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

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.

That isn't sufficient, unfortunately. A third-party's treatment of an individual depends a lot on the appearance of the individual. (i.e., beautiful people are treated differently than ugly people.) That is, one's appearance can have a causal effect on one's 'environment.'

Thus, even if raised separately, the 'environment' to which each identical twin is exposed will be more similar than the 'environments' of two non-identical people.

Simple example....

Adam and Bob are raised separately. Yet, both develop to be confident and strong leaders. Is this leadership quality 'genetic'? Or, is it because Adam and Bob are each 6'5" and handsome?

It would be very difficult indeed to statistically tease out these things.

Well if you take a single instance yes, it's hard to claim. But the effect is born out by thousands upon thousands of instances.

Thousands?

That is incorrect.

The number of identical twins that have been raised separately (and studied) is tiny.

With a sample size that small, it is difficult to get statistically significant results. Obviously. I was just pointing out that one of the issues with which a researcher must grapple is the fact that not only is one shaped by one's environment, but one's environment is shaped -- to some degree -- by one's appearance... a tricky issue when dealing with two subjects who are identical in every outward respect.

Hundreds of such twins have been studied, and hundreds more of fraternal twins. Combined with data from thousands of other twins which have contact (UK twin registry alone tracks 12,000 twins), this enables significant comparative analysis. You're seriously underestimating the amount of data available.

A single twin-study cited in this article [0] surveyed 8,280 twins.

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4653279/

That is incorrect. The sample consisted of 8,280 monozygotic (identical) and dizygotic (fraternal) twins. Twins separated at birth were not involved, if I'm reading the study correctly. In any case, it's nothing worth arguing about. (I will say, though, that if there are 8,280 pairs of identical twins in Denmark who were raised separately, there is something very strange happening in Denmark.)