Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status

Abstract

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. marijuana | longitudinalT o what extent does cannabis use have permanent and substantial effects on neuropsychological functioning? The question has received renewed and worldwide attention with the recent evidence of correlations between, on the one hand, persistent cannabis use initiated in adolescence and, on the other hand, a decline in IQ-scores between the ages of 13 and 38 (1). Although Meier et al. (1) note the possibility of remaining confounders, they conclude that the findings are suggestive of a neurotoxic effect of cannabis on developing brains that permanently lowers IQ. This conclusion seems premature in light of likely confounding from socioeconomic status (SES). After a brief description and discussion of the Meier et al. study design, I present evidence regarding the relationship between IQ trajectories and SES. Simulation results indicate that SES-correlated cognitive decline is sufficient to reproduce the Meier et al. results. I conclude by sketching empirical analyses that can distinguish between the causal and confounding model. Meier et al., 2012: Study Design and Methodological IssuesThe Meier et al.(1) study uses data from the high-quality Dunedin cohort, a "prospective study of a birth cohort of 1,037 individuals followed from birth (1972/1973) to age 38 y." As part of this study, participants were scored for use of-and dependence on-cannabis at ages 18, 21, 26, 32, and 38. Using these data, Meier et al. sort participants into cannabis-exposure groups: nonusers, users who never scored as dependent, and users who scored as dependent once, twice, or three or more times. Controlling for sex, ordinary least-squares regressions find IQ-declines increasing linearly with cannabis exposure (a dose-response relationship). The correlations persist within a number of subsets cleared of various possible confounders (e.g., subsamples with no alcohol dependence or no schizophrenia), and are driven ...

Prenatal exposure to cannabinoids evokes long-lasting functional alterations by targeting CB 1 receptors on developing cortical neurons

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“…In this seminal paper, the authors demonstrated a dramatic drop in intelligence for those with persistent cannabis dependence. Although Meier et al's study has been the largest and most complete longitudinal examination of IQ decline and marijuana use, there is disagreement as to whether this decline is a direct consequence of marijuana involvement or perhaps attributable to confounding variables (19,24).Although studies have demonstrated that heavy marijuana use may impact IQ test performance even a month after cessation (16,25), deficits seem to be more related to recent use rather than reflecting a permanent insult to cognition (22,26,27). Indeed, some studies find no long-term association of marijuana use and IQ (22,28) or, if so, only on measures of verbal ability (19).…”

“…In this seminal paper, the authors demonstrated a dramatic drop in intelligence for those with persistent cannabis dependence. Although Meier et al 's study has been the largest and most complete longitudinal examination of IQ decline and marijuana use, there is disagreement as to whether this decline is a direct consequence of marijuana involvement or perhaps attributable to confounding variables (19,24).…”

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