In previous posts, I have written about how access to alcohol has very little effect on students' academic performance, but legalised marijuana appears to be associated with worse performance. Why the difference? It may come down to how access to these products affect students' time use. So, I was interested to read this 2018 article by Yu-Wei Luke Chu (Victoria University of Wellington) and Seth Gershenson (American University), published in the journal Economics of Education Review (ungated earlier version here). They use data from the American Time Use Survey, and compare the time use of high school and college students in states with and states without medical marijuana laws (MML), before and after the laws were implemented (in a difference-in-differences strategy). The time use data looks at the number of minutes spent on each activity on the particular day of the survey (the diary day).
Chu and Gershenson find small and insignificant effects on high school students' time use (which should be no surprise, because that age group wouldn't have legal access to marijuana even after medical marijuana laws come into effect). However, for college students they find that there is:
...a decrease of 25 minutes in education-related activities such as attending class and doing research or homework after the passage of MMLs, or equivalently, a 23% (25/107) decrease relative to the pre-MML average. The Poisson estimates... suggest that college students spend 20%... less time on education activities.
In contrast, college students spend more time on leisure activities, where:
...the OLS estimates suggest an increase of approximately 30 min in leisure activities after the passage of MMLs.
So, after medical marijuana laws come into force, college students spend nearly half an hour less time on education activities, and an equivalent amount of time more on leisure activities. In other words, it appears that they substitute education time for leisure time.
When Chu and Gershenson look separately at the extensive margin (whether the student spent any time that day on the activity) and the intensive margin (how much time students who spent any time that day spent on the activity), they find that:
...behavioral changes on the extensive margin are an important channel through which total education time decreased... the OLS estimate indicates that MMLs decrease the likelihood of spending any time on education activities by 7.5 percentage points (19%)... In contrast... the increase in leisure time among college students... is primarily driven by behavioral changes on the intensive margin.
So, college students appear to respond to medical marijuana laws by engaging in education activities on fewer days (and having more days where they do no studying at all), and spend more time on leisure on all days. However, before we get too far ahead of ourselves, Chu and Gershenson also show that:
...behavioral responses during summer can account for most of the changes in education and leisure time. The reduction in education time is mostly driven by changes in summer time use... A similar pattern emerges for leisure time.
In other words, it may be that legalised marijuana induces college students to give up summer school. Finally, Chu and Gershenson show that the results are concentrated among part-time, rather than full-time, students.
All of this is consistent with rational behaviour for students. For students who are part-time, and students who are in summer time, the cost of devoting more time to 'leisure activities' is lower than for full-time students or students in regular semester times. So, if the benefit of 'leisure activities' increases, part-time and summer students are more likely to shift their time towards leisure activities than full-time students or students during a regular semester.
Of course, this study only evaluated the introduction of medical marijuana laws. Full legalisation of marijuana use might have different, and potentially larger effects. With legalisation progressing across states, no doubt we will see analyses along those lines in the future.
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