Study of Twins Finds No Evidence Marijuana Affects IQ

Photo Credit: Jason McELweenie

In the first study of its kind, scientists have found initial evidence that marijuana has no measurable effect on a person’s IQ. Researchers were able to determine the long-term effects of marijuana on IQ by comparing the IQs of adolescent twin siblings who either smoked marijuana or abstained from the act over 10 years. Looking at 789 pairs of adolescent twins over that amount of time, researchers found that both the marijuana smokers, and those that did not smoke marijuana, lost about four IQ points over the course of the study.

This demonstrated to researchers that something other than marijuana contributed to the loss in IQ. While the study’s lead author, Nicholas Jackson, sees this as a breakthrough, he says that more research will be needed to truly get to the bottom of marijuana’s cognitive effects on the brain. Nevertheless, Valerie Curran, a psychopharmacologist at the University College London says that “this is a very well-conducted study … and a welcome addition to the literature.”

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