Identical twins share a womb, and therefore even if you separate them at birth have already been subjected to identical "nurture" for ~9 months.

Let's say you had four, all identical, split into two groups ran in parallel simulations. Two in the same womb then split at birth, the other two split at conception implanted in two separate mothers. Of the two borne to separate mothers, one mother is heavily stressed, chronically exposes the fetus to cortisol, while the other is very stable.

When looking at outcomes between these pairs, how do you account for the difference in nature vs nurture when almost all of our twin studies rely on a shared womb, shared first 9 months of nurture?