The problem with studies of adopted twins, as critics have noted, is that they usually involve children of fairly comfortable socioeconomic means. Even if the siblings are raised apart, they’re unlikely to be at the poorest end of society, where the lack of good nutrition and a stable home life may be factors in their upbringing. They tend to go to fairly good schools. There’s a risk, then, that these studies underestimate the role of environment across the true range of how people live.
