Test scores and household income

Greg Mankiw called this graph “the least surprising correlation of all time”:

(SAT scores by family income)

I agree that it’s unsurprising, but for different reasons. Certainly, this correlation shouldn’t imply direct causation, as Mankiw states:

Suppose we were to graph average SAT scores by the number of bathrooms a student has in his or her family home. That curve would also likely slope upward. (After all, people with more money buy larger homes with more bathrooms.) But it would be a mistake to conclude that installing an extra toilet raises yours kids’ SAT scores.

Agreed, but I don’t agree with this:

The key omitted variable here is parents’ IQ. Smart parents make more money and pass those good genes on to their offspring.

While I’ll admit that parents’ IQ would show some correlation with test scores, I don’t think that this is the “key variable”. First of all, the SAT does not measure IQ, it attempts to measure language and math skills. I’d expect there to be a better correlation with some measure of the parents’ educational achievements.

August 14th’s must-listen episode of This American Life touched on this subject in the first act. Childrens’ academic success (and success in general) tracks very well with how much their parents talk with them when they’re very young and how much they read to/with them. Parents who read on their own are more likely to do these things.

Additionally, there’s plenty of training that goes into doing well on the SAT. More affluent parents can afford to pay for tutoring for their children and, more importantly, move to a town with a better school system. If standardized test scores were just measures of innate ability, then it would be absurd to measure the performance of a school system based upon them, and you could just test a child once. (Not saying that they’re a great way to measure, but I think that there are other problems with that.)

Growing up in Avon (suburban CT), I saw plenty of very wealthy homes produce terrible students, but the student body as a whole did well largely because we had a good school system, involved parents, and a culture that said it was OK to be smart.

So, yeah, I’m in the ‘nurture’ camp on this one.

Tags: ,

One Response to “Test scores and household income”

  1. […] Mankiw, whose post about test scores and household income elicited my response, seems to be bewildered about why people are pushing back against his post. It’s really too […]

Leave a Reply