Duke University researchers recently completed a study suggesting that regular long-term marijuana use could decrease IQ by eight points. The news spread quickly, thanks in part to the current debate about legalizing the drug in the U.S. According to The Washington Post, however, other scientists questioned the research. For instance, the study only had a small sample of heavy users (38), which drew concerns about generalization. In an effort to set the record straight, researchers at University College London revisited the issue of cannabis and IQ by conducting new research.
The (lack of) cannabis effect
The UCL study drew from a larger sample than the Duke one, including 2,612 children born between 1991 and 1992. Researchers had the children take an IQ test at ages 8 and 15, and fill out a questionnaire on marijuana use at age 15. The study also accounted for other behaviors, such as alcohol consumption and smoking tobacco. The results revealed that no link between marijuana use and decreased intelligence exists.
However, the research does not let cannabis off the hook, as smoking pot was linked to other risky behaviors. Those who smoked were more likely to drink and smoke cigarettes, which did impact IQ.
"This is a potentially important public health message – the belief that cannabis is particularly harmful may detract focus from and awareness of other potentially harmful behaviors," Claire Mokrysz, lead author of the UCL study, said in a statement.
Researchers also noted that separating pot usage from other drugs, drinking alcohol and having a low IQ is difficult. It's the classic chicken-and-egg question – do the teens have a lower IQ because they smoke, or do they choose to smoke because they have a low IQ?
Educational impact
While children who smoked pot did not experience decreased IQ scores because of it, those who used the drug at least 50 times by their 15th birthday performed worse on school exams. In fact, they scored about 3 percent worse than peers who didn't smoke. Again, such results may be linked more to lifestyle factors associated with cannabis use than the drug itself.
Refocusing research
Researchers continually pointed to important trends in their study. They warned that scientists should focus less on cannabis and more on the lifestyle smokers often exhibit. Alcohol consumption, cigarette use other behaviors could be more dangerous than smoking pot, but all tend to be linked. Researchers noted that society should not overlook this kind of conduct. Alcohol consumption in particular had a greater link to IQ than smoking pot.
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