Why is there no discussion of the Flynn Effect? If environment isn't a factor in IQ, the Flynn Effect wouldn't exist. Also, one thing most environmental twin studies fail to account for is that identical twins share possibly the most important environment: the womb. They are finally linking low Vitamin D exposure in utero to lower IQ and mental illnesses. (The evidence of correlation has been around for a long time.) There is another problem with identical twin studies: the kids, whether they are adopted by wealthy or middle class parents, tend to be adopted by parents who are highly motivated and trained. Also, there is a big difference between a middle class environment and a poor environment, but between a middle class and upper class environment, the differences may seem large but be inconsequential. The important things--safety, access to books--might be there, even if the fancy bells and whistles--expensive summer camps and trips abroad, might not.