Oh oh here is a reply to the above article.
http://www.pnas.org/content/110/11/E980.extract
We reported that persistent cannabis use was associated with neuropsychological decline, from adolescence to midlife (1). Two commentators suggested alternative explanations; we tested these and report the results here.
Rogeberg (2) wonders whether socioeconomic differences explain the association between cannabis and neuropsychological decline. His argument is based on his assumption that cannabis use is more common in youngsters of low socioeconomic status (SES). He also believes that the intelligence quotients (IQs) of low-SES children are temporarily boosted by schooling but that when they leave school and choose their own niches, their IQs rebound to their former low baseline. If many cannabis users were low-SES children, Rogeberg says this coincidence would create the false impression that cannabis use was responsible for their IQ drop in adulthood.
Rogeberg’s (2) idea and simulated data are interesting, but actual data exclude the possibility that the IQ drop we observed was attributable to SES differences. First, adolescent cannabis users are not concentrated in the lower classes; cannabis is used by young people from all social strata. In the Dunedin cohort, low SES did not significantly predict adolescent-onset cannabis dependence (χ2 = 1.15;
P = 0.56); only 23% of the adolescent cannabis users were from low-SES families (whose breadwinners had low-skill occupations such as foodpacker), making it unlikely that low SES explains why adolescent-onset cannabis users’ IQs decline. Second, as previously reported (3), the IQ scores of children from low-SES families did not change from the beginning of schooling to adolescence, nor did they change from adolescence to adulthood