The most rigorous study yet of the effects of marijuana legalization has identified a disturbing result: College students with access to recreational cannabis on average earn worse grades and fail classes at a higher rate.

Economists Olivier Marie and Ulf Zolitz took advantage of a decision by Maastricht, a city in the Netherlands, to change the rules for “cannabis cafes,” which legally sell recreational marijuana. Because Maastricht is very close to the border of multiple European countries, drug tourism was posing difficulties for the city. To address this, the city barred noncitizens of the Netherlands from buying from the cafes.

This policy change created an intriguing natural experiment at Maastricht University, because students there from neighboring countries suddenly were unable to access legal pot, while students from the Netherlands continued.

The research on more than 4,000 students, published in the Review of Economic Studies, found that those who lost access to legal marijuana showed substantial improvement in grades. Specifically, those banned from cannabis cafes had a more than 5 percent increase in the odds of passing courses. Low performing students benefited even more, which the researchers noted is particularly important because these students are at high risk of dropping out. The researchers attribute their results to the students who were denied legal access to marijuana being less likely to use it and to suffer cognitive impairments as a result.

The Maastricht study examined similar people in the same location — some of them even side by side in the same classrooms — making it easier to isolate the effect of marijuana legalization. Marijuana policy researcher Rosalie Pacula, of RAND Corporation, pointed out that since voters in U.S. states are the ones who approve marijuana legalization, it creates a chicken and egg problem for researchers (i.e. does legalization make people smoke more pot, or do pot smokers tend to vote for legalization?). This methodological problem was resolved in the Maastricht study because the marijuana policy change was imposed without input from those whom it affected.