Marijuana is one of the most commonly used drugs in the United States, and it has been proposed that its use during adolescence—when the brain is still developing—is a cause of declining intellectual function.
But a University of Minnesota-led study indicates that while adolescent users underperform on some measures of IQ compared to nonusers, marijuana use and lower IQ scores co-occur due to other factors. The work is published online this week in the Proceedings of the National Academy of Sciences.
“We found that marijuana users in our study showed decline in two aspects of intelligence—vocabulary and general knowledge,” says Joshua Isen, who led the research as a postdoc at the U of M and is now a lecturer at Loyola Marymount University in Los Angeles. “But when we looked closer, we noticed that some users were lower in these scores before they began use. It didn’t seem to matter if they had just used it a few times or every day, they still had similar IQs at followup several years later.
“Thus, the amount of use didn’t matter—what made a difference was whether they had initiated use. This got us thinking that it wasn’t the marijuana itself causing the lower IQ, but rather something else making adolescents more likely to use marijuana and to score lower on those measures of IQ.”
The study was not designed to uncover that hidden factor or factors, but Isen speculated that it could be conditions related to delinquency, such as truancy or a lack of parental monitoring.
Setting the stage
The study drew on two datasets on young twins: the Risk Factors for Antisocial Behavior study at the University of Southern California and the Minnesota Twin Family Study. It followed 789 adolescents in the USC set and 2,277 in the Minnesota set. Twins were both monozygotic (“identical”) and dizygotic (“fraternal”).
The twins’ baseline intelligence was measured at ages 9-12, prior to any marijuana use. They were asked about marijuana use every 2 or 3 years through middle and high school, then took followup intelligence tests at ages 17-20. The tests covered vocabulary and general knowledge, both related to learning from school and reading; and also culture-free questions that measured spatial reasoning, such as the ability to arrange pictures in logical chronological order or to note similarities in objects.
The twins were also asked whether they had used marijuana:
- Not at all; these were termed nonusers
- Fewer than 30 times
- More than 30 times
- Daily over at least one six-month period
Those who had ever used marijuana were termed users.
How users and nonusers differed—and didn’t
A clear difference appeared in vocabulary scores. Nonusers’ increased or stayed level between the baseline and followup assessments, but users’ scores took a downward trajectory equivalent to a drop of four IQ points. This difference was significant even after adjustment for variables like age, sex, race and socioeconomic status.
But the most telling results were negatives.
- Those who had used marijuana daily for six months or longer showed no greater IQ change than those who had tried marijuana fewer than 30 times.
- In tests of spatial reasoning, users in neither dataset showed significant decreases relative to nonusers.
- In the strongest test for a direct causal effect on intellectual functioning, the researchers examined twin pairs in which one had used marijuana frequently (more than 30 lifetime uses and/or a period of daily use) and the other had never used. They found no significant differences in IQ trajectory between members of twin pairs in any of the test scores.
“If marijuana had a direct effect, the twin who smoked it should show a greater IQ decline, but they don’t,” says Isen. “We conclude that marijuana use is not directly causing lower IQs and infer that there exists some other factor that’s common to marijuana use and low intellectual attainment.”
“[This finding demonstrates] that use of marijuana does not cause this decline,” says study coauthor William Iacono, University of Minnesota Regents Professor of Psychology. “Instead, it appears to be due to a combination of biological and environmental vulnerabilities that are present in the absence of marijuana use and that are shared by both members of a twin pair.”
Thus, says Isen, it would be expected to lower the IQs of both twins and to put the nonusing twin at risk for other deviant behaviors like skipping school, binge drinking, getting into fights or smoking tobacco.
“Parents should be just as concerned about those behaviors,” he says. “Simply preventing a child who has other problems from using marijuana is probably not going to change their intellectual trajectory.”
Other U of M coauthors were Matt McGue, Regents Professor of Psychology; and postdoc Daniel Irons.