Scientists have found that smoking weed does not make you stupid after all

Jan 19, 2016

Photo credit: AP/David Zalubowski

By Christopher Ingraham

You might have heard that smoking marijuana makes you stupid.

If you grew up in the ’80s or ’90s, that was more or less the take-home message of countless anti-drug PSAs. In more recent years, it’s a message we’ve heard — albeit in more nuanced form — from Republican candidates on the campaign trail and from marijuana opponents at the state-level.

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.

Other researchers have since criticized that study’s methods. A follow-up study in the same journal found that the original research failed to account for a number of confounding factors that could also affect cognitive development, such as cigarette and alcohol use, mental illness and socioeconomic status.

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.


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