“We were surprised to see substantial declines in marijuana use and abuse,” Grucza said in a statement. “Whatever is happening with these behavioral issues, it seems to be outweighing any effects of marijuana decriminalization.”

Grucza and his colleagues analyzed data from the National Survey on Drug Use and Health, an annual federal survey. Their research, forthcoming in the Journal of the American Academy of Child & Adolescent Psychiatry, was funded by the National Institute on Drug Abuse.

“The reduction in the past-year prevalence of marijuana use disorders among adolescents took place during a period when 10 U.S. states relaxed criminal sanctions against adult marijuana use and 13 states enacted medical marijuana policies,” the study found. “During this period, teenagers also became less likely to perceive marijuana use as risky, and marijuana use became more socially acceptable among young adults.”