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."
The UK study does find evidence, however, of slightly impaired educational abilities among the very heaviest marijuana users. This group of students scored roughly 3% lower on school exams taken at age 16, even after adjusting for confounding factors.
In a press release accompanying the study, lead author Claire Mokrysz noted that "this is a potentially important public health message- the belief that cannabis is particularly harmful may detract focus from and awareness of other potentially harmful behaviours." Reviewer Guy Goodwin of Oxford University agreed: "the current focus on the alleged harms of cannabis may be obscuring the fact that its use is often correlated with that of other even more freely available drugs and possibly lifestyle factors. These may be as or more important than cannabis itself."
This is a key point. Many skeptics of legalization in the United States focus on the potential harms of marijuana use alone. But marijuana use is just one of many behaviors that can possibly affect life outcomes. In many cases these other behaviors are likely to play a much larger role in determining a person's trajectory through life.
It also partly explains why even as we've seen increasingly permissive laws regulating marijuana use in the past decade, there has been no corresponding uptick in negative outcomes.
It's perfectly reasonable to assume that heavy use of marijuana - or any substance - in your teen years will lead to poor educational performance. Thus for states that are legalizing the drug -- Colorado and Washington so far - it makes sense to limit legal marijuana use to 21-and-over.
But this study is the latest in a growing, robust body of evidence suggesting it makes little sense to focus on marijuana use to the exclusion of all other factors.
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