You don't need to restrict to demographic cross sections. You can take a set of 30-40 twins (of all demographics) and compare them to a set of N>40 randomly selected twin-like pairs (of all demographics). As long as your twins and twin-like pairs have the same demographic mix, you have an adequate sample.
This sample will average out demographic variation, but the twin-induced correlations are controlled for.
Womb environment can be controlled for by comparing identical to fraternal twins.
It may be the case that most twin studies are performed badly. I'm just pointing out that most of the issues you raise are not a real problem for anyone who understands basic statistics (admittedly, this may exclude most psychologists).
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.
As long as your twins and twin-like pairs have the same demographic mix, you have an adequate sample.
Proof of this statement? There are mathematical definitions of what size of sample is "adequate" for given effect sizes. (What are the effect sizes of some of the differences in IQ we are talking about here, and how do they compare to error of estimation in the most reliable brands of IQ tests?)
It may be the case that most twin studies are performed badly.
That would definitely be an important matter to look at.
This sample will average out demographic variation, but the twin-induced correlations are controlled for.
Womb environment can be controlled for by comparing identical to fraternal twins.
It may be the case that most twin studies are performed badly. I'm just pointing out that most of the issues you raise are not a real problem for anyone who understands basic statistics (admittedly, this may exclude most psychologists).