A funny thing happened in the Dutch city of Maastricht in the fall of 2011. A policy went into effect banning the sale of marijuana at the city’s 13 legal cannabis shops to visitors from most other countries. The goal was to discourage disruptive drug tourism in a city close to several international borders. The policy had its intended effect, but also a remarkable unintended one: foreign students attending Maastricht University starting getting better grades. According to an analysis published earlier this year in Review of Economic Studies, students who had been passing their courses at a rate of 73.9% when they could legally buy weed were now passing at a rate of 77.9% — a sizeable jump.
The effect, which was based on data from 336 undergraduates in more than 4,000 courses, was most dramatic for weaker students, women, and in classes that required more math. Some of this falls in line with past research: marijuana use has been linked to inferior academic achievement (and vice versa), so it makes sense that poorer students might benefit most from a ban, and the drug is known to have immediate effects on cognitive performance, including in math.
But what’s really unusual about the study, notes one of its authors, economist Ulf Zoelitz of the Briq Institute on Behavior and Inequality, is that rather than merely correlating academic performance with cannabis use, as much past research has done, “we could cleanly identify the causal impact of a drug policy.” Zoelitz co-authored the study with Olivier Marie of Erasmus University Rotterdam.
The findings come in the wake of a movement to liberalize marijuana laws in the U.S. and some other countries. A total of 29 states and Washington, D.C. now allow the use of medical marijuana and eight states have decriminalized recreational use. Research shows that the legalization movement has coincided with a shift in attitudes among teenagers over the past decade: a steep decline in the perception of marijuana as risky. Use of all illicit drugs by adolescents has been dropping in the U.S., but smoking weed remains broadly popular: more than a third of high school seniors report some use and 6% admit they partake daily.
How much harm are they wreaking on their brains and academic performance? Obviously, students who are actively high in class are not doing themselves any favors. There’s a rich scientific literature demonstrating that learning, memory and attention all take a hit from cannabis intoxication.
The bigger question concerns cumulative damage. A substantial body of research – much of it summarized in 2016 review paper in JAMA Psychiatry – shows that heavy marijuana use that begins in adolescence and continues over many years predicts lower achievement and less overall happiness in life. It has also been linked to a variety of changes in brain anatomy and function, such as altered patterns of connectivity between the two hemispheres and within the prefrontal cortex — the center for higher thinking and learning. Brain imaging studies of habitual marijuana users have revealed a smaller amygdala and hippocampus — structures that, among other things, play a role in regulating emotions and forming memories. Marijuana also appears to be a trigger for schizophrenia in people with underlying vulnerabilities.
But it’s maddingly difficult to separate the impact of cannabis from other factors, such as poverty and childhood trauma, which increase the odds of using marijuana and which themselves can directly alter the brain. In an attempt to eliminate the role of poverty, psychologist Madeline Meier and colleagues at Arizona State University published a study in 2015 that looked at the relationship between smoking pot and grade-point average among upper-middle-class high school students. Sure enough, she found a correlation, but it was impossible to disentangle it from the effects of drinking and smoking. “The problem is that adolescents who use marijuana tend to also use alcohol and smoke tobacco,” she explains.
A study published the same year by epidemiologist Amelia Arria and colleagues at the University of Maryland, managed to separate these influences and found that as students’ weed consumption rose, their GPA fell — just as the experience in Maastricht would suggest. In addition, they found that the frequency of marijuana use during freshman year predicted how likely students were to graduate on time.
Perhaps the biggest source of worry, where teenagers are concerned, stems from scientists’ growing appreciation of how much the body’s own cannabis-like chemicals shape the brain. These “endocannabinoids,” which are made in nerve cells throughout the body, play a key role in wiring the brain during fetal development and also in guiding major changes that occur from puberty through the mid-20s, when the brain reaches full maturity. Throughout life, this system influences reward-seeking behavior around food, sex and social interaction and regulates our emotions, memory, movement and sleep.
“The fact that this system is changing significantly during adolescence is of major importance,” says neuroscientist Yasmin Hurd, director of the Addiction Institute at Mount Sinai in New York City. She wonders if young people would be quite so cavalier about “flooding their brain with marijuana” if they knew that this system is “one of the most important players in regulating communication between nerve cells and therefore in determining who we are on multiple levels — from motivation and cognition to motor function.”
Still, no one can say for sure that cannabis disrupts this system, and even less clear is the impact of short-term or moderate use. But answers are coming. The National Institutes of Health has launched the massive Adolescent Brain Cognitive Development Study (ABCD), which will track more than 10,000 nine- and ten-year-olds for 10 years — before and after the period when substance use typically begins. The children will undergo brain scans every two years to examine both structure and function, says Nora Volkow, director of the National Institute on Drug Abuse. In addition, she says, “we will be very carefully recording exposure to drugs, social environment and performance at school.” Researchers cannot wait to get their hands on the data, which should answer many of the unresolved questions about marijuana in the teenage years.
For now, though, it makes sense to talk to kids about what we do know: the startling role of the endocannabinoid system and the findings in Maastricht and elsewhere about the impact of marijuana on achievement in school. Even if the ABCD study winds up showing that marijuana does not directly harm the brain, it certainly hits kids in another place that matters: their GPA.