Does marijuana lower IQ?

mqdefaultRecent estimates show that about 1 in every 10 teens in grades 9 though 12 smoke marijuana at least 20 times per month.  Given the recently reported popularity of marijuana amongst teens, the findings of a new study conducted by postdoctoral researchers at Duke University’s Center for Child and Family Policy should be of grave importance to both teens and parents.

In the study more than a thousand participants from New Zealand were tested for IQ at 13 (likely before any significant marijuana use) and then tested again at 38.  Researchers compared the IQ scores of participants at each age, and found a drop in IQ for participants to reported having significant marijuana use before the age 18.  Among participants who’d been dependent at 18, quitting didn’t remove the problem.  IQ declines showed up even if they’d largely or entirely stopped marijuana use during adulthood.

Dr. Richie Poulton, a study co-author and professor of developmental psychopathology at the University of Otago in New Zealand, said the message of the research is to stay away from marijuana until adulthood if possible. “For some it’s a legal issue, but for me it’s a health issue,” he said.  Essentially, this study suggested that regular marijuana use  is harmful to the adolescent brain.  This study, conducted in 2006, has become the crux of arguments for those opposed tot he legalization of marijuana in the U.S.

However, new publications have emerged which assert that marijuana might not have anything to do with the parents_i_want_my_kids_to_smoke_pm-thumb-270x270IQ dip seen in the original study, and that a slew of other factors could be to blame.  New analyses, released this January, found other differences among the study group (outside of their levels of marijuana consumption) including their education, occupation, and other socioeconomic factors which are just as likely to have contributed to the drop in IQ.

Ole Rogeberg of the Ragnar Frisch Center for Economic Research in Oslo used computer simulations to trace the potential effects of these socioeconomic factors of IQ.  He found patterns which very closely resembled the findings the Duke researchers observed for smoking marijuana.

In an interview, Rogeberg said he’s wasn’t claiming that his alternative explanation for the IQ drop i definitely right, but that the methods and evidence implemented in the original study aren’t sufficient to rule it out.  The Duke scientists recently learned of Rogeberg’s paper and have vehemently disagreed with his new findings, saying that they conducted new statistical tests which rule out his explanation (no record of these statistical tests have been released to the public or published in scholarly journals).

Additional support has surmounted in response to the researchers debate. 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.

imagesAs the original study’s findings become more and more disputed, the stock argument that marijuana use amongst teens cannot be directly linked to a drop in IQ — and therefore, can no longer be referenced as a reason why the U.S. should continue to criminalize marijuana use.


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