Dope-smoking study questioned

A study last year found that dope-smoking had negative effects:

Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education. Informants also reported noticing more cognitive problems for persistent cannabis users. Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline. Further, cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users. Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents.

This study has now been criticized by statisticians who argue that not all of the confounders were factored out. That is, maybe some personality trait led to both the dope-smoking and the cognitive decline.