In one study, Eric Turkheimer and colleagues studied 320 pairs of 7-year-old twins who were raised in extreme poverty. Among the poorest, the shared environment accounted for most of the differences in IQ (60%), and the genes accounted for very little; consequently, in this study, the heritability of IQ was reported to be close to zero! Among the richest, however, the heritability of IQ approached what Bouchard found: variations in the genes accounted for most of the differences in IQ scores, and the shared environment accounted for very little of the variance. This study points to the fact that estimates of heritability depend on the sample that is studied, and the environment of that sample.It would seem that these studies of twins raised largely in upper-middle class homes (US adoptions have almost exclusively been to well-off, two parent homes) may be poisoned by the uniform environment of employed parents and decent schools. As Turkheimer's along with various French studies (described here) have discovered, growing up in a poor environment has a marked effect on outcomes.
Straight Talk about Twin Studies, Genes, and Parenting: What Makes Us Who We Are, Psychology Today
The appeal of the Minnesota study can be the same as that of eugenicist works like The Bell Curve: they comfort us with seeming proof that our comfortable lives are if not self-made, at least predestined. Our responsibility for reaching out to others who have not shared our luck - say through more equitable school funding - can safely be ignored.