“Far from leaning against economic inequality, U.S. schools make it worse.” This sentiment, from a recent Clive Crook op-ed, expresses a view that’s commonplace on both the left and the right, and among both proponents and opponents of school reform.

It’s wrong. Americans do leave the schooling system more unequal in cognitive and noncognitive skills than when they enter it. Yet that inequality is less — probably much less — than it would be in the absence of schools. Schools don’t increase inequality; they just don’t do enough to overcome the inequality produced throughout childhood by differences in families, neighborhoods, peers, and other influences.

How do we know that? First, children are vastly unequal in ability when they enter the school system at age five or six. This is due partly to genetics and partly to environmental differences.

Second, we have evidence from the natural experiment that is summer vacation. During those three months out of school, the cognitive skills of children in lower socioeconomic status (SES) households tend to stall or actually regress. Kids in high-SES households fare much better during the summer, as they’re more likely to spend it engaged in stimulating activities. In his book Intelligence and How to Get It, cognitive psychologist Richard Nisbett concludes that “much, if not most, of the gap in academic achievement between lower- and higher-SES children, in fact, is due to the greater summer slump for lower-SES children” (p. 40).

Without schools this pattern would be magnified, and the gap in cognitive and noncognitive abilities at age 18 almost certainly would be much greater than it now is.

This by no means implies our educational system is doing fine. It could and should do much better at helping children from disadvantaged environments. But saying it currently makes things worse suggests the situation is hopeless. Instead of promoting reform, that undercuts it.