There's not much known because it's almost impossible to research, surely? You'd have to have one identical twin developing naturally, and the other brought to term inside a surrogate mother.

This probably doesn't happen very often. (TBH I don't know if it's even possible.)

> That still wouldn't separate genetics vs. common pre-birth environmental influences (e.g. mother's hormones and nutrition).

Sure it would, if you study enough of them.