Scientists have linked teenage marijuana use with a host of undesirable outcomes: difficulty in paying attention, weaker memory and lower verbal ability and intelligence.
But is the drug itself to blame?
Two long-term studies of twins published Monday suggest that other factors are at fault, at least as far as vocabulary skills are concerned.
In one study, children who went on to become marijuana users weren’t as bright to begin with as their abstinent peers. And in both studies, drug-using teens fared no worse on IQ tests than their non-using twins in the same household, suggesting that some other factor was to blame, the authors wrote.
Something in the family environment perhaps, or simply the fact that kids who gravitate toward pot use may be less motivated to try hard in school.
Results of the two studies, described in a paper in Proceedings of the National Academy of Sciences, surely won’t settle the debate.
In interviews, two of the authors cautioned that even if marijuana does not directly impair intelligence in the developing brain, there are plenty of other reasons that teenagers should steer clear of weed.
The studies didn’t measure executive function – an umbrella term that covers the ability to focus and organize, among other skills – and previous research suggests that pot use can weaken it.
And despite the push to legalize marijuana in some states, it remains illegal for teens, said the University of Pennsylvania’s Adrian Raine, who helped design the studies with the University of Southern California’s Laura A. Baker, the paper’s senior author.
“We’re only talking about verbal ability,” said Raine, a professor of criminology, psychiatry and psychology at Penn. “There’s more to life than verbal ability.”
Joshua D. Isen, a co-lead author of the paper, cautioned that teen use of the drug also is associated with poorer performance in school.
“I’d think marijuana use could be a symptom of a larger problem,” said Isen, a University of Minnesota post-doctoral fellow and a lecturer at Loyola Marymount University in Los Angeles.
That kind of talk bothers one prominent marijuana advocate: Ed Forchion, owner of a Trenton, N.J., restaurant called NJ Weedman’s Joint. (Slogan: “Healthy, hearty, home-made food for Trenton’s ‘high society.’”)
“Anybody can look at me,” Forchion said. “I’m far from being stupid. I consume a lot of marijuana. I’ve always done very well on everything from SAT scores to IQ tests.”
To the extent that drug users have trouble getting ahead in the world, Forchion blames others.
“It’s other people’s stereotypes, and other people’s accepting of the government’s propaganda,” he said.
It’s difficult to study how pot affects kids because it would be unethical (and illegal) to undertake the gold-standard study, a randomized controlled trial in which one group would be given an illegal substance and compared to another group. The paper’s authors acknowledged as much, calling their work “quasi-experimental.”
One of the studies started with 2,277 Minnesota youths age 11 to 12, 93 percent of whom were white. The other tracked 789 adolescents from the Los Angeles area, age 9 to 10, three-quarters of whom were racial minorities or of mixed ethnicity.
By the time they reached their late teens, 36 percent of the Minnesotans said they used marijuana. A subset of them was included in the study because they had shown disruptive behavior and “academic disengagement.”
In the California group, 60 percent of the teens reported they had used the drug, but they were 20 when surveyed – about two years older than their Minnesota counterparts.
In Minnesota, the future users started out two points lower on the vocabulary portion of an IQ test, and six years later the gap had widened by 4 points. In California, the future users and non-users started out with similar scores as young adolescents, but the users scored 3.4 points lower when tested at age 20.
The researchers did not find similar before-and-after disparities in some other IQ subtests, such as one called “block design,” which involves rearranging colored blocks to match a pattern.
The researchers cited two results in concluding that the drug does not directly impair intelligence:
In a subset of households in which one twin smoked pot and the other did not, the smokers did not score any worse. And overall, those who reported using pot more than 30 times scored no worse than users who had tried it fewer than 30 times.
Not good enough, said Madeline H. Meier, an Arizona State assistant professor of psychology.
Meier led a previous study in which participants were followed up to age 38, in which marijuana use did appear to lead to cognitive decline.
“The problem is that for some teens,” she said, “short-term low-level cannabis use leads onward to long-term dependence on cannabis when they become adults.”
Raine, the Penn professor, agreed that the drug is not benign.
“I’m not convinced that there are no effects at all of taking marijuana,” he said.
Raine said he had never tried the drug, and that if either of his 13-year-old twins were to try it, he would object.
“Are you going to end up in jail or not?” he said. “Who are you hanging out with to get the cannabis in the first place? From a parenting perspective, I wear a different hat.”