Yes, I generally agree that it would be theoretically possible to do a separated at birth identical twin study. In practice, the requirements are so stringent t

Yes, I generally agree that it would be theoretically possible to do a separated at birth identical twin study. In practice, the requirements are so stringent that they've never (to my knowledge) been met. The setup you describe could indeed separate genetic/genetic+environment feedback component from the purely environmental component. Albeit with caveats: - Identical twins given up for adoption and then separated at birth are probably not drawn at random from the population. - You can't draw any conclusions at all about which genetic variance is causing the issue, unless you restrict the demographic cross sections. So it can't possibly give you evidence that one race has a higher average IQ, for example. - The conclusions you draw may be much more about environmental feedback than genetics - maybe with a different environment, the differences will go away. For example, maybe you've found genes that code for susceptibility to iodine deficiency, and your society hasn't discovered iodized salt yet. In that case it's hard to say that you've determined the degree to which intelligence is "genetic" except in the most general sense. In practice, locating identical twins separated at birth is hard. Finding a representative sample of them across demographic mix is harder. Doing it again for fraternal twins to control for womb environment makes it yet more work. I'm not sure how you control for the fact that the identical twins who are given up and then separated at birth are probably a different population from random adoptees, but I don't doubt with sufficient cleverness it could be done. The problem is that real twin studies almost always fail to control all these factors, and failing to control even one factor dooms your entire experiment.