And we don't exactly know why yet, though we have a few ideas:
This is the kind of news that school-choice advocates and skeptics alike need to pay attention to: The Economist magazine reports that a team of academic economists found that students who won a lottery in Louisiana to receive vouchers to go to the public or private school of their choice did worse than students who didn't win the lottery.
This outcome flies in the face of the predictions of many economists, who often tout school choice as a way to improve the U.S. educational system while also increasing equality of opportunity. Economists typically assume that people are rational and well-informed, and will make decisions that benefit them. If giving students and their parents more school choice hurts the students academically, then something is seriously wrong with the theory.
The Louisiana finding is just a more extreme version of a fairly typical finding. School-voucher programs rarely actually hurt students, but most produce little if any positive effect...
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But a third possibility is that parents, even when given the chance, simply don’t choose schools that raise their children’s test scores. Parents might be incapable of discerning which schools provide a better education. Or they might simply care about factors other than academic quality. Although evidence shows that parents do care about a school’s overall level of academic performance, that doesn’t necessarily mean that parents care about a school’s ability to boostperformance. They might simply want their children to be around a bunch of smart high-achievers -- but this might not actually help their own kids achieve more.
This highlights a big and fundamental problem with the idea of school vouchers. Parents might decide that they prefer schools that let their kid get away with doing little real work. Grade inflation and other perks might become rife, lowering schools’ real educational power while the government foots the bill. This unpleasant possibility is perfectly consistent with the rational-choice theories preferred by most economists. What parents and students want from schools may simply be different from what we want them to get.