My Cato Unbound exchange with my critics is over, but something’s still bugging me.  In her critique, Betsey Stevenson suggests that kids could easily be inferior goods:

Caplan is entirely focused on the substitution effect: having kids becomes cheaper relative to buying TVs. So he says buy more kids, and fewer TVs. But what about the income effect? As people become richer, they tend to “buy” fewer children, not more. So there’s an offsetting income effect.

It’s a common view, but is it true?  I decided to check with the General Social Survey.  On the surface, there’s a marginal case that Betsey’s right.  Results from a regression of number of children on log income and a year trend:



The main problem with regressing anything on income, though, is that income, education, and IQ are all highly intercorrelated.  Like look-alike siblings, income, education, and IQ endure endless mistaken identity issues.  So what happens if we run a horse race?  Results from a regression of number of children on log income, years of education, a ten-question IQ test (WORDSUM), and a year trend:


IQ turns out to be a red herring, but look: log income and education, though highly correlated, push in opposite directions – a classic “masking effect.”  Controlling for education, higher income predicts higher fertility.  (Adding age as a control variable slightly reduces the magnitude, but not too much).  The simplest story is that the elite values typical of the well-educated depress fertility – but regardless of your values, the higher your income, the more kids you want.

I’ve heard several people claim that kids are a normal good for men, but an inferior good for women.  Does this hold up?  No.  Results for men alone:


Results for women alone:


It is true that education depresses female fertility more, and income boosts female fertility less.  But it looks like kids are normal goods for both genders.  If you’re prone to futurist speculation, trying re-imagining Idiocracy.  The twist: in the real world, the most fertile people aren’t those with low IQ; they’re people who counter-stereotypically combine low education with high income.  Plumbers shall inherit the earth!