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.