They found large negative results in both reading and math. Public elementary school students who started at the 50th percentile in math and then used a voucher to transfer to a private school dropped to the 26th percentile in a single year. Results were somewhat better in the second year, but were still well below the starting point.

This is very unusual. When people try to improve education, sometimes they succeed and sometimes they fail. The successes usually register as modest improvements, while the failures generally have no effect at all. It’s rare to see efforts to improve test scores having the opposite result. Martin West, a professor at the Harvard Graduate School of Education, calls the negative effects in Louisiana “as large as any I’ve seen in the literature” — not just compared with other voucher studies, but in the history of American education research.

There’s always the chance that a single study, no matter how well designed, is an outlier. Studies of older voucher programs in Milwaukee and elsewhere have generally produced mixed results, sometimes finding modest improvements in test scores, but only for some subjects and student groups. Until about a year ago, however, few if any studies had shown vouchers causing test scores to decline drastically.

In June, a third voucher study was released by the Thomas B. Fordham Institute, a conservative think tank and proponent of school choice. The study, which was financed by the pro-voucher Walton Family Foundation, focused on a large voucher program in Ohio. “Students who use vouchers to attend private schools have fared worse academically compared to their closely matched peers attending public schools,” the researchers found. Once again, results were worse in math.

Three consecutive reports, each studying one of the largest new state voucher programs, found that vouchers hurt student learning. Researchers and advocates began a spirited debate about what, exactly, was going on.

Mark Dynarski of the Brookings Institution noted that the performance gap between private and public school students had narrowed significantly over time. He argued that the standards, testing and accountability movement, for all its political shortcomings, was effective. The assumed superiority of private schools may no longer hold.

Some voucher supporters observed that many private schools in Louisiana chose not to accept voucher students, and those that did had recently experienced declining enrollment. Perhaps the participating schools were unusually bad and eager for revenue. But this is another way of saying that exposing young children to the vagaries of private-sector competition is inherently risky. The free market often does a terrible job of providing basic services to the poor — see, for instance, the lack of grocery stores and banks in many low-income neighborhoods. This may also hold for education.