Following up on the "No Rich Kid Left Behind" discussion, I wanted to get some (ideally dispassionate) discussion of the twin and adoption study based estimates of IQ heritability. One summary of them is on p. 85 of this article:
http://www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdfThis is not my field, but let me start with a couple thoughts:
1. With the twin adoption studies, the result seems to be that monozygotic twins who are raised together have IQ that is almost as correlated as MZ twins who are raised apart (presumably because both were adopted by different parents). So same DNA and same home yield almost as high a correlation as same DNA and different home.
It seems like one, and perhaps the main, critique of the external validity of estimates of the relative role of heredity and environment to a broader population (e.g., the entire U.S.) is that the variation in environments among families who adopt children may be smaller than the variation in environments across all families. Which seems plausible, given that adopting families are screened pretty heavily.
2. There are also studies comparing correlations among mono and di-zygotic twins. So same DNA and same home compared with half of the same DNA and same home. The MZ twins have higher correlations than the DZ twins, and if one assumes a functional form for the relationship between DNA similarity and IQ similarity, one can back out the role of heredity.
It seems like the grounds for criticizing this approach would focus on either the functional form assumption or on the some "MZ twins get more correlated environments than DZ twins" (dressed alike, mixed up at school, etc.) type of story.
Is that a fair summary of the evidence and the grounds for criticism? What am I missing?
