Rogeberg offers a critique of a recent suggestion by Meyer et. al. of a neurotoxic effect of cannabis on developing brains that permanently lowers IQ, based on on a correlation between persistent cannabis use initiated in adolescence and a decline in IQ-scores between the ages of 13 and 38. The data come the" Dunedin cohort," 1,037 individuals followed from birth (1972/1973) to age 38 y. An alternative confounding model can be based on time-varying effects of socioeconomic status on IQ.
Does cannabis use have substantial and permanent effects on neuropsychological functioning? Renewed and intense attention to the issue has followed recent research on the Dunedin cohort, which found a positive association between, on the one hand, adolescent-onset cannabis use and dependence and, on the other hand, a decline in IQ from childhood to adulthood [Meier et al. (2012) Proc Natl Acad Sci USA 109(40):E2657–E2664]. The association is given a causal interpretation by the authors, but existing research suggests an alternative confounding model based on time-varying effects of socioeconomic status on IQ. A simulation of the confounding model reproduces the reported associations from the Dunedin cohort, suggesting that the causal effects estimated in Meier et al. are likely to be overestimates, and that the true effect could be zero. Further analyses of the Dunedin cohort are proposed to distinguish between the competing interpretations. Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal inference drawn from the results premature.