Associations between Adolescent Cannabis Use and Neuropsychological Decline: A Longitudinal Co-Twin Control Study. - PubMed - NCBI

We know from other studies that kids who abuse marijuana have lower than average IQs. What this study did for the first time was ask if the low IQ was as a result of marijuana abuse. The answer was no, actually these kids had lower than average IQ before they started abusing marijuana. So much for demon weed.