It's known that birth order matters. "The more older brothers a male has from the same mother, the greater the probability he will have a homosexual orientation."[1] Interestingly, this occurs only in right-handed males.
See [2]: "Mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons." There's something going on during pregnancy, and it's starting to be identifiable, but it's not understood yet.
Sexual orientation has a known association with handedness (same-sex-attracted people of either gender have higher odds of being left-handed/ambidextrous than do straight people) so it’s reasonable a priori for a correlation to exist in one sub-population and not the other. Splitting by handedness reduces variance for many sexuality questions — it’s not a sign of p-hacking.
> Sexual orientation has a known association with handedness
That sounds so strange I had to fact-check, and it appears you're right, there does seem to be a measurable association [0].
Perhaps even more interesting, that same article reports: '...the strongest handedness nding for both sexes was a marked tendency for participants who described themselves as ambidextrous also to describe themselves as bisexual'. How weird.
The mechanism is thought to be a maternal immune response to male fetuses, whereby antibodies neutralize male Y-proteins thought to play a role in sexual differentiation during development. This would leave some regions of the brain associated with sexual orientation in the 'female typical' arrangement – or attracted to men. Biochemical evidence for this hypothesis was identified in 2017, finding mothers with a gay son, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein than mothers with heterosexual sons
Are there any studies looking for genetic differences in mothers of gay men? Is it possible for the mothers to have a genetic trait that increases the chance of a gay son, but doesn’t get passed down to that gay son?
See [2]: "Mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons." There's something going on during pregnancy, and it's starting to be identifiable, but it's not understood yet.
[1] https://en.wikipedia.org/wiki/Fraternal_birth_order_and_male...
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5777026/