My understanding is that separating children from their biological parents has wide-reaching consequences, even if done in a non-traumatic way, and even if they are ultimately raised by a different set of parents. I would imagine the trauma originating from having to be adopted could be a uniquely triggering factor for genetic predisposition in the case of only one of the twins. How would twin studies be able to account for that?

I agree. Also, the prenatal environment (9 months of development!) and circumstances of birth, which both twins share, is not accounted for at all. Or rather, it is accounted for as "heritability" by twin studies, which is plainly wrong.

https://williamjbarry.substack.com/p/the-first-1000-days