Link between pot smoking and IQ drop challenged

NEW YORK — A new analysis is challenging a report that suggests regular marijuana smoking during the teen years can lead to a long-term drop in IQ.

The author of the new paper says pot might not have anything to do with the mental decline seen in the original study, and that other factors may be to blame.

The original study included more than 1,000 people who’d been born in the town of Dunedin, New Zealand. Their IQ was tested at ages 13 and 38, and they were asked about marijuana use periodically between those ages.

Participants who said they were dependent on pot by age 18 showed a drop in IQ score between ages 13 and 38, according to researchers at Duke University and elsewhere. Their report, which got wide attention last August, suggested pot is harmful to the adolescent brain.

Not so fast, says the new analysis, published online Monday by the Proceedings of the National Academy of Sciences.

Ole Rogeberg of the Ragnar Frisch Center for Economic Research in Oslo, says the IQ trend might have emerged from differences among the study participants in socioeconomic factors like income, education and occupation.

He based his paper on a computer simulation. Drawing on results of earlier research, It traced the potential effects of those socioeconomic factors on IQ. He found patterns that looked just like what the Duke study found for smoking marijuana.

In an interview, Rogeberg said he’s not claiming that his alternative explanation is definitely right, just that the methods and evidence in the original study aren’t enough to rule it out. He suggested further analyses the researchers could do with their data.

The Duke scientists, who learned of Rogeberg’s paper late last week, say they conducted new statistical tests that ruled out his explanation.

Rogeberg says they need to do still more work to truly rule it out.

Experts unconnected to the two papers said the Rogeberg paper doesn’t overturn the original study. It “raises some interesting points and possibilities,” but provides “speculation” rather than new data based on real people, said Dr. Duncan Clark, who studies alcohol and drug use in adolescents at the University of Pittsburgh.

Dr. Nora Volkow, director of the National Institute on Drug Abuse, said observational studies of people like the Duke work can’t definitively demonstrate that marijuana causes irreversible effects on the brain. In an email, she said Rogeberg’s paper “looks sound” but doesn’t prove that his alternative explanation is correct either.

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