ADHD meds like Ritalin, Adderall, Concerta, and Vyvanse have been called "smart pills" for their ability to bestow superhuman powers of concentration. In the U.S. especially, where about 11 percent of schoolchildren have an ADHD diagnosis, parents and teachers embrace the drugs as a way to get kids to sit still and pay attention.

Which makes it all the stranger that there has never been proof that ADHD meds make you smarter or more likely to succeed in school. And a new study that looks at the rise of Ritalin use in Quebec suggests exactly the opposite.

The NBER working paper, from Princeton economist Janet Currie and colleagues, charts the fascinating consequences of a 1997 law that made prescription drug insurance mandatory for all of Quebec—but nowhere else in Canada. In the decade after the law kicked in, the number of children on ADHD medication tripled. By 2010, the 23 percent of Canadians living in the province accounted for 44 percent of the country's prescriptions for the stimulant drugs.

This set up a neat natural experiment. Because they had more access to drug insurance, children with ADHD symptoms in Quebec were more likely to be on medication than their counterparts in other provinces. But when researchers compared educational records, test scores, teacher evaluations, they didn't see any improvement. In fact, the ADHD Quebec children seemed to be worse off in several ways. In the short term, they had lower math scores and were more likely to repeat a grade. In the long term, the boys continued to struggle with math and were more likely to drop out, while the girls were more likely to have been diagnosed with a psychological disorder.

"I thought there might be some disconnect between the short run and the long run—maybe in the long run the beneficial effects fade," Currie told me on the phone. "I didn't expect to find no beneficial effects at all."

It's hard to square these results with the mountain of evidence that ADHD meds improve attention and performance on classroom tasks. But those were all relatively brief studies, while Currie’s is one of the first to look at years of school records. Besides, Ritalin’s medicated hyper-focus can be as much a curse as it is a gift. Though the pills make you concentrate, they can’t control what you concentrate on. Anecdotes abound of Adderall-addled college kids helplessly organizing their closets at 2 a.m. instead of working on that last-minute paper.

Currie and her co-authors speculate that something similar might be happening in the classroom. Ritalin can hush a hyperactive kid, but it can just as well leave him in a trance sorting M&Ms in a corner. “It could be these kids are out-of-sight, out-of-mind, and they don’t get the additional help they need,” Currie said.

A more sinister explanation is that the meds actually worsen ADHD symptoms by causing long-term changes in the brain. Scientists think that ADHD is caused by low levels of a chemical called dopamine. Drugs like Ritalin act by slowing down how fast the body removes dopamine from the brain. But a study from May found that after twelve months of taking Ritalin, subjects' bodies were working harder to clear the dopamine, partly counteracting the effects of the drug.

The lesson from Quebec seems to be twofold. First, moral hazard abounds, especially for a drug with a such a hazy and permissive diagnosis process. In the course of their research, Currie and her colleagues compared Ritalin prescriptions to asthma drug prescriptions. Only Ritalin prescriptions in Quebec shot up after insurance was made mandatory. The rate of asthma prescriptions remained the same as in the rest of Canada. It’s becoming clear that more Quebec kids got more ADHD meds than they should have—just because they could—and it did not do them any good.

The other lesson is that Ritalin skeptics have been right to question why so many children are given the drug when the problem is not their health, but their behavior. Is hyperactivity and lack of focus a disease, or just the way that some minds function? It may be that the ideal of a quiet and organized classroom runs counter to the reality of how children are wired. And medicating such a world into existence, especially if the drugs don’t seem to work long-term, shortchanges kids of the skills and practice they need to stay focused on their own.

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