Blowing Up the Big Marijuana IQ Myth - The Science Points to Zero Effect on Your Smarts

“Marijuana makes people retarded, especially when they’re young.” So claimed conservative commentator Ann Coulter while speaking at Politicon last week. 


But while such inflammatory claims by culture warriors like Coulter are to be expected – and may readily be dismissed – the notion that smoking pot will have lasting negative impacts on intelligence is a longstanding one, and a claim that is all too often made by those on both sides of the political spectrum. Yet the latest science finds little to no factual basis for this contention.

Longitudinal data just recently published online in the journal Addiction reports that pot smoking is not independently associated with adverse effects on the developing brain. A team of investigators from the United States and the United Kingdom evaluated whether marijuana use is directly associated with changes over time in neuropsychological performance in a nationally representative cohort of adolescent twins. Authors reported that “family background factors,” but not the use of cannabis negatively impacted adolescents’ cognitive performance.

They wrote: “[W]e found that youth who used cannabis … had lower IQ at age 18, but there was little evidence that cannabis use was associated with IQ decline from age 12 to 18. Moreover, although cannabis use was associated with lower IQ and poorer executive functions at age 18, these associations were generally not apparent within pairs of twins from the same family, suggesting that family background factors explain why adolescents who use cannabis perform worse on IQ and executive function tests.”

Investigators concluded, “Short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence.”

They’re not alone in their conclusions. In 2016, researchers at the University of California, Los Angeles and the University of Minnesota performed a similar longitudinal analysis regarding marijuana’s potential impact on intelligence quotient in a separate cohort of adolescent twins. They reported no dose-response relationship between pot exposure and IQ decline at age 20, and observed no significant differences in performance among those who used marijuana and their non-using twins. 

Investigators concluded: “In the largest longitudinal examination of marijuana use and IQ change, … we find little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline. … [T]he lack of a dose–response relationship, and an absence of meaningful differences between discordant siblings lead us to conclude that the deficits observed in marijuana users are attributable to confounding factors that influence both substance initiation and IQ rather than a neurotoxic effect of marijuana.”

The UCLA findings mimicked those of separate longitudinal data published earlier that year in the Journal of Psychopharmacology. Investigators in that study assessed IQ and educational performance in a cohort of 2,235 adolescent twins. They too reported that after adjusting for potential confounds (such as the use of tobacco and alcohol), teens who used cannabis “did not differ from never-users on either IQ or educational performance.”

Florida State researchers similarly examined the issue earlier this year. Writing in the journal Drug and Alcohol Dependence, they reported on the impact of marijuana exposure on intelligence scores in subjects over a 14-year period (ages 12 to 26). They concluded, “[O]ur findings did not reveal a significant association between cumulative marijuana use and changes in intelligence scores.”

Nonetheless, political opponents of cannabis policy reform continue to opine that pot smoking “reduces IQ by 6-8 points.” This claim is derived from a widely publicized 2012 New Zealand study published in The Proceedings of the National Academy of Sciences. It reported that the persistent use of cannabis from early adolescence to adulthood was associated with slightly lower IQ by age 38. 

However, a followup review of the data published later in the same journal suggested that the observed changes were the result of investigators’ failure to properly control for confounding variables, primarily the socioeconomic differences between users and non-users, and were not unduly influenced by subjects’ cannabis use history.

A later paper by the lead investigator of the New Zealand study similarly reported that the presence of confounders makes it difficult to impossible to attribute changes in teens’ academic performance on pot use alone, finding that the effects of persistent adolescent cannabis use on academic performance are “non-significant after controlling for persistent alcohol and tobacco use.”

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