Over the past 10 years, study after study has shown the damaging effect of marijuana on the teenage brain. Northwestern School of Medicine researchers reported in the Schizophrenia Bulletin in December that teens who smoked marijuana daily for about three years showed abnormal brain-structure changes. Marijuana use has clearly been linked to teen psychosis as well as decreases in IQ and permanent brain damage.

The response of those who support legalization: Teenagers can be kept away from marijuana. Yet given the dismal record regarding age-restricted use of tobacco and alcohol, success with barring teens from using legalized marijuana would be a first.

The reason such a large number of teens use alcohol and tobacco is precisely because those are legal products. The reason more are now using marijuana is because of its changing legal status—from something that was dangerous and forbidden to a product that is now considered “medicinal,” and in the states of Colorado and Washington recreational. Until recently, the illegality of marijuana, and the stigma of lawbreaking, had kept its use below that of tobacco and alcohol.