Young Brains on Weed

Via the BBC:  Young cannabis smokers run risk of lower IQ, report claims

An international team found those who started using cannabis below the age of 18 – while their brains were still developing – suffered a drop in IQ.


For more than 20 years researchers have followed the lives of a group of people from Dunedin in New Zealand.

They assessed them as children – before any of them had started using cannabis – and then re-interviewed them repeatedly, up to the age of 38.

Having taken into account other factors such as alcohol or tobacco dependency or other drug use, as well the number of years spent in education, they found that those who persistently used cannabis – smoking it at least four times a week year after year through their teens, 20s and, in some cases, their 30s – suffered a decline in their IQ.

The more that people smoked, the greater the loss in IQ.

The effect was most marked in those who started smoking cannabis as adolescents.

The average decline was 8 points for persons who started young and continued to smoke regularly.  Ceasing smoking did not restore said points.

Intuitively, it make sense such regular use of the substance, especially from a young age, would have deleterious effects.  One thing that does strike me, however, is that that this seems to treat IQ points as a static measure, but that is not the case.   Still, one presumes that the researchers have a firmer grasp on what they are measuring and how than do the journalists doing the write up.  At a minimum, it seems that pre-adult indulgence in any number of substances is a bad idea, both because of maturity issues and brain development ones.

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Steven L. Taylor
About Steven L. Taylor
Steven L. Taylor is a Professor of Political Science and a College of Arts and Sciences Dean. His main areas of expertise include parties, elections, and the institutional design of democracies. His most recent book is the co-authored A Different Democracy: American Government in a 31-Country Perspective. He earned his Ph.D. from the University of Texas and his BA from the University of California, Irvine. He has been blogging since 2003 (originally at the now defunct Poliblog). Follow Steven on Twitter


  1. Reminds me of the dopey kid down the street who borrows my tools ….

    For what it’s worth Steven, I think you are on to something, that IQ might be more malleable in general. See also how beautiful people don’t have to study so hard, etc.