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.