Adoption and twin studies are very important for determining the impact of genes versus environment in the modern world (and hence the likely impact of various interventions). Other types of studies tend to show larger effects for some types of latter interventions, but these studies are seen as dubious, as they may fail to adjust for various confounders (eg families with more books also have more educated parents).
But adoption studies have their own confounders. The biggest ones are that in many countries, the genetic parents have a role in choosing the adoptive parents. Add the fact that adoptive parents also choose their adopted children, and that various social workers and others have great influence over the process, this would seem a huge confounder interfering with the results.
This paper also mentions a confounder for some types of twin studies, such as identical versus fraternal twins. They point out that identical twins in the same family will typically get a much greater shared environment than fraternal twins, because people will treat them much more similarly. This is to my mind quite a weak point, but it is an issue nonetheless.
Since I have very little expertise in these areas, I was just wondering if anyone knew about efforts to estimate the impact of these confounders and adjust for them.
The biggest problem is that twins raised apart are actually pretty rare, so almost any study of them goes to desperate lengths to just get enough of them for the study. This often involves fudging what they're willing to accept as "raised apart" to a degree no unbiased observer would be comfortable with, just to get sufficient numbers.
Yes, that's a real problem, but Stuart didn't actually mention studies of identical twins raised apart, probably because they are rare. He mentioned two types, adoption studies and twin studies, not (twin adoption) studies. Examples: comparing siblings raised apart for the one, and comparing identical twins to their siblings for the other.