Smoking Pot Interferes With Math Skills, Study Finds Researchers studying the effects of marijuana faced an obstacle: they couldn't create an exact control group. But a change in drug laws in the Netherlands offered a perfect laboratory.

Smoking Pot Interferes With Math Skills, Study Finds

Smoking Pot Interferes With Math Skills, Study Finds

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Researchers studying the effects of marijuana faced an obstacle: they couldn't create an exact control group. But a change in drug laws in the Netherlands offered a perfect laboratory.

ARI SHAPIRO, HOST:

It can be difficult to study the effects of legalizing marijuana. And now, as many states are making marijuana more available, there's new research from - surprise - the Netherlands that might address an important question. NPR's social science correspondent Shankar Vedantam is here to explain. Welcome.

SHANKAR VEDANTAM, BYLINE: Hi, Ari.

SHAPIRO: Tell us about this Dutch study.

VEDANTAM: Well, the Dutch study was addressing a perennial problem that researchers have in studying marijuana laws, Ari. From a research perspective, what you really want to study the effect of a law is to have the law selectively apply to only some people. So if half the people in a city were given access to pot while the other half were not, you could compare the two groups and study the effects that marijuana has. Now, in most places, you don't have that kind of control group. Either all adults have legal access or no adults have access. The new Dutch study found a way out of the problem. It utilized a natural experiment in the city of Maastricht.

SHAPIRO: And what exactly did they do? How did they get some people who used marijuana and some who couldn't?

VEDANTAM: So here's what happened. Recreational pot has been legal in the Netherlands for many years, and since Maastricht is close to the border with Belgium, Germany, France and Luxembourg, lots of tourists come over the border to buy pot in Maastricht. In October, 2011, local authorities felt that tourists from France and Luxembourg were creating a nuisance, so they passed a local ordinance that said pot was legally available only to adults who had Dutch, German or Belgian forms of ID.

SHAPIRO: Oh.

VEDANTAM: So now you have some people who could get pot legally and some who couldn't, and now we have our control group. So an economist, Olivier Marie, decided to study the effect of the policy on students at Maastricht University, and he examined the test scores of more than 4,300 students before and after this partial ban went into effect.

SHAPIRO: All right. So big finale - what impact did smoking pot have on these students?

VEDANTAM: Well, the students who belonged to the groups that were banned from buying marijuana started to do better in their courses. Their test scores...

SHAPIRO: Oh, it's just what our parents always told us.

VEDANTAM: (Laughter) Their test scores improved by about 5 percent, and the effect was largely driven by improved performance in courses that involve numerical skills. Students started to do better at math.

SHAPIRO: OK, but what about courses that involved abstract painting?

VEDANTAM: (Laughter).

SHAPIRO: Or cooking, no?

VEDANTAM: I think that research is still to be done, Ari.

SHAPIRO: OK. Thanks, Shankar.

VEDANTAM: Thank you, Ari.

SHAPIRO: That's Shankar Vedantam who regularly joins us to talk about social science research. You can follow him on Twitter at @HiddenBrain. You can also follow this program at @MorningEdition, and I'm at @AriShapiro.

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