There are several reasons why, in theory, substance use might hurt academic performance. For instance, drinking or the recreational use of drugs could divert time and energy away from studying. It is also possible that substance use could hurt academic performance through its effects on cognitive functioning, attention span, short-term memory, or even self-esteem.
Although there is a clear negative association between substance use and academic performance (as measured by, for instance, grades and test scores), this association could simply reflect difficult-to-measure factors such as personality and the home environment. Accounting for the influence of these factors turns out to be quite challenging.
The cleanest, surest method of accounting for their influence would be to conduct a randomized controlled experiment. Imagine randomly assigning students to treatment and control groups, ensuring that neither group has a disproportionate share of impulsive risk-takers. The treatment group would agree to use marijuana (or some other substance) as directed, while the control group would have no choice but to abstain. After a few years, the effect of marijuana use on academic performance could be ascertained by simply comparing the outcomes of the two groups.
Because conducting such an experiment would be wildly impractical (and grossly unethical), researchers have turned to the next best thing: natural experiments. A natural experiment exploits an isolated change in conditions or incentives facing a sub-population but leaves an otherwise comparable subpopulation essentially untouched. If the change is sufficiently abrupt and large—and if the two sub-populations are essentially identical, the experiment can be used to produce causal estimates of the effect in which we are interested.
Studies relying on clearly defined natural experiments are, unfortunately, few and far between. A handful have exploited the minimum legal drinking age (MLDA) of 21 in the US, comparing the performance of university students immediately before and after reaching the MLDA. Their results provide strong evidence of a sharp decrease in grades when university students gain legal access to alcohol. These studies, however, are of limited value in trying to determine whether alcohol consumption affects academic performance outside the university setting, because most 21-year-olds have already graduated from high school.
A recent study using administrative data on Dutch university students provides exciting new evidence on the causal relationship between marijuana use and academic performance. The authors of this study exploited an extraordinary policy introduced by the Dutch city of Maastricht in 2011. Under this policy, residents of countries such as France and Luxembourg were prohibited from frequenting cannabis shops, while access to residents of Belgium, Germany, and the Netherlands was unaffected. The authors compared the grades of “treated” students (i.e. students from France, Luxembourg, and other countries) at Maastricht University to those of “untreated” students (i.e. students from Belgium, Germany, and the Netherlands). Before the policy came into effect, the grades of the treated versus untreated students changed in tandem from one semester to the next. After the policy came into effect, a substantial gap emerged: the treated (French, Luxembourgian, and other) students clearly outperformed their untreated counterparts (Dutch, Belgian, and German).
To sum up, a few recent studies by economists have produced credible evidence that alcohol and marijuana use negatively impact the grades of university students. Perhaps surprisingly, however, it is still not clear whether substance use affects educational attainment as measured by, for instance, years of education or the likelihood of dropping out of high school. Obviously, there is much more work to be done in this area, which for a researcher is both exciting and somewhat daunting.
© Daniel I. Rees
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