Neil Seeman
National Post

What's a pot smoker to think? He wakes up one morning and reads three incongruous headlines: Heavy Marijuana Use Lowers IQ, Study Finds, (Canadian Press/The Toronto Star); Effect of Pot on IQ Temporary, Study Says, (National Post); and Smoking Pot No Risk to IQ, Study Says, (The Globe and Mail). No, he thinks, this isn't April Fool's; that was the day before.

How do we explain the dueling headlines? In the immortal words of Buffalo Springfield, nobody's right if everybody's wrong. The study's results, reported by Carleton professor Peter Fried in the current edition of the Canadian Medical Association Journal, were largely inconclusive owing to a number of factors, not least the small sample size.

The study followed 70 youth from birth, evaluating their IQ in their preteens, before any were introduced to drugs, and again in early adulthood. After gauging how much pot the subjects now smoke as young adults, the investigators then assessed the differentials in IQ scores from preteen years to early adulthood for the following groups: current heavy users, current light users, former users and never-users.

Those people who now smoke more than five joints a week suffered, on average, a four-point decline in IQ from their preteen scores. Does that mean that smoking dope fries your brain, as the Star headline suggests? Not necessarily. First, the heavy users started out almost 5 IQ units less intelligent to begin with, suggesting something else besides pot-smoking may be to blame. Second, a four-point drop isn't all that much. (Most people would easily trade four IQ points for one beauty point).

Former users who have now stopped and those who now smoke lightly showed gains in intelligence of 3.5 and 5.8 units, respectively. Non-users (who had never smoked pot regularly) got smarter, too, but less so. Does that mean smoking a little marijuana makes you smarter? (That, incidentally, is the theme of last year's highly underrated film, How High, starring two rappers whose super-weed boosts their IQs and gets them into Harvard.) Probably not. The study only provides readers with average IQs. The report omits standard errors and deviations for the group, and omits individual IQs, so that we don't know whether one or two people could have swung the entire group mean.

Nor can we conclude that marijuana's effects on IQ are either temporary or harmless in the long haul. Many of the user groups studied are too small (nine former users, 15 current heavy users, and nine light current users) to be meaningful. Ideally there should have been as many in each of these groups as in the 37-person "non-user" group with whom they were compared. Even if that were the case, the category of "former users" makes comparison tricky, since former users started out several IQ points lower than non-users.

None of this is to dismiss the pot study as junk science. Its strength is that individuals are compared against their former selves, which is a plus. And the research is what epidemiologists call a longitudinal prospective study; any findings are by definition preliminary. But that didn't stop a Globe and Mail editorial from using the study to warn readers of the "brutal joys of addiction."