Report Revisiting data casts doubts on link between heavy cannabis use and declining IQ.


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Cannabis rots your brain — or does it? Last year, a paper published in Proceedings of the National Academy of Sciences (PNAS)1 suggested that people who used cannabis heavily as teenagers saw their IQs fall by middle age. But a study published today2 — also in PNAS — says that factors unrelated to cannabis use are to blame for the effect. Nature explores the competing claims.

Ole Røgeberg, a labour economist at the Ragnar Frisch Centre for Economic Research in Oslo and the author of the latest paper, ran simulations which showed that confounding factors associated with socioeconomic status could explain the earlier result. For example, poorer people have reduced access to schooling, irrespective of cannabis use.

The data used in the original paper came from the Dunedin Study, a research project in which a group of slightly more than 1,000 people born in New Zealand in 1972–73 have been tracked from birth to age 38 and beyond. As with all such birth-cohort epidemiological studies (also called longitudinal studies), there is a risk of inferring causal links from observed associations between one factor and another.

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