So what's the problem? These studies typically assume that the similarity of twins' shared environment is the same as that of regular siblings (highly unlikely) and that adoptive families are as diverse as families generally (in fact, parents that adopt tend to be better off and better educated). When these assumptions are relaxed, environmental factors start to loom larger. In this regard, consider a pair of French adoption studies that controlled for the socioeconomic status of birth and adoptive parents. They found that being raised by high-SES (socioeconomic status) parents led to an IQ boost of between 12 and 16 points - a huge improvement that testifies to the powerful influence that upbringing can have.
A study of twins by psychologist Eric Turkheimer and colleagues that similarly tracked parents' education, occupation, and income yielded especially striking results. Specifically, they found that the "heritability" of IQ - the degree to which IQ variations can be explained by genes - varies dramatically by socioeconomic class. Heritability among high-SES (socioeconomic status) kids was 0.72; in other words, genetic factors accounted for 72 percent of the variations in IQ, while shared environment accounted for only 15 percent. For low-SES kids, on the other hand, the relative influence of genes and environment was inverted: Estimated heritability was only 0.10, while shared environment explained 58 percent of IQ variations.This last bit seems to me the most important. The Turkheimer study suggests that while everyone has a genetically bounded intellectual capacity, that genetic capacity is not inevitable and can be powerfully limited by environmental factors. It is perhaps more simple to think of the example of height. You might have the genes to become 6’2’’ but if you don’t receive the proper nutrition as a child, you will likely never realize your full genetic height capacity. So in the case of low-income kids, where environment accounted for 58 percent of variations, in comparison to just 28 percent of the IQ of their high-income peers, you can see the clear effect of environment disrupting IQ development. Put another way, it doesn’t matter your genetic capacity for intelligence if you never learn the vocabulary or practice and develop the critical reasoning skills.
All in all these questions point to the importance of providing children with every opportunity to realize their full potential and to be careful about the conclusions we draw if they don’t. It also points to why exactly my post at Recess could be considered controversial. To state that students often have disparities in their academic level that align disproportionately with demographic factors such as race and socio-economic status can seem dangerously close to claims by the likes of Richwine and Murray that their is something inherently different about the capacity of certain groups that cause that alignment. And there is a lot of misleading analysis out there that can cause people to misinterpret the causes and implications of the achievement gap.
To my mind, to see the disparities in educational outcomes among demographic groups as anything but the result of a profound and systematic inequality of opportunity and the devastating effects of poverty is dangerously misguided. But is precisely because I believe that we have deprived those most in need of the educational opportunities that they need and deserve for so long that I believe we need to be clear and honest about the effect of those disparities and what they mean for how we teach students. To allow the potentially racist conclusions that might be drawn from a choice to track students by level to prevent us from doing what is best for students would grant too much power to theorists such as Richwine and Murray.