Study Posits Link Between Marijuana Use, IQ Drop May Not Exist


A new study reports that smoking marijuana during adolescence may not lead to declines in the smokers’ intelligence. Instead, say the scientists, pot smoking could be a symptom of “something else” that is responsible for the effect to the brain that has been noted in previous research, reports The Associated Press.

What that “something else” may be, according to Joshua Isen, one of the authors of the report, is not yet known. But the adolescent who is prone to smoke marijuana “is probably going to show this IQ drop regardless of whether he or she is actually smoking pot,” said Isen, a lecturer in psychology at Loyola Marymount University in Los Angeles.

Released by the Proceedings of the National Academy of Sciences and published in the Journal of Psychopharmacology, the study differs from previous research that suggested the developing brains of teenagers are especially defenseless against damage from marijuana.

Analyzing this thesis is problematic because it would be unethical to ask children to take illegal drugs or to ask them to refuse marijuana for a period of years in order to compare their outcomes. The only way scientists can study the situation is to assess people who choose to use or abstain from using cannabis based on their personal lifestyles.

The authors used two large collections of information regarding sets of twins for the study. Participants numbered 3,066 and were given intelligence tests at ages nine to 12, before any had smoked marijuana, and at ages 17 to 20. Researchers tracked test score changes and whether the changes were worse for marijuana users than for those who did not use pot.

Most of the tests resulted in no differences between the two groups, but tests on vocabulary and general knowledge showed lower scores for users than for those who had abstained from using.

David Downs of The San Francisco Chronicle writes that drug warriors have used information from a New Zealand study published in 2012 that found long-time pot smokers had an eight-point IQ drop. This data, he notes, has been used to support policies such as arresting young people, saddling them with a criminal record, suspending them from school, expelling them from school, and forcing them into mandatory rehab programs where they learn about hard-core drugs.

Further research, however, found that lower lifetime IQ can be blamed on poverty more than marijuana. Another study in 2015 found that cannabis alone does not affect academic performance when testing controls for persistent alcohol and tobacco use were employed.

As for this latest study, the researchers conclude:

“These findings suggest that adolescent cannabis use is not associated with IQ or educational performance once adjustment is made for potential confounds, in particular adolescent cigarette use. Modest cannabis use in teenagers may have less cognitive impact than epidemiological surveys of older cohorts have previously suggested.”

Weed may cause episodes of poor judgment, but co-author Claire Mokrysz said the study demonstrated the long-held notion that cannabis use can lower a person’s IQ is not as clear-cut as some may think.

In another hypothesis from the study, Mokrysz explained that people who use marijuana also are likely to have higher instances of childhood behavioral problems, depressive symptoms, and other substance abuse such as tobacco and alcohol. The report also pointed out that cannabis users were more likely to have mothers who used marijuana during pregnancy.

It is possible that some of the differences in lifestyles and personalities between cannabis users and non-users account for the dissimilarities in IQs, not their pot smoking, reports Jordyn Taylor for Tech.Mic.