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As sentiment shifts in favor of decriminalizing pot, some worry that more teens will start using it and suffer a drop in IQ (Tampa Bay Times photo) As sentiment shifts in favor of decriminalizing pot, some worry that more teens will start using it and suffer a drop in IQ (Tampa Bay Times photo)

As sentiment shifts in favor of decriminalizing pot, some worry that more teens will start using it and suffer a drop in IQ (Tampa Bay Times photo)

Jon Greenberg
By Jon Greenberg July 27, 2014

Marcus: If teens use marijuana 'regularly,' they drop 8 IQ points

About two dozen states have moved to loosen the reins on marijuana. The legal ground is shifting and a New York Times Sunday editorial tried to shake things up a bit further. For more than 40 years, federal law has banned pot and lawmakers in many states have voiced reluctance to run afoul of Washington. The paper’s editorial board said the federal government should be silent on marijuana and let states decide for themselves.

Washington Post columnist Ruth Marcus said she was okay with letting states experiment, but in a discussion on NBC’s Meet the Press on July 27, 2014, Marcus said she was against letting states "go the full legalization route." Her main concern with state’s loosening the rules had to do with teenagers.

"It is a vast social experiment," Marcus said. "We do not know the outcome except that the best evidence is that you lose -- if you use marijuana as a teenager regularly -- eight IQ points."

We didn’t hear from Marcus, but we’re pretty sure she is citing a 2012 study from Duke University researchers published in the Proceedings of the National Academy of Sciences. The Duke team led by Madeline Meier studied nearly every child born in the New Zealand town of Dunedin born in the early 1970s. By the time they turned 13, these children had taken their first IQ tests. Follow-up interviews as they grew older determined if they were using marijuana, alcohol or other drugs. They took another battery of IQ tests when they turned 38.

The Duke researchers screened out those who had problems with hard drugs, alcohol or schizophrenia. They controlled for the kids who dropped out of school. What they found was the ones who began smoking pot weekly before they turned 18 showed an average drop of about eight IQ points. Some lost more and some lost less but all lost at least a bit.

"Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline," the researchers wrote.

The study didn’t say this proved that that early and long-term use caused the drop in IQ results. Researchers acknowledged that some unknown variable might be responsible. At the same time, they said it was plausible that pot could be disrupting brain development in teenagers.

The report attracted a fair bit of attention. Then, about a year later, another analysis emerged, again published in the Proceedings of the National Academy of Sciences. This one came from Ole Rogeberg, a Norwegian economist, who said he could get the same results as Meier by factoring in socioeconomic status. In short, Rogeberg said the kids from poorer households would lose ground on IQ tests over time because they tended to end up doing work that was less mentally demanding.

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"The causal effects estimated in Meier et al. are likely to be overestimates and the true effect could be zero," Rogeberg wrote. "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."

Meier shot back that Rogeberg had relied on an economic model and that when her team looked at the incomes of the families, they still found a loss in intelligence that was tied to marijuana. In particular, children from middle-class households were more likely to see a drop in their IQ scores if they started smoking pot early and continued as they aged. However, there was no mention of how large the drop was or how statistically robust it was.

Annette Dobson, a statistician at the University of Queensland, commented on the debate. Dobson largely weighed in on Rogeberg’s side, saying the number of long-term pot users in the Dunedin group was small, only 124 reported regular use at any time.

"I suspect that the small sample size limited the extent to which the authors were able to take possible confounding factors, such as socioeconomic status, into account," Dobson wrote. "So are the original results by Meier et al. correct? I don’t believe we can say whether they are or not."

Our ruling

Marcus said the best evidence shows that teenagers who start smoking marijuana will lose eight IQ points. This overstates the actual underlying report. It would be more accurate to say the study showed a higher risk of losing that many points on an IQ test, not that a person necessarily would. More fundamentally, an equally reputable analysis found that IQ loss could be tied to household incomes as much as marijuana use.

Clearly, a scientific consensus has yet to emerge, and Marcus ignored the ongoing debate.

We rate the claim Half True.

Our Sources

NBC, Meet the Press, July 27, 2014

New York Times, Repeal Prohibition, Again, July 27, 2014

Time, Does Marijuana Use by Teens Really Cause a Drop in IQ?, Aug, 28, 2012

Proceedings of the National Academy of Sciences, Persistent cannabis users show neuropsychological decline from childhood to midlife, July 30, 2012

Proceedings of the National Academy of Sciences, Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status, January, 2013

Time, New Research Questions Marijuana’s Impact in Lowering IQ, Jan. 15, 2013

Journal of the American Medical Association, The Residual Cognitive Effects of Heavy Marijuana Use in College Students, Feb. 21, 1996

Neurospychopharmacology, Effects of Acute Smoked Marijuana on Complex Cognitive Performance, April, 2001

Drug and Alcohol Dependence, Early-onset cannabis use and cognitive deficits: what is the nature of the association?, April 2003

Science Media Centre, Cannabis and IQ debate in the media, Jan. 15, 2014

U.S. News and World Report, Pot Use-Low IQ Link Challenged in Study, Jan. 14, 2013

The Conversation, Teen cannabis use lowers IQ, despite claims to the contrary, Jan. 14, 2013

Science Media Centre, Teen cannabis IQ impact questioned – experts respond, Jan. 15, 2013


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