I haven't read the book, but I've been reading Caplan's blog for years, so I think I'm sufficiently familiar with his positions to comment.

It seems to me that the elephant in the room here are the peer effects. I don't think even the strongest hereditarian theories would imply that it doesn't matter for your kids' future outcomes if they socialize with peers who display low-class or antisocial habits and behaviors.

Now, if you live in North America, making sure your kids are isolated from low-class kids is extremely expensive. If nothing else, you must be able to afford a house in a nice neighborhood. Unless you are extravagantly wealthy, or perhaps enjoy some very unusual combination of an upper-middle class income and high job security, this means getting into an enormous debt, which you won't be able to pay off for decades, and living a stressful and anxious existence on the edge of solvency, in which a fit of bad luck can easily send you into ruin. And this latter possibility doesn't mean just falling back to a more frugal but still respectable lifestyle -- it means being thrown, together with your kids, right into the dreaded underclass in which all sorts of frightful social pathologies are rampant. It's like precariously holding onto a rope above a pond full of crocodiles. Needless to say, the marginal expense of raising each additional kid will make your situation only more precarious!

Of course, this may be nonobvious to a professor who proudly admits to living in a bubble (and who is presumably not rich, but does enjoy the extremely unusual position of having an upper middle class income and full job security). However, while Caplan is certainly right that people should relax and stop the ridiculous helicopter parenting, he seems to be oblivious to the problem of ensuring quality peers for your kids.