Smoking dope doesn't actually make you a dope. That's the takeaway from a new study on hundreds of pairs of twins—one of whom smoked pot as an adolescent and continued for over 10 years while the other abstained. Though a 2012 study linked marijuana use with a decline in IQ, critics say it failed to account for certain factors. Hoping to avoid such complaints, scientists at UCLA and the University of Minnesota reviewed data from 789 pairs of twins. They found marijuana users lost about four IQ points over time, but their twins showed a similar decline, reports Science. When it came to an apparent decrease in a marijuana user's vocabulary and information ability, the alteration was reduced or "completely eliminated" when researchers adjusted for a participant's reported binge drinking and use of other drugs including tobacco, which may affect cognitive development, per Meta.
Researchers also found no link between heavier or more frequent marijuana use and greater IQ decline. This fails to back up the notion that "marijuana exposure in adolescence causes neurocognitive decline," researchers say, per the Washington Post. Instead, it suggests "children who are predisposed to intellectual stagnation in middle school are on a trajectory for future marijuana use." A British researcher says she reached "broadly the same conclusions" in a recent study of 2,235 adolescent Brits. "Cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance," researchers noted in that study, concluding that modest cannabis use "is not by itself causally related to cognitive impairment." Critics say the American study did not properly question adolescents on their marijuana use. (This study revisited teen pot smokers in their 30s.)