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 == PNAS study == 

 Here's a good review in Nature of the debate over whether cannabis causes a decline in IQ or is just associated with a decline in IQ.  

 http://www.nature.com/news/pot-smokers-might-not-turn-into-dopes-after-all-1.12207 
 Nature | News: Explainer 

 Pot smokers might not turn into dopes after all 
 Revisiting data casts doubts on link between heavy cannabis use and declining IQ. 

 Arran Frood 

 14 January 2013 

 Last year, a paper published in Proceedings of the National Academy of Sciences (PNAS)1 suggested that people who used cannabis heavily as teenagers saw their IQs fall by middle age. But a study published today2 — also in PNAS — says that factors unrelated to cannabis use are to blame for the effect.  

 Ole Røgeberg, a labour economist at the Ragnar Frisch Centre for Economic Research in Oslo and the author of the latest paper, ran simulations which showed that confounding factors associated with socioeconomic status could explain the earlier result. For example, poorer people have reduced access to schooling, irrespective of cannabis use. 

 Past research on the Dunedin cohort shows3 that individuals from backgrounds with low socioeconomic status are more likely than others to begin smoking cannabis during adolescence, and are more likely to progress from use to dependence. Røgeberg says that these effects, combined with reduced access to schooling, can generate a correlation between cannabis use and IQ change. 

    

 :This should be included per WP:NPOV. If factors unrelated to cannabis are the cause, then the cohort study (Meier, 2012) would undue weight.    

 ::Refs: ABSTRACT mainstream media: CBS   "