I'm surprised that you are so ready to abandon your common sense in the face of a psychology book (Judith Rich Harris's book specifically, which asserts that how a parent treats a child has almost no influence on how the child turns out). Psychology papers and psychology books misuse and misapply statistics all the time. Surely someone as well educated as you knows this? (Maybe your wife is a psychologist, so you are overly accepting of psychology results?) The basic mistake being made here is to ignore the possibility that a parent has treated different children differently: one kid is shy: a good parent will nudge him into making friends, but avoid forcing him into unstructured situations with many children because that will tend to overwhelm him. I.e., a good parent is part of the so-called "unshared environment": the shy kid's non-shy sister is not treated the way I just described. (There is for example no need to nudge her into making friends.)