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.
"How can we expect our children to reject drugs when some authorities are telling them that illegal drugs should no longer remain illegal, but should be used instead to help the sick?" Thomas Constantine, then head of the Drug Enforcement Administration, asked just before the California vote in 1996. "We cannot afford to send ambivalent messages about drugs."
John Walters, George W. Bush's drug czar, likewise cited the purported threat to teenagers when he urged voters to reject medical marijuana initiatives. Gil Kerlikowske, President Obama's first drug czar, took up the same theme. "We have been telling young people, particularly for the past couple years, that marijuana is medicine," he complained in 2010. "So it shouldn't be a great surprise to us that young people are now misperceiving the dangers or the risks around marijuana."
Three years later, Kerlikowske sounded the alarm again. "Young people are getting the wrong message from the medical marijuana and legalization campaigns," he told USA Today. "If it's continued to be talked about as a benign substance that has no ill effects, we're doing a great disservice to young people by giving them that message."
Kerlikowske was troubled by the rising percentage of teenagers who rejected the idea that people who smoke pot run a "great risk" of harming themselves. Since people who smoke pot do not actually run a great risk of harming themselves, he was in effect bemoaning the fact that adolescents' perceptions of marijuana had become more accurate. The less harmful teenagers believed pot to be, Kerlikowske worried, the more likely they would be to use and abuse it. The Kerlikowske Conjecture sounds plausible, but it has proven to be off the mark.
According to the Monitoring the Future Study, which surveys students in the eighth, 10th and 12th grades, the share of high school seniors who saw "great risk" in occasional marijuana use fell from 26 percent in 1996 to 16 percent in 2015. For regular marijuana use, the drop was even bigger, from 60 percent in 1996 to 32 percent in 2015. Disapproval of occasional or regular marijuana use also fell significantly. Yet during the same period, the share of high school seniors who reported using marijuana in the previous month fell slightly, from 21.9 percent to 21.3 percent. Past-year use fell from 40.2 percent to 38.6 percent.
The trends for eighth-graders and 10th-graders are similar. Perceived harmfulness is down, disapproval is down (albeit only slightly among eighth-graders), and so is marijuana use. Between 1996 and 2015, past-month use fell from 20.4 percent to 14.8 percent among 10th-graders and from 11.3 percent to 6.5 percent among eighth-graders. Past-year use fell from 33.6 percent to 25.4 percent among 10th-graders and from 18.3 percent to 11.8 percent among eighth-graders.
Results from the National Survey on Drug Use & Health (NSDUH), which began in 2002 and covers Americans 12 and older, cast further doubt on the Kerlikowske Conjecture. According to an analysis of NSDUH data published this month in the Journal of the American Academy of Child and Adolescent Psychiatry, past-year marijuana use by 12-to-17-year-olds fell from 15.8 percent in 2002 to 13.2 percent in 2013—a 16 percent drop. Even more strikingly, the prevalence of "marijuana use disorders" among adolescents fell by 24 percent, which means the risk that any teenager who used marijuana experienced serious problems as a result fell by 14 percent.
"The magnitude of the decline in the prevalence of marijuana use disorders is encouraging," Washington University epidemiologist Richard Grucza and the other authors of the study write. The researchers found that the decline could be explained by a drop in the percentage of teenagers reporting conduct problems, which are correlated with marijuana abuse.
"We found that scores on a measure of self-reported past-year conduct problems declined substantially over the 2002 to 2013 period, and that this phenomenon statistically accounted for the trend toward lower risk for marijuana use disorders," Grucza et al. say. Why conduct problems became less common during this period is not clear, although "some possible contributing causes mentioned in the existing literature include reduced lead exposure, more frequent diagnosis and treatment of childhood behavior disorders, a rise in school-based programs to prevent violence and bullying, and the emergence of state anti-bullying laws."
One thing that seems clear is that liberalization of state marijuana policies did not have the impact that Kerlikowske and his predecessors feared. "The reduction in the past-year prevalence of marijuana use disorders among adolescents took place during a period when 10 US states relaxed criminal sanctions against adult marijuana use and 13 states enacted medical marijuana policies," Grucza and his colleagues note. "During this period, teenagers also became less likely to perceive marijuana use as risky, and marijuana use became more socially acceptable among young adults."
As Kerlikowske et al. feared, less oppressive marijuana policies were accompanied by less fear and less disapproval of marijuana use (although it is hard to say which caused which). But contrary to pot prohibitionists' predictions, those changes in attitudes did not drive up adolescent marijuana use.
It is possible that easier availability of marijuana through diversion from adult buyers in states that have legalized the drug for recreational use will lead to more underage consumption. Since the first state-licensed marijuana store serving recreational consumers opened just a couple of years ago, the jury is still out on that question. But the experience of the last two decades pretty decisively refutes the idea that lies and intolerance are necessary to deter teenagers from using marijuana.
This article originally appeared at Forbes.com.