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New Study Casts Doubts On Claims That Marijuana Use Lowers IQ


study lower iq teen marijuana useWhile prohibitionists have hung their hat on a study that claimed that marijuana use by teens lowers IQ by an average of 8 points as a reason to continue prohibition, cannabis law reformers realize that the best way to keep marijuana out of the hands of teens is to take the substance out of the underground market.  Today, kids find it much easier to get a hold of marijuana than alcohol and cigarettes, demonstrating that prohibitionists are actually making it easier for kids to use marijuana by fighting cannabis regulation.  Now, a new study even calls into question whether marijuana use by teens leads to a decrease in IQ.  Regardless of how the science pans out, it is clear that young people with developing brains should refrain from recreational drugs.  However, our drug policies need to be based upon science and fact and not on emotion and fear-mongering.

From Time:

Last August, Madeline Meier of Duke University and her colleagues published the results of a study assessing the effect of marijuana use on cognition, as measured by IQ. The findings, published in the Proceedings of the National Academy of Sciences (PNAS), found that people who started using cannabis weekly before they turned 18 and continued to use heavily into adulthood lost an average of 8 IQ points over that period. That’s enough to move someone with an average IQ of 100 from the 50th percentile of scores down to the 29th percentile.


But now another analysis published in the same journal calls the IQ findings into question. Although the initial research involved a relatively large number of participants and controlled for factors such as alcohol dependence and schizophrenia that might also affect cognitive development, the new study suggests that the original one did not account for the effects of poverty, which can affect the way IQ changes over time. Using mathematical modeling, the new research found that because education can affect the trajectory of IQ development differently in people of different socioeconomic status, the environment, and not marijuana, may be the source of the poorer cognitive development.

“[Their] statistical models are unable to distinguish between a causal effect of cannabis on IQ-development and a non-causal correlation,” Ole Rogeburg, the author of the study and a research economist at the Ragnar Frisch Centre for Economic Research in Norway, said in an email discussing the work. In the paper itself, he puts it more bluntly, “[The] estimated effect on of adolescent-onset cannabis use on IQ is likely biased and the true effect could be zero. It would be too strong to say that the results have been discredited but fair to say that the methodology is flawed and the causal inference drawn from these results is premature.”

The new study makes sense to me as there are just too many factors that can affect one’s IQ to pin any increase or decrease on just one possible variable.  Also, another study demonstrated that alcohol damaged the brain tissue of teens while cannabis did not.  Teens should simply be told the truth about drugs and the available scientific developments.  Adults, likewise, should implement policies based upon science and facts, instead of holding onto a Reefer Madness mentality and cultural wars from a bygone era.  Cannabis prohibition simply doesn’t work and we are doing a disservice to our entire country by continuing a policy that leaves marijuana sales unregulated.

Cannabis prohibition leads to an unregulated, underground market, where people don’t necessarily check for identification before making a sale.  Our country has made progress decreasing cigarette and alcohol use by young people through regulation and education.  If prohibitionists truly care about our nation’s young people they should support a licensed and regulated cannabis market instead of our current prohibition model that actually increases the likelihood that underage kids have access to illegal substances.

Republished with special permission from the National Cannabis Coalition


About Author

Anthony Johnson is the director of New Approach Oregon, the PAC responsible for Measure 91, that ended cannabis prohibition for all Oregon adults in 2014. In addition to helping organize the International Cannabis Business Conference & the Oregon Medical Marijuana Business Conference, he also serves as a Board Member of the National Cannabis Coalition, working to legalize cannabis across the country and Show-Me Cannabis Regulation, an organization specifically working to end cannabis prohibition in Missouri. As President of the University of Missouri Law School ACLU Chapter, Anthony co-authored the measures that legalized medical cannabis possession and decriminalized personal possession for all adults within the city limits of Columbia, Missouri, in 2004. Following law school, Anthony practiced criminal defense for two years before working full time in the political field to help improve and protect civil liberties. You can follow Anthony on Twitter and also friend him on Facebook by following the links below as he posts mostly about civil liberties and politics with dashes of sports, music, movies and whatnot.


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