In discussing his Selfish Reasons to Have More Kids, Bryan Caplan addresses the old nature-nurture argument:
Admittedly, there’s a sense in which upbringing is all-important: If a baby is raised by wolves, he won’t know any words. (There’s also a sense in which genes are all-important: If you had wolf DNA, you wouldn’t know any words either.) But twin and adoption research focuses on questions that are much more relevant for parents: how your child will turn out if you switch to another parenting style.