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How do we know that school makes kids smarter? In this post we get the answer from cognitive scientist Daniel Willingham, professor and director of graduate studies in psychology at the University of Virginia.

Willingham is the author of “Why Don’t Students Like School?” His newly published book is “When Can You Trust The Experts? How to tell good science from bad in education.” This post appeared on his Science and Education blog.

 By Daniel Willingham

 Does going to school actually make you smarter (at least, as measured by standard cognitive ability tests)? Answering this question is harder than it would first appear because schooling is confounded with many other variables.

Yes, kids cognitive abilities improve the longer they have been in school, but it’s certainly plausible that better cognitive abilities make it more probable that you’ll stay in school longer. And schooling is also confounded with age – – kids who have been in school longer are also older and therefore have had more life experiences, and perhaps those have prompted the increases in intelligence.

One strategy is to test everyone on their birthday. That way, everyone should have had the same opportunity for life experiences, but the student with a birthday in May has had four months more schooling than the child with the January birthday.

That solves some problems, but it entails other assumptions. For example, older children within a grade might experience fewer social problems, for example.

A new paper (Carlsson, Dahl, & Rooth, 2012) takes a different approach to addressing this difficult problem.

The authors capitalized on the fact that every male in Sweden must take a battery of cognitive tests for military service. The testing occurs near his 18th birthday, but the precise date is assigned more or less randomly (constrained by logistical factors for the military testers). So the authors could statistically control for the time-of-year effect of the birthday and in addition investigate the effects of just a few days more (or less) of schooling. The researchers were able to access a database of all the males tested between 1980 and 1994.

Students took four tests. Two tests (one of word meanings and one of reading technical prose) tap crystallized intelligence (i.e. what you know). Two others (spatial reasoning, and logic) tap fluid intelligence (i.e., reasoning that is not dependent on particular knowledge).

The authors found that older students scored better on all four tests — no surprise there. What about students who were the same age, but who, because of the vagaries of the testing, happened to have had a few days more or fewer of schooling?

More schooling was associated with better performance, but only on the crystallized intelligence tests: an extra 10 days in school improved by about 1% of a standard deviation. Extra non-school days had no effect.

There was no measurable effect of school days on the fluid intelligence tests. This result might mean that these cognitive skills are unaffected by schooling, but it might also mean that the “dose” of schooling was too small to have an impact, or that the measure was insensitive to the effect that schooling has on fluid intelligence.