For years, the image of the silly, spaced-out stoner seemed to embody marijuana use in popular culture. But with new, relaxed marijuana policies becoming law in many states, the picture is changing. High-functioning executives, government officials and even grandparents are openly discussing smoking pot.
Still, the question remains: Does marijuana negatively impact intelligence? Researchers have long disagreed. A new study using many years of data tries to settle the debate.
An academic study worth reading: “Examining the Influence of Adolescent Marijuana Use on Adult Intelligence: Further Evidence in the Causation Versus Spuriousness Debate.” In Drug and Alcohol Dependence, 2017.
Study summary: Criminologists Cashen Boccio and Kevin Beaver of Florida State University review the available — and contradictory — research on the relationship between adolescent marijuana use and intelligence. Unsatisfied, and concerned that previous research did not control for enough factors, they examine a large, longitudinal dataset funded for decades by the U.S. government: The National Longitudinal Study of Adolescent to Adult Health. “Add Health”, as the survey is known, began tracking over 20,000 middle school and high school students in 1994-1995 and has followed these students well into adulthood through three subsequent waves (in 1996, 2001-2002 and 2008). It is, say its organizers, “the largest, most comprehensive longitudinal survey of adolescents ever undertaken.”
Add Health studies a number of factors. Boccio and Beaver are interested in verbal intelligence, which is often used as a proxy for overall intelligence, and marijuana use. In order to test the relationship, the researchers examined frequency of use and cumulative use over time (by comparing different waves of the study).
The researchers did not consider respondents who indicated using marijuana during the first wave (when some were as young as 12), so they could focus on those who started using the drug as adolescents. This also allowed them to measure any changes to intelligence between the first wave in 1994-1995 and the third wave in 2001-2002. They control for a number of factors, including socioeconomic status and personality traits.
- Trying marijuana was associated with lower intelligence scores.
- There was, however, no clear relationship with the amount of marijuana consumed: “There is no dose-dependent relationship between marijuana use and changes in intelligence scores.”
- Neither personality factors nor socioeconomic status affected these findings.
- Compared with someone who had never tried marijuana, having tried as an adolescent was associated with a 2.098 percentage point decrease in intelligence scores (which the authors call “relatively small”).
- Compared with someone who had never tried marijuana, having tried (for the first time) as a young adult was associated with a 1.06-point decrease in intelligence scores.
Limitations: The authors were unable to measure the effects of marijuana dependence because each wave only recorded 30 days of use. “The findings of this study can only be generalized to low/average levels of marijuana use and not heavy users or individuals who are marijuana dependent,” they write. They also warn that the findings could be confounded by the abuse of other substances.
- We wrote an extensive review of scientific literature on marijuana and health in 2017. We have also reviewed research on marijuana use, crime and distracted driving.
- Is marijuana a gateway drug? A 2017 study in The B.E. Journal of Economic Analysis & Policy suggests it is not.
- Low doses of marijuana may make us relax, while higher doses are associated with stress, a forthcoming study in Drug and Alcohol Dependence indicates.
- Teenagers from upper-middle class suburbia are substantially more likely to abuse alcohol and drugs than the national average and to carry these abusive patterns into adulthood, according to a forthcoming study in Development and Psychopathology. The authors also observe that stricter parenting at age 17 and 18 is associated with lower rates of substance abuse in adulthood.