Proceedings of the National Academy of Sciences of the United States of America—otherwise known as the clinical journal with the long-ass title—published a study in 2012 that suggested “persistent cannabis users show neuropsychological decline from childhood to midlife.” The Duke University researchers utilized data from the Dunedin Study of 1,037 individuals from birth (1972 or 1973) to age 38, and they found that “persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education.” Likewise, the study concluded, “Cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users. Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain.”
In other words, smoking blunts causes a permanent drop in your IQ.
The study quickly picked up traction, and one of the many references included a Washington Post columnist on Meet the Press. In her argument against full cannabis legalization, she said, “The best evidence is that you lose—if you use marijuana as a teenager regularly—eight IQ points."
The columnist apparently missed the update. A few months after the study, the same journal published a refutal of the study’s findings.
“Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status”—published in January 2013 in the same journal—said this about the previous findings: “Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal inference drawn from the results premature.”
In a nutshell, the 2013 refutal said the earlier study did not account for confounders, that is, other circumstances that might account for lower IQs. In particular, the new study suggested “an alternative confounding model based on time-varying effects of socioeconomic status on IQ.” The study added that “heavy, persistent, adolescent-onset cannabis use involves a culture and norms that raise the risk of dropping out of school, getting entangled with crime, and other such behaviors. Unlike a neurotoxic effect, however, this effect would be nonpermanent and mediated by the cognitive demands of different environments.”
In a press release for the 2013 refutal, the lead author said, “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.”
A reviewer at Oxford University seconded this notion, stating, “The current focus on the alleged harms of cannabis may be obscuring the fact that its use is often correlated with that of other even more freely available drugs and possibly lifestyle factors. These may be as or more important than cannabis itself.”
In 2017, the Addiction journal published a study that looked at 1,989 twins involved in the Environmental Risk (E-Risk) Longitudinal Twin Study, a nationally representative birth cohort of twins born in England and Wales from 1994 to 1995. Once again, the findings showed no evidence of neuropsychological decline.
"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," the authors wrote. "Family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests."