I would also ask myself if such ideas as are in the book have become widely accepted, and if not, why.

So, given that it's a parenting book that argues "heredity dominates, so don't worry too much about how you parent so long as you don't do any huge damage," I would expect it to undersell relative to parenting books that take the opposite tack- because it appeals to lazier parents who are less likely to buy books. (As far as I can tell, Caplan is reading the literature correctly.)