A new study challenges previous work that found a link between marijuana use and lower IQ, but the authors of the original paper stand by their findings.
Last August, Madeline Meier of Duke University and her colleagues published the results of a study assessing the effect of marijuana use on cognition, as measured by IQ. The findings, published in the Proceedings of the National Academy of Sciences (PNAS), found that people who started using cannabis weekly before they turned 18 and continued to use heavily into adulthood lost an average of eight IQ points over that period. That’s enough to move someone with an average IQ of 100 from the 50th percentile of scores down to the 29th percentile.
The study also showed that this level of decline was seen mostly in about 8% of people who started using cannabis early and qualified for a diagnosis of marijuana addiction no fewer than three times between ages 18 and 38. The results suggested that marijuana was having a negative effect on brain development.
But now another analysis published in the same journal calls the IQ findings into question. Although the initial research involved a relatively large number of participants and controlled for factors such as alcohol dependence and schizophrenia that might also affect cognitive development, the new study suggests that the original one did not account for the effects of poverty, which can affect the way IQ changes over time. Using mathematical modeling, the new research found that because education can affect the trajectory of IQ development differently in people of different socioeconomic status, the environment, and not marijuana, may be the source of the poorer cognitive development.
“[Their] statistical models are unable to distinguish between a causal effect of cannabis on IQ-development and a non-causal correlation,” Ole Rogeberg, the author of the study and a research economist at the Ragnar Frisch Centre for Economic Research in Norway, said in an e-mail discussing the work. In the paper itself, he puts it more bluntly, “[The] estimated effect on of adolescent-onset cannabis use on IQ is likely biased and the true effect could be zero. It would be too strong to say that the results have been discredited but fair to say that the methodology is flawed and the causal inference drawn from these results is premature.”
The authors of the original paper disagree. In a statement they released by e-mail responding to the criticism, they write: “Dr. Rogeberg’s…challenge is based on simulations. We used actual data on 1,037 people to carry out the analyses he suggested. While Dr. Rogeberg’s ideas are interesting, they are not supported by our data.” Meier and her colleagues also note that in their investigation, only 23% of the participants were from lower-income families, which made it unlikely that the effect of poverty was strong enough to skew the results for the entire group. They examined both middle and low socioeconomic groups and saw IQ changes linked to marijuana use in both. “By restricting our analysis to only include children from middle-class homes, our findings of IQ decline in adolescent-onset cannabis users remain unaltered, thereby suggesting that the decline in IQ cannot be attributed to socioeconomic factors alone,” they wrote.
Still, Rogeberg claims that poor children tend to start smoking pot at a much younger age than rich or middle-class kids do. Early exposure to marijuana and other recreational drugs is linked to a higher risk of addiction, which the Duke study found connected to IQ loss.
Marijuana may also lead to higher dropout and expulsion rates from school, which may be an indirect way that marijuana is connected to IQ. “As you consider groups with higher and higher exposure to cannabis, these groups will have higher and higher shares of participants from low SES backgrounds,” says Rogeberg, who also notes that other research has not found a connection between teen marijuana use and lowered IQ.
“The issues raised by Rogeberg are those that confound all observational studies: no matter how carefully controlled a study appears to be, there are always other variables that may alter the conclusions once they are uncovered,” says Dr. Nora Volkow, director of the National Institute on Drug Abuse. “In fact, for something like IQ, it would be surprising for one variable to be 100% causal to a particular outcome.”
Neither research group is insisting that is the case; clearly IQ is affected by myriad factors that scientists are only beginning to tally. But whether marijuana is directly affecting brain development — or whether factors like school dropout or expulsion are more influential — is not yet clear.
MORE: Does Marijuana Use by Teens Really Cause a Drop in IQ?