Now, a new study out from the University College of London provides even stronger evidence that the Duke findings were flawed. The study draws on a considerably larger sample of adolescents than the Duke research – 2,612 children born in the Bristol area of the U.K. in 1991 and 1992. Researchers examined children’s IQ scores at age 8 and again at age 15, and found “no relationship between cannabis use and lower IQ at age 15,” when confounding factors – alcohol use, cigarette use, maternal education, and others – were taken into account. Even heavy marijuana use wasn’t associated with IQ.
“In particular alcohol use was found to be strongly associated with IQ decline,” the authors write. “No other factors were found to be predictive of IQ change.”
The UK study does find evidence, however, of slightly impaired educational abilities among the very heaviest marijuana users. This group of students scored roughly 3% lower on school exams taken at age 16, even after adjusting for confounding factors.
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