Does Smoking Pot Make You Dumber?

Once dubbed “the Devil’s lettuce”, doctors and politicians alike are beginning to see marijuana as less detrimental to a person’s health. In fact, a few states have already passed laws making the drug essentially legal. While there is much controversy over pot, one of the most frequently asked questions is “Does smoking marijuana make you dumber?” While it sounds simple, this question is extremely complex and I believe it could have a number of answers.

Null Hypothesis: Marijuana use is not at all related to a person’s IQ

Alternative Hypothesis: There are confounding factors between marijuana use and a person’s IQ


I began research on this topic and found a surplus of articles. After sifting through and getting the main ideas, I locked in on an experiment conducted at the University of London by Claire Mokrysz. In her experiment she studied data from 2,235 teens from southwest England, who made up what she called the “Children of the 90’s”. She sought out to test the relationship between how many times someone used marijuana by age fifteen, and how high they scored on an IQ test at that same age. Importantly, Mokrysz also tested these kids at age eight, before any of them had any idea what marijuana was. At first glance, the results seemed to link pot use to lower scores immediately. However, as Mokrysz explained, the kids who smoked pot at that age were also far more likely to use cigarettes, alcohol, and other drugs. For example, of the fifteen year olds categorized in the “heavy smoker” group, (those who had used 50 or more times) 84% had admitted to using cigarettes in excess of 20 times. The rate of smoking 20+ cigarettes for children who had never used pot was a mere 5%. Taking into consideration all the other factors that came with children who used marijuana before the age of fifteen, Mokrysz concluded that smoking marijuana does not lower a person’s IQ.
I found this experiment to be interesting, but questioned the limitations of Mokrysz’ work. These limitations, to me, included the young ages of participants and the relatively moderate level of pot use.

Another article I read regarding marijuana use and a person’s IQ was published earlier this year by Emily Underwood. In this article Underwood refers to an experiment in which scientists compared IQ changes among twins who either used or abstained from weed for a ten year period. They monitored 789 pairs of twins from Los Angeles and Minnesota. Each pair was enrolled sometime from the ages of nine to eleven, and were given five tests each over the ten year period. In addition, the scientists monitored alcohol and other drug use. From test four to five, the scientists noted that marijuana users went down an average of four IQ points. This would be very conclusive evidence if not for the second part- their pot-free twins regressed about the same amount during that time period! This suggests that other factors also play a role in brain development. The head of this experiment, professor Nicholas Jackson, concluded “Our findings lead us to believe that ‘something else’ it related to something about the shared environment of twins, which would include home, school, and peers.”


While these are solid, more recent studies, I still believe it is appropriate to reject the null hypothesis. Despite claims that the effects are indirect, marijuana clearly acts as a gateway drug for many teens and these results cannot be completely denied. Based on my research, I think it is safe to accept the alternative hypothesis in this instance. Marijuana use and a person’s IQ are seemingly impossible to measure accurately because there are so many other factors that go into a person’s life in the time necessary to conduct a convincing experiment. I don’t think it is safe to say marijuana has no effect on a person’s brain; but I also don’t agree with the saying “Marijuana use makes you dumber”. However, I do believe that marijuana use, particularly at a young age, often leads kids to make poor decisions that can hurt them in the long run.

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