Nora Volkow is the top neuroscientist in the US, and the Director of NIDA and former lead researcher at the national PET scan lab in Brookhaven. All covariants were addressed through regressional analysis and odds ratio estimates are statisically significant. This methodilogy is sound. If you dont like the small sample size try this: A recent NIDA-supported a longitudinal study conducted by researchers from Duke University. The team collected data from the Dunedin Multidisciplinary Health and Development Study that has followed 1,000 New Zealanders born in 1972. Participants were interviewed at 18, 21, 26, 32 and 38 and also underwent neuropsychological testing at ages 13 and 38.   The results were shocking. Persistent marijuana use was linked to a significant decline in IQ, (1/2 standard deviation) even after controlling for educational differences and other covariates. The most persistent users, experienced a drop in neuropsychological functioning equivalent to about six IQ points others a much as 8 points. Participants with more persistent cannabis dependence generally showed greater neuropsychological impairment.  Inspection of the means suggests that the highest impairment was found in domains of executive functioning and processing speed. To test whether impairment was relatively greater for certain domains, cannabis-associated neuropsychological impairment was compared across the four Wechsler Adult Intelligence Scale-IV (WAIS-IV) indexes (i.e., working memory index, processing speed index, perceptual reasoning index, and verbal comprehension index), which share psychometric properties. The results showed that associations between persistent cannabis dependence and the four WAIS-IV indexes could be equated without a deterioration in model fit (Δχ2 = 2.13, df = 3, P = 0.55). This suggests that impairment was not significantly different across neuropsychological domains. Average IQ is 100, or 50th percentile, marijuana use over time dropped individuals who were at the 50th to the 29th percentile. Meaning that 71% of Americans are now smarter than you.