthless. It doesn’t disprove the leaky-epigenetics theory, it’s just no evidence at all.
What you want to do is look for is a difference in the brain between those discordant identical twins. Sure, geneticists want to do genetics, but that’s not the way to go in this case. Look at neuron subpopulations, something like narcolepsy. More likely in the hypothalamus than the frontal lobes. While you’re at it, you’ll likely find out quite a bit about the neuroanatomical basis of male sexual behavior.
Of course, no one could get funding for this, and if someone did succeed, he would become an unemployable pariah… Not every researcher is that curiosity-driven.
Here is one scenario for the discovery of the cause that actually could happen – although there are others.. Consider the case of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), a synthetic street drug that, by accident, zapped the neuron subpopulation involved in Parkinson’s disease. Instant Parkinson’s, in young people. We learned a lot about the disease from those brave junkies.
Posit a new synthetic heroin, another China White, that by sheer accident zaps the particular neuron subpopulation that triggers/implements male sexual attraction (and nothing else) . Suddenly, Bob’s not just your uncle any more. With that kind of clue, we’d get to the bottom of things in no time. There would be no fundamental difficulty in determining the naturally-occurring causes of that particular neural ablation, such as infection, toxins, or in rare cases mutation (all of which we have seen with Parkinson’s disease).
Then we would understand.