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Whether marijuana use during adolescence affects cognitive function over time is controversial. To investigate this issue, researchers analyzed data from two separate longitudinal studies of twins. One study followed 614 ethnically diverse families from California with 1241 twins or triplets. Siblings were assessed at ages 9–10, 11–13, 14–15, 16–18, and 19–20 years. The other study included two population-based samples, enriched for twins with externalizing behaviors, comprising 2527 twins from Minnesota recruited at age 11 and evaluated every 3 years until age 18.
Mean intelligence quotient (IQ) decreased significantly more in marijuana users compared with nonusers — by about 3 to 4 points — from baseline to follow-up in both studies; howev…