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.
http://www.washingtonpost.com/blogs/...lower-your-iq/
http://www.washingtonpost.com/blogs/...lower-your-iq/