The contemporary version of argument can be traced to a 2012 Duke University study, which found that persistent, heavy marijuana use through adolescence and young adulthood was associated with declines in IQ.
Two new reports this month tackle the relationship between marijuana use and intelligence from two very different angles: One examines the life trajectories of 2,235 British teenagers between ages 8 and 16, and the other looks at the differences between American identical twin pairs in which one twin uses marijuana and the other does not.
Despite vastly different methods, the studies reach the same conclusion: They found no evidence that adolescent marijuana use leads to a decline in intelligence.
I wrote about the study of British teenagers before, when it was still a working paper. It has been peer-reviewed and accepted for publication, and its findings still stand: After adjusting for a range of confounding factors, such as maternal health, mental health and other substance use, the researchers found that "cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance. These findings therefore suggest that cannabis use at the modest levels used by this sample of teenagers is not by itself causally related to cognitive impairment."
They did find, though, a distinct relationship between cigarette use and poor educational performance, which is in line with what other research has found. The researchers did not find a robust link between cigarette use and IQ.
The authors of this study stress that their results don't necessarily invalidate the findings of the 2012 Duke University paper. That paper focused on persistent heavy use over a long period of time, while this study looked only at low to moderate levels of adolescent use. "While persistent cannabis dependence may be linked to declining IQ across a person’s lifetime," the authors write, "teenage cannabis use alone does not appear to predict worse IQ outcomes in adolescents."
But the researchers in the study of American twins tackle the Duke University findings head-on. Examining the life trajectories of twin pairs in which one uses marijuana while the other doesn't, they found that those who used marijuana didn't experience consistently greater cognitive deficits than the others.
Identical twin comparisons are a powerful tool for this kind of analysis, because their genetic makeup is nearly identical and their early home environment is consistent. This automatically controls for a lot of the confounding factors that can make sussing out causality difficult.
The twin data "fails to support the implication by Meier et. al. [the authors of the Duke study] that marijuana exposure in adolescence causes neurocognitive decline," the study concludes. The numbers suggest, on the contrary, that "children who are predisposed to intellectual stagnation in middle school are on a trajectory for future marijuana use." In other words, rather than marijuana making kids less intelligent, it may be that kids who are not as smart or who perform poorly in school are more inclined to try marijuana at some point in their lives.
Also, if marijuana use were responsible for cognitive decline, you might expect to find that the more marijuana a person smokes, the less intelligent they become. But this paper found that heavier marijuana use was not associated with greater decreases in IQ.
None of this is to say, though, that you can smoke all the weed you want and not have to worry about negative outcomes. There are any number of negative physical and mental health outcomes linked to marijuana use — especially heavy use. Some research suggests that heavy marijuana use may increase the risk of psychosis or suicide. These risks are further compounded among people who start using marijuana early in their lives. And people who use heavily or start at an early age are at a high risk for cannabis-use disorder, a form of drug dependency.
Marijuana is a drug. And just like any other drug — alcohol, nicotine, caffeine — there are risks and benefits associated with use. But exaggerating the extent of those risks and benefits won't help create smarter policies. For proof of this, simply review the history of the drug war.