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Research Challenges Marijuana’s Supposed Impact on IQ

Tue, 01/15/2013 - 12:28pm
Associated Press, Malcolm Ritter

Cannabis. Image: Hupu2, WikimediaA 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 analysis says the statistical analysis behind that conclusion is flawed.

The original study, reported last August, 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.

Researchers at Duke Univ. and elsewhere found that participants who'd reported becoming dependent on pot by age 18 showed a drop in IQ score between ages 13 and 38. The findings suggest pot is harmful to the adolescent brain, the researchers reported.

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

Ole Rogeberg, of the Ragnar Frisch Center for Economic Research, says the IQ trend might have nothing to do with pot. Rather, it may have emerged from differences among the study participants in socioeconomic status, or SES, which involves factors like income, education and occupation, he says.

He based his paper on a computer simulation. It traced what would happen to IQ scores over time if they were affected by differences in SES in ways suggested by other research, but not by smoking marijuana. He found patterns that looked just like what the Duke study found.

Rogeberg says 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 suggests further analyses the researchers could do.

The Duke scientists, who learned of Rogeberg's analysis late last week, say they conducted new statistical tests to assess his proposed explanation. Their verdict: it's wrong. Rogeberg says they need to do still more work to truly rule it out.

Experts unconnected to the two papers say 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, says Duncan Clark, who studies alcohol and drug use in adolescents at the Univ. of Pittsburgh.

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

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