I think we need to be careful with the evidence from adoption and twin studies. Such studies rely on the assumption that twins adopted into different homes experience different environments, but in fact twins who are reared apart are often raised in quite similar homes. Another confounding factor is that the influence of genes seems to be stronger in more advantaged settings than it is in more disadvantaged settings. Think of corn planted in fertile, well-watered soil; the main thing that will matter in that context is genetic variation in seeds. Because adopted children more often grow up in advantaged homes, studies of adopted children will therefore tend to overstate the influence of genetics.