Timberlake said cannabis helped him relax during times when endless responsibilities swirl around in his head, which is a far cry from the stoner stereotype that depicts amotivational Jeff Spicoli types banging their heads with a shoe. That stoner stereotype, however, sometimes gets a public boost, which happened a year after Timberlake’s confession.
In August 2012, Proceedings of the National Academy of Sciences of the United States of America (PNAS) published “Persistent cannabis users show neuropsychological decline from childhood to midlife,” a Duke University-led study that claimed chronic teen cannabis use led to a permanent drop in IQ points. This bombshell study led to headlines like “Drop in IQ linked to heavy teenage cannabis use” (US News & World Report), “Teens Beware: Using marijuana could make you dumber” (Wired) and “Teen cannabis use may damage brain for life” (The Independent). A Washington Post columnist even went on Meet the Press and said, “The best evidence is that you lose—if you use marijuana as a teenager regularly—eight IQ points.” The study spread like wildfire providing a scientific basis, many argued, for the decades-old argument that smoking cannabis makes you dumb.
The researchers, led by Dr. Madeline Maier, examined data from the Dunedin Longitudinal Study, which has been tracking 1,037 individuals born at the same maternity hospital in Dunedin, New Zealand between April 1, 1972 and March 31, 1973. The study members, who are reassessed every few years, include a near-identical number of men and women, plus a dozen pairs of twins. The study provides nearly half a century of data, and researchers commonly analyze the information to look for health trends and correlations. The PNAS study, which looked at data through age 38, found that “persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education” and that “Cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users.”
Cannabis, according to the researchers, likely had “a neurotoxic effect … on the adolescent brain,” and the average teen smoker lost eight IQ points by age 38. This landmark study is still widely cited by Drug War advocates today, but do the findings hold up in scientific circles? Point in fact, the same journal published a strong rebuttal mere months later.
Approved in December 2012 and published in early 2013, PNAS published a follow-up that concluded, “Although it would be too strong to say that the results [of the previous study] have been discredited, the methodology is flawed and the causal inference drawn from the results premature.” The rebuttal, titled “Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status,” said the Maier-led study was “likely biased” and criticized the research because it “only controlled for sex” in its analysis.
“The risk with this approach is that it overestimates the effect of cannabis use if cannabis use is correlated with a number of confounders that have small individual, but large collective, impact,” wrote the authors of the second study. “For example, the researchers could run a regression of adult IQ on childhood IQ, cannabis exposure, childhood traits and environmental characteristics, educational level, occupational status (as a proxy for a job’s cognitive demands), and employment history.”
Likewise, the study noted that teen cannabis use is commonly associated with “a culture and norms that raise the risk of dropping out of school, getting entangled with crime, and other such behaviors.” In other words, a teen who chronically smokes cannabis is probably not in the math club and could possibly be self-medicating an undiagnosed disorder or dealing with socioeconomic hardship. In such cases, chronic use is likely a symptom of deeper issues that bear responsibility for the neuropsychological decline.
“This is a potentially important public health message: The belief that cannabis is particularly harmful may detract focus from and awareness on other potentially harmful behaviors,” wrote the lead author in a press release.
A clinical reviewer at Oxford University added, “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.”
The point: Blaming cannabis for such ills can be dangerous as it obscures the real problems.
More studies followed that contradicted the 2012 findings, including the following:
Journal of Psychopharmacology (2016): “Findings [from more than 2,000 adolescents] 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.”
Medical Research Council (2014): “The findings [from a different longitudinal study] do not support the [2012 PNAS study] hypothesis that cannabis use in adolescence leads to persistent decline in cognitive functioning, once other possible confounding variables are accounted for…. The analyses therefore do not support the findings of Meier et al.”
PNAS (2015): “Marijuana-using twins failed to show significantly greater IQ decline relative to their abstinent siblings. Evidence from these two samples suggests that observed declines in measured IQ may not be a direct result of marijuana exposure but rather attributable to familial factors that underlie both marijuana initiation and low intellectual attainment.”
A 2013 study in Schizophrenia Research, interestingly enough, actually found that psychosis patients who smoke cannabis had “significantly higher” IQs than those who don’t. Psychiatry Research published a similar study in 2015 that found no difference in IQ between youth who smoke or abstain, but it did find that youth at high risk for psychosis who start smoking late showed “significantly higher IQ.”
Several studies suggest that alcohol, not cannabis, is the real IQ threat. Still, it begs the same question: A teen alcohol abuser likely has multiple issues that contributed to his or her neuropsychological decline, so how does one truly pinpoint the cause? Any type of extreme substance use, even overeating and vitamins, can involve health risks and suggest the possible presence of confounding issues. The same can be said of chronic behavior—gaming, social media, shopping, gambling—which potentially involve process addictions and adverse effects on developing teen minds that might otherwise focus on academics and athletics.
Another Dr. Meier-led study published in 2016 highlights similar difficulties in differentiating between correlation and causation. Three years after her controversial PNAS study, the former Duke researcher—now at Arizona State University—published another cannabis study based on Dunedin data in the Journal of the American Medical Association (JAMA) Psychiatry. The study concluded, “Our findings showed that cannabis use over 20 years was unrelated to health problems in early midlife. Findings [also] showed that cannabis use was associated with slightly better metabolic health (smaller waist circumference, lower body mass index, better lipid profiles, and improved glucose control).”
In every metric but one (i.e., gum health), cannabis users had equal or better physical health than people who abstained. In particular, long-term cannabis users were less likely to be overweight. Does this mean cannabis makes people skinny and helps prevent a wide range of weight-related health issues? Or does it suggest cannabis often replaces alcohol consumption thus reducing a person’s caloric intake? Or is there another option? Unfortunately, government restrictions on cannabis research make it difficult to identify much more than correlations and associations.
So, does cannabis make you dumb? The evidence suggests no, at least not directly. A teen who smokes all day everyday will likely see an IQ drop, but the behavior, not the plant, would be the culprit, and the behavior suggests other issues where health professionals should focus. Otherwise, if basing theories purely on disputed corollary data, one could argue there’s just as much evidence that people lose IQ points by chronically watching Fox & Friends.