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.
Associations between Adolescent Cannabis Use and Neuropsychological Decline: A Longitudinal Co-Twin Control Study. AIMS: This study tested whether adolescents who used cannabis or met criteria for 'cannabis dependence' showed neuropsychological impairment prior to cannabis initiation and neuropsychological decline from before to after cannabis initiation. DESIGN: A longitudinal co-twin control study. SETTING AND PARTICIPANTS: Participants were 1,989 twins from the Environmental Risk (E-Risk) Longitudinal Twin Study, a nationally representative birth cohort of twins born in England and Wales from 1994-1995. MEASUREMENTS: Frequency of cannabis use and 'cannabis dependence' were assessed at age 18. Intelligence quotient (IQ) was obtained at ages 5, 12 and 18. Executive functions were assessed at age 18. FINDINGS: Compared with adolescents who did not use cannabis, adolescents who used cannabis had lower IQ in childhood, prior to cannabis initiation and had lower IQ at age 18, but there was little evidence that cannabis use was associated with IQ decline from age 12-18. For example, adolescents with 'cannabis dependence' had age-12 and age-18 IQ scores that were 5.61 (t=-3.11, p=.002) and 7.34 IQ points (t=-5.27, p<.001) lower than adolescents without 'cannabis dependence', but adolescents with 'cannabis dependence' did not show greater IQ decline from age 12-18 (t=-1.27, p=.20). Moreover, adolescents who used cannabis had poorer executive functions at age 18 than adolescents who did not use cannabis, but these associations were generally not apparent within twin pairs. For example, twins who used cannabis more frequently than their co-twin performed similarly to their co-twin on 5 of 6 executive function tests (ps>.10). The one exception was twins who used cannabis more frequently than their co-twin performed worse on one working memory test (Spatial Span Reversed; β=-0.07, p=.036). CONCLUSIONS: Short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence. Family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests. https://www.ncbi.nlm.nih.gov/pubmed/28734078