Jay Greene has an excellent piece in the Wall Street Journal this week revealing that the teacher workforce has grown dramatically over the past forty years—and at enormous cost—without improving student achievement by the end of high school. And he rightly disparages President Obama for arguing that even more teachers would somehow do the trick. Even better, Greene notes that American education will not reverse its productivity collapse and become efficient until we allow it to benefit from the freedoms and incentives of the marketplace.

But then Jay cites Governor Romney’s goal of “voucherizing federal education funds so that parents can take those resources and use them to send their children to schools of their choice,” and he does so with apparent approbation. Even ignoring the fact that the Constitution does not empower Congress to run education programs, this is a very dangerous idea.

There has been no civilization in the history of humanity in which governments have paid for private schooling without ultimately controlling what was taught and who could teach, erecting barriers to entry and thereby crippling market forces.

For that reason, I recommended against a federal voucher program under the Bush administration. Since then, additional evidence has come to light. When I studied the regulatory impact of U.S. private school choice programs last year I found that even the small existing U.S. voucher programs do indeed impose a heavy and very statistically significant additional burden of regulation on participating private schools.

Perhaps a way will be found to enact and maintain minimally regulated voucher programs in the coming years. Until that time comes, it would be the height of folly to introduce a federal voucher program whose regulations would suffocate educational freedom from coast to coast.

In my statistical study of choice program regulation, I found that K‑12 tax credit programs do not impose a statistically significant extra burden of regulation on private schools. But even a national K‑12 tax credit program would be far too dangerous. By leaving education policy to the states and the people, we can see which programs flourish and which become sclerotic. We must encourage and learn from that policy diversity, not squelch it with federal programs or mandates.