Popular wisdom says people who smoke a lot of pot are dumber, on average, than people who don’t. A well-hyped study by Duke University researchers, released in 2012, proclaimed that regular pot smoking by teenagers leads to lower IQ scores.

Turns out, they were wrong. In a big way.

Marijuana TeenagerA new, much larger study, with thousands of teenage participants, reached the opposite conclusion, finding no correlation between marijuana use and intelligence scores. Researchers from the University College of London reported in October that even heavy marijuana use had no significant effect on intelligence.

Anecdotally, you don’t need to go far to find evidence that this is true. Carl Sagan smoked a lot of dope in his youth, and he wasn’t exactly an intellectual lightweight in his later years.

But until now there has been little if any reliable empirical data to help policy makers, educators, parents, and kids make smart decisions. Most of the hype is exactly that: hype. It’s based in a deeply held cultural belief that anything that feels good must be bad for you.

The British study examined 2,612 children born in the area around Bristol in the U.K. IQ levels were tested twice, at ages eight and 15, and researches reported they found “no relationship between cannabis use and lower IQ at age 15.” Even heavy tokers didn’t see a notable decline in their intelligence scores.

The study accounted for a number of other variables: alcohol use, cigarette smoking, and maternal education, among others. Researchers determined that alcohol use was a much bigger influence on intellect than marijuana.

“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.”

Anyone who has ever gotten within sniffing distance of marijuana can tell you it’s less dangerous than alcohol in every conceivable way. People who only use weed don’t generally break into liquor stores, rob gas stations, or start bar fights. They don’t die from ODs, and they don’t develop life-threatening diseases.

Marijuana ScientistThe scientists in the U.K. did find evidence that chronic toking can lower scores on school exams by about 3 percent by age 16, but the difference was too small to matter. The study’s authors said their findings would allow politicians and other scientists to make informed decisions about pot, alcohol, and other drugs.

“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 behaviors,” the authors said in a press release.

Guy Goodwin, a professor of psychiatry at Oxford University, reviewed the study and agreed with the authors’ conclusion.

“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,” Goodwin said. “These may be as or more important than cannabis itself.”


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