Australasian Science: Australia's authority on science since 1938

Getting Stoned May Not Be Making Teens Stupid

A previously reported causal link between teenage marijuana use and a reduction in IQ may be erroneous. A Norwegian researcher re-examined data from the Dunedin longitudinal study of cannabis users in New Zealand and found that the decrease in IQ observed may instead be linked to lower socioeconomic status.

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“The approaches differ between these two papers, reflecting the authors’ backgrounds.

“The original paper is led by authors specialising in psychology and psychiatry. They do not consider variables such as socio-economic status (SES), and their statistical analyses rely heavily on strong and untested assumptions. For example, they assume a simple linear relationship between the duration of marijuana smoking and the change in IQ, and their results do not show clearly how confident we can be in the magnitude of the results.

“In contrast, Rogeberg, an economist who authored the more recent study*, takes SES into account, and has a more thorough approach to the statistical methods he applies. He is also clearer about the uncertainty of his results, using simulations to explore the potential effects of different assumptions.

“My perspective as a biostatistician accustomed to working in epidemiological settings is different again.

“Firstly, there is the issue of statistical power. The Dunedin study is not large, with only 874 participants in the original analysis by Meier et al., of whom just 153 ever met the criteria for cannabis dependence and 124 used cannabis regularly at one or more of the five surveys conducted. I suspect that the small sample size limited the extent to which the authors were able to take possible confounding factors, such as SES...

The full text of this article can be purchased from Informit.

*Rogeberg, O. Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status. PNAS Early Edition, 14–18 January 2013.
Source: AusSMC