Article by Ben Taub, IFL Science
Smoking marijuana during one’s teenage years may not have a direct impact on cognitive development, although those who do use cannabis may be more at risk of stunted intelligence due to a range of other environmental factors. This is according to a pair of studies investigating how the IQ of adolescent marijuana users compares to that of their abstinent peers, while accounting for other factors that may contribute to any differences.
The first of these appeared in the Journal of Psychopharmacology last week, and found that in a sample of 2,235 British teenagers, IQ levels were lower among those who had smoked marijuana at least 50 times by the age of 15. However, their results also revealed higher rates of childhood behavioral problems, depressive symptoms, and other substance use – particularly alcohol and cigarettes – among cannabis smokers, raising the possibility that these factors may have been responsible for their reduced IQ.
Through statistical analysis, the researchers report that cannabis use could not be used to predict reduced teenage IQ scores once these other variables had been accounted for. Interestingly, however, they discovered that even after excluding those who had never tried cannabis, cigarette smokers exhibited lower educational performance, dropping by up to two grades in any given academic subject. Since cannabis users were 17 times more likely to also smoke cigarettes, the study authors posit that cigarette use may be a more reliable predictor of reduced IQ in this group.
By way of explanation, they propose that those who smoke both tobacco and cannabis may do so due to “social adversityduring adolescence,” possibly stemming from behavioral problems or an unstable family environment. This, in turn, could have a negative effect on IQ and academic attainment. While this idea is backed up by previous research, currently it is just speculation as they have no supportive data.
Building on this research, a second study appeared in the Proceedings of the National Academy of Sciences, in which researchers examined the effects of marijuana on the intelligence of twins, where one twin was a regular user while the other was not.