Twins study finds no evidence that marijuana lowers IQ in teens
Roughly half of Americans use marijuana at some point in their lives, and many start as teenagers. Although some studies suggest the drug could harm the maturing adolescent brain, the true risk is controversial. Now, in the first study of its kind, scientists have analyzed long-term marijuana use in teens, comparing IQ changes in twin siblings who either used or abstained from marijuana for 10 years. After taking environmental factors into account, the scientists found no measurable link between marijuana use and lower IQ.
“This is a very well-conducted study … and a welcome addition to the literature,” says Valerie Curran, a psychopharmacologist at the University College London. She and her colleagues reached “broadly the same conclusions” in a separate, nontwin study of more than2000 British teenagers, published earlier this month in the Journal of Psychopharmacology, she says. But, warning that the study has important limitations, George Patton, a psychiatric epidemiologist at the University of Melbourne in Australia, adds that it in no way proves that marijuana—particularly heavy, or chronic use —is safe for teenagers.
Most studies that linked marijuana to cognitive deficits, such as memory loss and low IQ, looked at a single “snapshot” in time, says statistician Nicholas Jackson of the University of Southern California in Los Angeles, lead author of the new work. That makes it impossible to tell which came first: drug use or poor cognitive performance. “It's a classic chicken-egg scenario,” he says.
To better probe whether marijuana erodes IQ or inflicts harm in other ways, scientists have started following large groups of teenage drug users over time. The first study to do so, in Dunedin, New Zealand, in 2012 reported significant declines in IQ between ages 13 and 38 in heavy users compared with those who used marijuana occasionally before age 18 or not at all. The paper “had a major effect on thinking about the risks of early heavy exposure to cannabis,” says Patton, a co-author. Critics, however, pointed out that the study failed to rule out other potential explanations for the decline in IQ, such as a teen’s family environment or whether they dropped out of school.
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Marijuana users lost about four IQ points over the course of the study. But their abstinent twin siblings showed a similar pattern of decline, suggesting that the loss of mental sharpness was due to something other than pot, Jackson says. “Our findings lead us to believe that this ‘something else’ is related to something about the shared environment of the twins, which would include home, school, and peers,” he says.
More at: http://www.sciencemag.org/news/2016/01/twins-study-finds-no-evidence-marijuana-lowers-iq-teens
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