The “stoner” stereotype isn’t just a myth. Heavy use of marijuana in the teenage years dulls intelligence—and the loss of IQ points may last for life, according to a new study. Researchers tested the IQs of more than 1,000 New Zealand schoolchildren at age 13, before most had tried marijuana, and again at age 38. In between, they regularly surveyed each subject about his or her drug use.They found that those who had started smoking pot regularly before age 18, and continued into adulthood, lost an average of eight IQ points—the difference between scoring in the 50th percentile (average) and the 29th (well below average).
Those who waited until after age 18 to make a habit of weed showed no drop in intelligence. “Our hypothesis is that we see this IQ decline in adolescence because the adolescent brain is still developing,” Duke University researcher Madeline Meier tells Time .com. She says marijuana’s active ingredients may interfere with that development, with negative effects on memory and the ability to plan. It’s bad news for American Toking their way to lower IQ’s teens, who are smoking pot more than ever before. In a recent survey, 23 percent of high school students said they’d recently smoked pot, and about one in 10 gets high 20 times or more per month.