Following last week's post about marijuana legalisation and student time use, I followed up by reading this 2020 article by Adam Wright and John Krieg (both Western Washington University), published in the journal Economic Inquiry (sorry, I don't see an ungated version online). They look at the case of Western Washington University, comparing the academic performance of students before and after attaining age 21, before and after 2012, when marijuana was legalised in Washington State (and became legally available to people aged 21 years and over). This difference-in-differences analysis is necessary in order to extract the effect of marijuana access, separate from the effect of alcohol access (since alcohol becomes legally available at age 21 as well).
Wright and Krieg use student-level data from 2003 to 2017, which allows them to control for student and class (course-by-instructor-by-quarter) fixed effects, as well as dealing with the 'seasonality' of grading (more on that in a moment). Their sample contains over 1.1 million student-course grade observations, from over 29,000 students. The key change in average grades between the period before, and the period after, marijuana legalisation in 2012 are captured in Figure 1 from the paper:
There are a few things to note about this figure. First, you can see the seasonality in grades. Average grades are highest in the summer quarters (Western Washington University operates four quarters per year, rather than semesters or trimesters). Second, there is a distinct drop in average grades between just before, and just after, marijuana legalisation. Third, there is obvious grade inflation both before, and after, marijuana legalisation (notice the upward-sloping dashed trend lines).
Wright and Krieg control for the 'seasonality' and grade inflation through their use of fixed effects. Moreover, they:
...include various controls for experience designed to capture expected grade changes as a student makes progress toward degree completion. We include these controls to separate phenomena such as changes in motivation as students approach the end of their college career from the effect of turning 21, which also tends to happen near degree completion. These experience controls include the overall number of accumulated credits, the number of credits a student has accumulated within the course’s academic department, and student age at the beginning of the term (in months)...
They then find that:
Prior to legalization, students’ grades are estimated to fall by approximately 0.03 standard deviations after turning 21 relative to their earlier grades. This decline is nearly identical to Lindo et al.’s estimate of the effect of turning 21—an effect that they (and we) attribute to legal alcohol access. After marijuana legalization, our estimates indicate that the post-21 effect grows by about half to 0.046 standard deviations, suggesting that legalization further reduces student performance by 0.016 standard deviations.
That is quite a substantial effect. Incidentally, the Lindo et al. paper that they refer to is one that I discussed here. Wright and Krieg then look at specific grades and find that:
...legal access to alcohol decreases the likelihood that a student receives an A grade and increases the likelihood that a student receives a C, D, or F grade—exactly the same pattern attributed to legal alcohol access by Lindo et al. (2013). Legal access to marijuana appears to exacerbate these shifts in the grade distribution with a statistically significant (at the 5% level) increase in the probability that a student earns a D or F grade by 0.3 percentage points. In our sample, 4.4% of students receive a D or F grade so an increase of 0.3 percentage points equates to an increase in the probability of receiving a D or F by about 7%.
So far, so unsurprising (at least, in light of the other literature on this topic). Wright and Krieg then go on to show that the effect is almost entirely concentrated among male students, among students in quantitative (rather than non-quantitative) classes, and that when the sample is separated into high-ability and low-ability students (based on students being above or below the median SAT or ACT scores) that:
...while the grades of both ability groups were negatively impacted by legal alcohol access, only low-ability students’ grades suffered after gaining legal marijuana access. The point estimates for low-ability students suggest that the marijuana effect was nearly as large as the alcohol effect (−0.024 vs. −0.028 standard deviations) for this group.
Finally, Wright and Krieg show that:
...after gaining legal access [to] marijuana, students attempt fewer course credits and enroll in courses that are expected to offer higher grades.
Now, the headline effect (a reduction in student performance of 0.016 standard deviations) is a bit smaller than that observed in this earlier study by Marie and Zölitz (ungated version here, and I blogged about it here). However, the difference may be entirely down to the nature of the analysis in this paper. Since Wright and Krieg don't know which students are marijuana users, their analysis is essentially an 'intent-to-treat' analysis. That is, it looks at the average treatment effect across both students who do, and students who do not, use marijuana. Once you take that into account, Wright and Krieg note that:
Using this as our estimate for the proportion of students who consumed marijuana in response to the change in policy, the treatment effect on the treated would be about 0.23 standard deviations (0.016/0.07).
That effect is very similar in size to the effect reported by Marie and Zölitz. Between all these studies then, we are getting a clear picture that legalised access to marijuana has an appreciable negative impact on student performance. It isn't irrational for students to use marijuana, but the debate on legalisation should take these negative impacts into account.
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