A new report from the Marijuana Policy Project, tied to the expected release this week of the latest numbers from the Monitoring the Future Study of drug use by students, concludes that "marijuana prohibition has not curbed marijuana use by young people." I think this may overstate what the evidence shows, but various kinds of data reviewed in the report do indicate that variations in drug policy have little or no impact on pot smoking by teenagers:
1) The number of marijuana arrests in the U.S. has risen dramatically since the early 1990s, but reported availability of marijuana among high school seniors has remained more or less flat (with about 85 percent saying pot is "easy to get").
2) States that have "decriminalized" marijuana possession (which in the U.S. generally has meant replacing criminal penalties with modest fines) do not have significantly higher rates of adolescent marijuana use. This is true of Australia as well as the U.S.
3) The Netherlands did not see an increase in teenage marijuana use for years after it started allowing the open sale of cannabis by "coffeeshops" in 1976, and even today the rate there is substantially lower than in the U.S. according to most surveys (about the same according to one).
4) Since Britain essentially stopped arresting people for marijuana possession in 2004, pot smoking among 16-to-19-year-olds has dropped.
5) Marijuana use in the U.S. is much more common today than it was prior to the federal ban on marijuana in 1937.
The last point, which is based on retrospective data from surveys that asked Americans how old they were when they first tried marijuana, is probably the weakest, if it is meant to support the conclusion that "marijuana prohibition has not curbed marijuana use by young people." Leaving aside possible problems with the data, drug warriors can always argue that marijuana use would have risen even more without prohibition. "During the era of marijuana prohibition," MPP estimates, "use of marijuana by Americans under 35 (who have traditionally been the largest proportion of users) increased by more than 4,000%." That sounds like impressive evidence of prohibition's failure, and MPP suggests that the "forbidden fruit" effect, plus reaction against anti-drug propaganda, may have made teenagers more likely to smoke pot than they would have been had marijuana remained legal.
Since the 1930s, the U.S. has gone from a situation where smoking cannabis as an intoxicant was virtually unknown (some legislators who voted for the ban had never even heard of the drug) to one where, according to the 2005 National Survey on Drug Use and Health, 40 percent of Americans 12 or older have tried marijuana. But only 6 percent of the respondents in that survey said they'd used cannabis in the previous month, compared to more than 50 percent who said they'd consumed alcohol. Without prohibition, maybe the rate of past-month marijuana use would be closer to the rate of past-month drinking, among teenagers as well as adults. That possibility obviously cannot be addressed with historical data.
Still, the comparisons across states, across countries, and across shorter periods of time in the U.S. (looking at variations in drug law enforcement, as opposed to the impact of prohibition itself) make a pretty compelling case that significant changes in marijuana policy do not have a noticeable effect on marijuana use by teenagers. When drug warriors oppose liberalization of marijuana laws in the name of protecting children, they should not benefit from a presumption that marijuana use by teenagers will rise if the government is more tolerant of adult pot smokers.