[tomrod]

I'm not sure I follow the causality argument here.

A twin study should normalize everything genetically. Ergo epigenetic expression and environment are the only two causal levers. It sounds like you're arguing that parental involvement has a causal impact, which is the point of the studies to a degree.

Perhaps my original question was unclear: is there something the twins do or have done to them that cause a latent selection bias before adoption, thus invalidating the twin studies approach as a whole?

[aidenn0]

We are questioning how much the parenting has an affect on the child. The parents are self-selected and so are more likely to be similar to each other.

[tomrod]

Agreed. That was my point. Not all twins under adoption are adopted, meaning there is nothing particularly invalidating about the twin study framework.