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.

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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 their grades. Specifically, those banned from cannabis cafes had a more than 5 percent increase in their odds of passing their 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 (e.g., in concentration and memory) as a result.

Other studies have tried to estimate the impact of marijuana legalization by studying those U.S. states that legalized medicinal or recreational marijuana. But marijuana policy researcher Rosalie Pacula of RAND Corporation noted that the Maastricht study provide evidence that “is much better than anything done so far in the United States.”

States differ in countless ways that are hard for researchers to adjust for in their data analysis, but 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. Also, Pacula 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.

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Although this is the strongest study to date on how people are affected by marijuana legalization, no research can ultimately tell us whether legalization is a good or bad decision: That’s a political question and not a scientific one. But what the Maastricht study can do is provides highly credible evidence that marijuana legalization will lead to decreased academic success — perhaps particularly so for struggling students — and that is a concern that both proponents and opponents of legalization should keep in mind.