Teens, researchers, reporters, and how cannabis affects their IQ

One of the big discussions on the internet (and here in comments) yesterday was this study from New Zealand that was reported in the media as proving that use of marijuana by teens resulted in lowered IQ.

Check out the “reporting” by NBC’s Tia Ghose:

Teens who smoke marijuana see their IQs drop as adults, and deficits persist even after quitting, according to a new study.

“The findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” study researcher Madeline Meier of Duke University said in a statement.

The study followed 1,037 New Zealand children for 25 years. Subjects took IQ tests at age 13, before any of them had smoked marijuana, and again at age 38. Throughout the study, participants also answered several surveys about their drug use.

My first reaction was “New Zealand? Why is it always New Zealand when they find something bad about cannabis?”

And it does seem to be true. New Zealand was where they did the tiny study that seemed to find a lung cancer connection despite the huge study in the U.S. finding no negative links at all. And there have been others.

Maybe it’s a difference in how the rigor of research is applied in Kiwiland.

Or could it have something to do with New Zealand in general? Maybe things are just really different there. After all, it’s a part of the world where some mammals have pouches, where a cute bear-like creature spends its entire life stoned on eucalyptus leaves, where birds that look like crows sound like babies crying and another bird laughs at you, where a vine can eat a tree and become a new tree, and where hobbits and wizards alike extoll the virtues of the local smoking materials. Who knows what that does to pot or pot-smokers?

Of course, the truth is that the study isn’t quite as big a deal as some of the reporters would lead you to believe. For instance, the lead sentence by Ghose: “Teens who smoke marijuana see their IQs drop as adults” is false. The implication is that 18 or 19-yeaar-olds who tried marijuana could see a drop in IQ, whereas the study only found differences in younger teens (ie, those under 18), who were regular or heavy smokers.

Maia Szalavitz explains why the breathless reporting is out of place here. Does Weekly Marijuana Use By Teens Really Cause a Drop in IQ?

Not all experts agree, however. “Scientifically, these are extremely preliminary findings,” cautions Carl Hart, associate professor of psychology at Columbia University, who has studied the cognitive effects of marijuana in humans in the lab and was not associated with the research.

Hart notes that because only 38 people in the study— around 8% of those who ever tried marijuana— used it heavily enough to get diagnosed with dependence during several follow-up periods, he is skeptical about how generalizable the results are. He says that in his studies of people who smoke at least three times a week, “When you compare these people’s scores to a normative database on a wide range of domains including executive function, memory, and inhibitory control, they score dead smack in the middle, in the 50th percentile.”

Of course, Tia Ghose never mentioned that only a small group in the study actually fit the criteria. (Just as they seldom mention that only a handful met the cancer criteria in the infamous New Zealand cannabis/cancer study).


Reacting to the study, Wim van den Brink, Professor of Psychiatry and Addiction at the University of Amsterdam said it was interesting research but its findings should not be overestimated. Speaking to Dutch daily de Volkskrant he pointed to the results from a sub-group in the study who stopped smoking the week before being tested. The effect on their IQ’s was much less pronounced. “The researchers are right to warn of the consequences of cannabis use at a young age,” he said, “but their results are probably being exaggerated.”

So, at this point, this is really nothing more than interesting research that should be viewed with caution in terms of any actual results.

However, even if the research is fully on target. If it is, in fact, true that heavy marijuana use by those under 18 can lead to lowered IQ, then it is an argument for legalization and regulation with age limits, not the status quo. I don’t know of any in the reform community who are pushing for heavy use of marijuana by children.

The more we have ramped up marijuana emforcement, the more we have ceded our ability to control age use of marijuana. School zones are a joke, because they have nothing to do with limiting use by children, but rather are a means of piling on charges for drug deals that didn’t involve them at all.

When I was in college in the 1970s, marijuana was illegal, but enforcement was much more lax. People regularly smoked pot in the dorms, and campus police walking by would merely sometimes ask that you close the door to limit the amount in the hallways. However, it was made very clear that if anyone sold pot to the High School students in town, there would be hell to pay. It was a clear line and one that was followed scrupulously. Oh, sure, I bet some High School students found a way to get some pot ocassionally (you’ll never be able to eliminate that entirely), but you can set up systems that say “above this age is deemed OK, and below this age is not” and make a real difference.

The best way to do that, of course, is through regulated legalization.

The one thing we know for sure from this study is that there is nothing in it that justifies arresting responsible adults.

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