Twins study finds no evidence that Cannabis lowers IQ in teens

Jan. 18, 2016

Roughly 25% of Australians use Cannabis 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 Cannabis use in teens, comparing IQ changes in twin siblings who either used or abstained from Cannabis 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, non-twin study of more than 2000 British teenagers, published earlier this month in the Journal of Psychopharmacology.

Most studies that linked Cannabis 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 Cannabis 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 Cannabis occasionally before age 18 or not at all. 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.

One “powerful” way to address such concerns is to study identical twins, who share genes and upbringing, Jackson says. In the new study, he and his colleagues looked at 789 pairs of adolescent twins from two ongoing studies—one from the Los Angeles, California, area and the other from Minnesota—who enrolled between the ages of 9 and 11. Over the course of 10 years, the team administered five intelligence tests and confidential surveys about Cannabis use. They also asked about other drug use such as opioid painkillers, cocaine, and binge drinking.

Cannabis 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 Cannabis, 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.

In the new study, teens who reported daily Cannabis use for 6 months or longer did not show any difference in how much their IQ changed, compared with teens who had tried pot fewer than 30 times. This is a “clear indication that cannabis is unlikely to be the cause of any IQ decline,” says Claire Mokryz, a Ph.D. student in Curran’s lab.

http://www.sciencemag.org/news/2016/01/twins-study-finds-no-evidence-marijuana-lowers-iq-teens