The data on this point has been consistent enough that longtime skeptics of the merits of marijuana legalization, like Nora Volkow of the National Institute on Drug Abuse, are expressing surprise at the findings. “We had predicted based on the changes in legalization, culture in the U.S. as well as decreasing perceptions among teenagers that marijuana was harmful that [accessibility and use] would go up,” Volkow told U.S. News and World Report earlier this month. “But it hasn’t gone up.”

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However, a study out Tuesday in the journal JAMA Pediatrics flies somewhat in the face of the new conventional marijuana wisdom. Examining marijuana use among high school students in Washington state two years before and after the vote to legalize in 2012, it finds that rates of marijuana use increased by about 3 percent among 8th- and 10th-graders over that period.

The authors posit that reduced stigma about marijuana use is one factor leading to the results that they observed.

“Our study suggests that legalization of marijuana in Washington reduced stigma and perceived risk of use,” said lead author Magdalena Cerdá of the University of California in Davis in a news release, “which could explain why younger adolescents are using more marijuana after legalization.”

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The findings are something of a puzzle. The study found no change in marijuana use among 12th-graders in Washington state, which the authors said could be because the 12th-graders in the study were old enough that “they had already formed attitudes and beliefs related to marijuana use” before the legal change.

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The study also found no change in use among students at any grade level in Colorado. The authors write that Colorado had a robust medical marijuana industry in place well before full legalization, which may have affected youth attitudes and behaviors there before the study period.

Here's a summation of what they found in chart form. It compares past-month marijuana use rates among 8th-, 10th- and 12th-graders in Washington (gray), Colorado (white) and states where recreational marijuana is illegal (dark blue).

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In an email, Kleiman pointed out that in Washington state, the recreational marijuana market didn't open until halfway through 2014, and then only in limited form. That's halfway through the “after” period (2013 to 2015) in the JAMA Pediatrics study.

“The effect of the [legalization] initiatives themselves on price and availability of cannabis really wasn't felt until after” the study's surveys were done, Kleiman said. “Any measured effect would be more likely the result of the political campaign around legalization than legalization itself.”

Indeed, the study's authors agree with that assessment. “Simply legalizing an activity can change people's views about it and can change their behaviors as well,” said co-author Deborah Hasin of Columbia University in an email.

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Still, the measured effect in the study is small — a 2 percent increase for eighth-graders and a 4 percent increase for 10th-graders. Given the small magnitude of the findings and the lack of effect among either 12th-graders or students in Colorado, Kleiman said it simply “remains too early” to say anything conclusive about the effect of recreational marijuana laws on teen marijuana use.

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The authors of the JAMA Pediatrics study said that, given the findings, states that legalize marijuana should also invest in substance-abuse-prevention programs for teens.

Kleiman said there's an even easier way to ensure that adolescent marijuana use remains at a minimum level — make sure marijuana doesn't become too cheap.

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“There's reason to think that adolescents are more price-sensitive than adults with respect to cannabis use,” he said, “so I'd advise states that legalize to do what they can to keep prices from falling.”