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.
This is a standard criticism of the twin studies. For example, Shalizi http://vserver1.cscs.lsa.umich.edu/~crshalizi/weblog/520.html
I will quote from an old paper by Bronfenbrenner [3, pp. 159--160], because it's handy and it makes the point:
The importance of degree of environmental variation in influencing the correlation between identical twins reared apart, and hence the estimate of heritability based on this statistic, is revealed by the following examples.
a. Among 35 pairs of separated twins for whom information was available about the community in which they lived, the correlation in Binet IQ for those raised in the same town was .83; for those brought up in different towns, the figure was .67.
b. In another sample of 38 separated twins, tested with a combination of verbal and non-verbal intelligence scales, the correlation for those attending the same school in the same town was .87; for those attending schools in different towns, the coefficient was .66. In the same sample, separated twins raised by relatives showed a correlation of .82; for those brought up by unrelated persons, the coefficient was .63.
c. When the communities in the preceding sample were classified as similar vs. dissimilar on the basis of size and economic base (e.g. mining vs. agricultural), the correlation for separated twins living in similar communities was .86; for those residing in dissimilar localities the coefficient was .26.
d. In the Newman, Holzinger, and Freeman study, ratings are reported of the degree of similarity between the environments into which the twins were separated. When these ratings were divided at the median, the twins reared in the more similar environments showed a correlation of .91 between their IQ's; for those brought up in less similar environments, the coefficient was .42.
(Let us pause a moment to contemplate the sense in which identical twins, growing up in the same town and attending the same school, are "raised apart".)
One confounder for twins is that they were in resource competition when they were in their mom's womb. This environmental effect probably causes twins to appear more different and results in us, when using them to study genetic influence, underestimating the role of genetics.
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.
It seems to me that there wouldn't be any selection pressure for identical twins to compete for resources (if someone has the same genome as you, you don't acquire any fitness advantage from competing with them), but I suppose there may be competitive instincts that are selected for in the case of fraternal twins for which there is no shutoff switch in the case of identical twins.
Why would this make them more different? Do you think one twin wins a resource competition and the other loses? Why do you think that?
No, resource competition is an environmental influence that twins share and it makes them similar to each other and different from single births. For example, it suppresses IQ. The most extreme similarity is that it makes them more likely to miscarry, especially boys, but this confounds other measurements.
Why would this make them more different? Do you think one twin wins a resource competition and the other loses? Why do you think that?
I don't understand your questions. Why would it not make them more different? The competition introduces another source of variability: what fraction of the resources a particular fetus gets. A singleton has no such randomness, since it just gets 100%, there's no competition.
And there's also the problem that you're sampling the environment of people who adopt kids. Conclusions drawn from twin studies don't necessarily hold outside similar families - though it very likely holds for mine :P
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.
One way to assess the confounders' impact is to estimate heritability using a method that doesn't rely on making assumptions about adoptees or twins, and see whether it gives higher/lower results. Here's a paper that did so to estimate height's heritability in a sample of 3,375 sibling pairs:
I'd personally delete the first two words from that paper's title, but it nonetheless avoided whatever issues there might be with using adoptees & twins, getting a heritability estimate of 80% (with a 95% confidence interval of 46% to 85%), broadly comparable to those from older twin studies.
Thanks! I still think they're overconfident at having solved the problem, but it's a useful piece of the puzzle.
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.