In the comments, Gary asks the $10,000 question:

Bryan, I’m mostly on board with Judith Harris’s hypothesis, but one
thing bothers me: why do parents believe so strongly that they can
influence their children? Perhaps parents’ intense efforts at
influencing their children has some informational value about the
parental profitability of such behavior.

This is a challenge for me, too, because my previous book argues that people tend to make the biggest mistakes in areas like politics and religion where the private cost of error is very low.  When parents falsely believe that they have to break their backs to raise decent human beings, though, the private cost is quite high.  Ever looked into the eyes of an exhausted parent at Tae Kwon Do?  The horror!  The horrror!

So how can billions of parents be so wrong about something so privately important? 

Here’s my best answer: The nature-nurture question is intrinsically hard.  Until twin and adoption studies came along, there was really no way to even start to resolve it.  What’s more, with the benefit of twin and adoption studies we have discovered that intrinsic difficulty is only the first stumbling block.  For the nature-nurture question first-hand observation actually turns out to be directly misleading.

How so?  One of the big lessons of twin and adoption studies is that the short-run effects of parenting are much bigger than the long-run effects.  So when a parent nags a kid and sees immediate improvement, his first-hand observation confirms that nagging works.  It’s very tempting to infer that the difference between an average kid and a great kid is several thousand hours of nagging. 

What twin and adoption studies have taught us, though, is that nagging isn’t cumulative.  It’s not like trying to hold back the ocean by building a sea wall brick by brick until it’s high enough to get the job done.  It’s more like building a sea wall out of sand – you have to keep building just to stay in place.  And once your kids grow up and start making their own decisions, the tide comes in whether you like it or not.  Or as I’ve put it before:

Instead of thinking of kids as lumps of clay that parents “mold,” we should think of kids as plastic that flexes in response to pressure – and springs back to its original shape once the pressure goes away.

To repeat myself, though, the glass is half-full.  You have little effect on your kid’s long-run prospects, but most kids’ long-run prospects are still bright.  If you’re the kind of parent who reads econ blogs, your kids’ prospects are probably very bright indeed, because they’re going to painlessly inherit your brains, charm, good looks, and modesty.