Honesty check

Does heavy, chronic pot-smoking starting in adolescence damage IQ? Maybe.

Any opponent of liberalizing cannabis laws who cites the Dunedin results showing persistent IQ declines in people who start to use cannabis heavily as adolescents and keep using heavily for the next 20 years and doesn’t:

(a) point out that what’s being studied is long-term drug abuse, not the mere use of cannabis and

(b) acknowledge another analysis, published in the same journal, showing that the results of the Dunedin study may not reflect a causal link from cannabis abuse to IQ decline

is trying to persuade, not to inform.

To be clear: It’s quite possible that very heavy and very prolonged cannabis use has measurable and lasting cognitive impacts. It’s also possible that some of those impacts might stem from actual cell- and tissue-level brain changes; you’d want measurements after more than a month of abstinence to figure that out. But there’s been entirely too much “It’s a scientific fact that pot-smoking makes you stupid” around. Note that the original paper found no measurable IQ change in those who didn’t start heavy cannabis use in adolescence, even if they smoked like chimneys as adults.

Author: Mark Kleiman

Professor of Public Policy at the NYU Marron Institute for Urban Management and editor of the Journal of Drug Policy Analysis. Teaches about the methods of policy analysis about drug abuse control and crime control policy, working out the implications of two principles: that swift and certain sanctions don't have to be severe to be effective, and that well-designed threats usually don't have to be carried out. Books: Drugs and Drug Policy: What Everyone Needs to Know (with Jonathan Caulkins and Angela Hawken) When Brute Force Fails: How to Have Less Crime and Less Punishment (Princeton, 2009; named one of the "books of the year" by The Economist Against Excess: Drug Policy for Results (Basic, 1993) Marijuana: Costs of Abuse, Costs of Control (Greenwood, 1989) UCLA Homepage Curriculum Vitae Contact: Markarkleiman-at-gmail.com

16 thoughts on “Honesty check”

  1. Question Mark: Is the evidence stronger that soccer (and especially “heading”) in the game results in more brain damage than cannabis use (or in the measures being studied, kinda more like abuse)?

    I am assuming the football to cannabis comparison is clearly that the former is more dangerous…

    But in fairness to both these athletic games, haven’t a lot more studies been done about their possible damaging affects than cannabis?

    I do recall you stating that if they had evidence for cannabis like the do for soccer showing brain damage, the drug warriors would be having a field-day…


    1. In defense of Fox News:
      Is it possible that rather than Fox News making you stupid it, simply, is a magnet for the already stupid?

  2. Does a similar long term use of Ethanol reduce IQ?
    The factoid whilst I was in college was ‘each drink of alcohol kills 20,000 brain cells’.

    We pretty much knew it was B.S., but I still have to wonder.

    1. Google THIS: “not only is marijuana safer than alcohol, it may actually protect”

  3. And none of this changes the fact that something being bad for you is not a justification for it being illegal.

  4. Research done in 2005 demonstrates adult marijuana use results in neurogenesis, that it stimulates the growth of hippocampus neurons thereby reducing anxiety and depression-like behavior:


    In 2008, researchers at Ohio State were confident that marijuana reduces memory impairment by reducing inflammation and producing new cells in the brain:


    Depression and reduced memory can lower I.Q. scores, so the expectation of a research study on the topic would be that marijuana raises I.Q. scores in those who exhibit depression and who use medical marijuana. I don’t believe that study has been done, as cannabis research in the U.S. is usually performed with the hope of finding something evil about cannabinoids.

    Meanwhile, a synthetic drug that creates the same type of neurogenesis as marijuana is slated for FDA approval in about five years as a new type of anti-depressant. And it’s been discovered that the positive effects of electroshock and magnetic therapy for depression works because it results in neurogenesis. So it would be no surprise to find that memory enhancement and anti-depressive capabilities combine to give more credence to marijuana studies showing a small rise or no change in I.Q. for adult consumers, as opposed to a decrease.

  5. Thank you. I’d also like to point out (as I have before) that their sample is small, their research preliminary, and their results barely rise to the level of statistical significance. I’d also like to point out that environmental factors, toxin exposure, the use of some generally prescribed psychiatric drugs can all cause far more significant effects on tested I.Q. Linky-linky-linky: http://www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdf

    1. Oh…my point being that they failed to rule out other factors of far greater significance.

    2. I think “barely rise to the level of statistical significance” is unfair. The t-statistic for overall IQ, adjusting for education, was more than 4.

  6. …one more proof read to catch the repetitive phrase, and I’d have been good….but you get my drift. You need an edit function, Mark.

  7. Mark,

    I agree in general, but I think it’s worth pointing out that more than 5% of the NZ population aged 15-64, and more than a third of past-year users, currently fall into this ‘heavy use’ category according the NZ Health Behaviours Survey (there’s a lot of cannabis use in NZ). And while Rogeberg had a good point, and the lack of socioeconomic status adjustment in the original paper was bad, the subsequent re-analyses are at least modestly convincing.

    Using the IPCC uncertainty terminology I think I’d put the evidence for cognitive function loss from heavy teenage use as either “likely” or “about as likely not”. Before seeing the analyses I would have rated it “unlikely”. But I certainly expect both papers to get overused as propaganda.

    You’ll be pleased to know I’ve been recommending y’all’s book, which unfortunately isn’t yet in the local public libraries.

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