Published in full on Washington Post Wonkblog.
A 2012 Duke University study made international headlines when it purported to find a link between heavy marijuana use and IQ decline among teenagers. Other researchers questioned the findings almost immediately: Columbia University’s Carl Hart noted the very small sample of heavy users (38) in the study, leading him to question how generalizable the results were.
Then, a follow-up study published 6 months later in the same journal found that the Duke paper failed to account for a number of confounding factors: “Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal inference drawn from the results premature,” it concluded.
Now, a new study out from the University College of London provides even stronger evidence that the Duke findings were flawed. The study draws on a considerably larger sample of adolescents than the Duke research—2,612 children born in the Bristol area of the U.K. in 1991 and 1992. Researchers examined children’s IQ scores at age 8 and again at age 15, and found “no relationship between cannabis use and lower IQ at age 15,” when confounding factors—alcohol use, cigarette use, maternal education, and others—were taken into account. Even heavy marijuana use wasn’t associated with IQ.
“In particular alcohol use was found to be strongly associated with IQ decline,” the authors write. “No other factors were found to be predictive of IQ change.”
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