In his arguments Caplan relies heavily on the behavior genetics findings I have discussed previously here and here. These findings show that it is very difficult for parents to shape their kids, and that genes have a larger impact than "shared environment" (i.e., effects from being raised in the same family). One can study these effects by varying genetic relatedness (e.g., identical twins vs fraternal twins) and environment (adoption into different families, variation of family characteristics such as SES, parental education, etc.).
This is from some correspondence (slightly edited) I had with a new father about Caplan's book.
The question is whether you accept the behavior genetics conclusions at the Tiger Mom/Dad extremes. That is, the twin/adoption data covers mostly normal people and probably cannot be extrapolated with confidence to exceptional cases like high IQ families with a strong focus on education and achievement.
I am very committed to helping my kids, although not in the Amy Chua way, and I wonder how well I could achieve that if I had, say, 4 instead of 2 kids. As it is I can think of stuff almost every day that I could have done with them if I wasn't so busy with other things.
I take my kids out and play with them as much as I can. But not just random play. For example, I run races with them and I notice that at this age they can improve their running ability a lot by practicing. They have probably run hundreds (or maybe thousands!) more flat out sprints (say 40 yards) than a typical Taiwanese kid of the same age. (Cities there are very crowded, so it's not easy even to find a place to do something like this.) I can imagine that their self-esteem and ability to do well in school sports might be improved by my willingness to not only spend time with them but to insist that we do something modestly constructive while we are having fun.
I see lots of US dads already teaching their kids how to hit a baseball or do other sports specific things. I've spent a lot of time in sports and athletics, and while genes matter, training also matters, especially at the K-12 level where the threshold for making the team is much lower than in college. Even in football, basic skills like accelerating out of a 3 point stance are things you learn through early repetition and are hard to pick up later in life.
My dad was a professor but not a natural teacher. We had a neighbor who was a math professor and very extroverted and passionate about his subject. His kids really didn't like to discuss math with him but I loved it and it was one of the best experiences of my life. I could offer that kind of thing to my kids (in many subjects), assuming the dynamics are right. But I certainly couldn't if I were too busy or had too many kids.
Early success in anything (sports, math, etc.) can be self-reinforcing and have non-linear effects down the line. I realize the behavior genetics data suggests that *averaged over large groups* such effects are small, but the studies are still crude and could easily miss some relatively significant strategies that you or I might take advantage of. Are you willing to take the risk of forgoing such positive impacts you might have on your kids?
I like Caplan in general but I think he's a little too hardcore libertarian and also a bit robotic (autistic economist) and simple-minded in his thinking.