Because it turns out they really shouldn’t. Or at least on a national average it doesn’t really seem to make kids perform better. Really.

That’s according to a new book by Sarah Theule Lubienski and her husband, Christopher A. Lubienski, both professors at the University of Illinois at Urbana-Champaign. The Lubienskis knew that a lot of the higher performance of students in private schools had to do with social class—in general rich people’s kids perform a little better in school no matter where they go—but they wanted to figure out how much of the higher education performance had to do with the school itself.

What they discovered was interesting. As Julie Ryan at the Atlantic puts it:

Studying the National Assessment of Educational Progress and the Early Childhood Longitudinal Study, they have found that, when controlling for demographic factors, public schools are doing a better job academically than private schools. It seems that private school students have higher scores because they come from more affluent backgrounds, not because the schools they attend are better educational institutions.

So why are parents still doing this? Well, as Sarah Theule Lubienski explains:

Why would somebody pay money for a service that is apparently inferior to one they could get for free? It flies in the face of economic logic. But there are other reasons for choosing schools, and we know this from other research about how parents make those decisions. And it’s things like reputations, convenience, safety, the value systems that are represented by schools. Those are all legitimate reasons, but also parents are making choices based on the peer group they are selecting for their students, which does have an impact on a student’s performance.

It’s not really that complicated, though. Parents don’t choose “private school” in some general sense; they choose one particular private school for their particular child. Sure they’re not always making the right decisions but “reputations, convenience, safety, the value systems that are represented by schools and… the peer group they are selecting for their students” matter a lot.

In the real world, that’s a kid’s whole life.

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Daniel Luzer is the news editor at Governing Magazine and former web editor of the Washington Monthly. Find him on Twitter: @Daniel_Luzer