Zen, I don't follow. The SD of IQ in the full population is ~15 pts. If we identify a group of pairs of twins whose IQs are correlated with coefficient 0.8, then I think the within-twin SD of IQ is 15*sqrt(1-.8^2) = 15*.6 = 9. For the normal distribution, mean(abs(x)) ~= 0.8 if SD(x) = 1. So mean absolute differences should be about 7 pts.
On the broader point, the "adoptive families are more similar than non-adoptive families" critique of the twin adoption studies seems to me more serious than the "MZ twins get more correlated environments than DZ twins" critique of the MZ-DZ twin studies. The latter I'm sure is true, but seems unlikely to be a big deal, since DZ twins are already getting very correlated environments in most families. Misidentifying MZ/DZ status will likely lead to underestimates of heritability. So is it fair to interpret the MZ-DZ estimates as a lower bound on heritability and the adoption study estimates as an upper bound? What would this be missing?
