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Smoking Marijuana May Not Lower IQ After All

A new paper that refutes a much-heralded study also highlights the difficulty of separating pot science from politics.


Marijuana hasn't been proved to lower IQ.
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By Tony O'Neill


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Remember the study that came out last year, researched in part by Duke University, which claimed that smoking marijuana in your teens leads to a long-term drop in IQ? It won blanket coverage at the time—but a new analysis is now crying foul. Media enthusiasm for any study suggesting a causal link between pot use and low IQ/psychosis/ impotence/cancer is nothing new. The proud tradition goes all the way back to when William Randolph Hearst used his press empire to suggest that marijuana caused Mexican migrant workers to go on violent rampages; the unfounded claims helped to bring about US pot prohibition. The Duke study looked at 1,000 people born in the town of Dunedin, New Zealand: Their IQs were tested at the ages of 13 and again at 38, and they were interviewed about their marijuana use. When a causal link between teen pot use and lower IQ was reported, the press wasted no time: “Smoking Cannabis When a Teen Makes You a Dope!” trumpeted UK tabloid The Sun. The Daily Mail went one further, suggesting that teenagers “addicted” to marijuana show signs of mental impairment normally seen “in early Alzheimer’s.”

The new paper, published by the Proceedings of the National Academy of Sciences, examines that research and finds its methodology to be flawed. Socioeconomic differences among study participants in terms of education level, occupation and income weren't taken into account, says Ole Rogeberg of the Ragnar Frisch Center for Economic Research in Oslo. These factors could, according to Rogeberg, have influenced the participants' varying IQs. His paper, based on on a computer simulation, traced what would happen to IQ scores over time if they were affected by socioeconomic factors (as suggested by other research), but not by smoking marijuana. He found that the patterns closely resembled those found by the pot-centric Duke study. This, says Rogeberg, suggests that the researchers of the initial study should have analyzed their results more thoroughly before jumping to conclusions. Dr. Nora Volkow, director of the less-than-progressive National Institute on Drug Abusegrudgingly admits that Rogeberg’s findings “look sound”—though she points out that socioeconomic factors aren't yet proven to be the cause of the IQ variations, any more than marijuana use is.

Unsurprisingly, Rogeberg's work doesn't excite the media as much as last year's study—far from the first example of a collective blind spot to stories that suggest drug use isn't as harmful as previously believed. Of course “Smoking Pot Makes You Stupid Forever” is a juicier headline than “Researcher Finds Pot Study Methodology May Be Flawed.” But if you need a reminder of the importance of remembering how loaded coverage of this subject can be, check out how Rupert Murdoch’s Fox News is spinning Rogeberg's work: “Pot Does Lower IQ, Study Finds.”

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