Ever since 1996, when California became the first state to recognize marijuana as a medicine, drug warriors have been warning that loosening legal restrictions on cannabis “sends the wrong message” to the youth of America, encouraging them to use a drug they would otherwise avoid. Twenty years later, with marijuana legal for medical or recreational use in two dozen states and the nation’s capital, there is little evidence that adolescents have responded in the way pot prohibitionists predicted. In fact, data from government-sponsored surveys show that teenagers are less likely to use marijuana and, if they do, less likely to abuse it than they were before this sea change in state policy.