Not quite. Identical and fraternal twins have different "intrauterine competition" issues, which are generally worse for identical: identical twins, in the original splitting, may get clumps of cells different in various ways (but fraternal twins, stemming from different eggs, get 100% of their respective egg); identical twins usually share the same placenta which causes a lot of problems & competition, while fraternals get separate placentas; and more obscurely, identicals may share an amniotic sac.
(Every time I read in detail about pregnancy, I can't help but think it's a really freaky and complex process.)
Of course, there are other biases. For example, identical twins aren't actually perfectly genetically identical, as they come with various new mutations and copy-errors and whatnot, so if you assume they are 100% the same, that may bias the estimate downward just like the 'identical womb environment as with fraternal and singles' assumption does, and there's measurement error in IQ scores, which generically leads to underestimates of anything to do with IQ. But there are other biases upward, and I don't know if there's any consensus on what the net is. People who hate hate hate the idea of IQ and there being any genetics there of, such as Shalizi, will certainly bend your ear about problems with the assumptions, but are they engaged in motivated cognition and making mountains of methodological moleholes? Dunno. I'm happy to wait for the GWAS studies. We'll see which emperor has no clothes.
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.