Largest study yet finds heavy adolescent cannabis use actually not related to IQ decline

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May 24, 2018
San Francisco, CA

Marijuana is used more than any other recreational drug, with recent trends toward greater social and legal acceptance in some regions. Concerns remain, however, about a possible causal relationship suggested in scientific studies between marijuana use and decline in IQ, particularly among adolescent users. Other analyses have found no or inconclusive evidence for a link between IQ decline and marijuana use. However, most prior research, regardless of conclusion, has suffered from small participant cohorts, absence of pre-use data, potential influence of confounding effects, or some combination of these factors.

A new study led by scientists from UCLA and the University of Minnesota attempts to examine the link between marijuana use and IQ in a larger context. Reporting in PNAS, the authors analyze results from two longitudinal studies of twins, with data from more than three thousand individuals from Southern California and Minnesota. Participants were tested in six cognitive areas at two time points: between 9-12 years of age, with follow-up between ages 17-20. Study subjects self-reported marijuana use and frequency, and additional information, including genetic data, family background, socioeconomic status and use of other substances was collected at each encounter.

The authors focused on three criteria they proposed as measures for evidence of a direct causal relationship between marijuana use and cognitive decline. First, if marijuana use causes IQ decline, as opposed to merely being associated, then poor cognition scores should only be evident after use begins, and not before. Second, with a causal link, a dose-response relationship – that is, higher decline with heavier marijuana use – would be expected. And finally, if the relationship is causal, then the association of marijuana use and IQ decline should remain, even after genetic and social factors are taken into consideration.

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