Studies of twins consistently show a heritability of about 85% for adult IQ and executive function (focus vs. ADHD). The postnatal environment has little effect unless it is severely neglectful or deprived.

Edit: added "postnatal".

Do these twins grow up in comparable environments? Another thing to consider: gene expression and brain development is kicked off in the womb, and twins most often share the same mother. I'm not sure your argument applies here.

That's the point of twin studies: comparing genetically identical subjects who grew up in different environments.

The prenatal influence cannot, practically, be investigated at this point in history.

The prenatal influence has been directly tested. They compare pairs of non-identical siblings, some who are twins and some who are not. The same pregnancy pairs are about as similar to each other as the other pairs. So the prenatal environment either has little effect, or it has a significant but highly nonlinear effect. (Once again, this assumes no extreme hardship.)

Interesting, thanks.

Some of the studies use twins raised in the same household, some use twins adopted by different families. They get very similar results.

Although you believe this is the case, a family should be allowed to select what they view as the "best" donor possible