Given the widespread liberalization of marijuana laws and huge changes in public acceptance of the drug, you might expect that by now we'd be seeing more marijuana use — and more problematic use, such addiction and dependency — among the nation's teens. But in fact the exact opposite has happened, according to a new study from Richard Grucza and colleagues at the Washington University School of Medicine in St. Louis.

The number of American teens with marijuana-related problems — such as dependency on the drug, or troubles with family and school due to marijuana use — fell by 24 percent between 2002 and 2013. The overall number of teens using marijuana fell, too. And the teens who do use marijuana are less likely to experience problems due to the drug.

"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."

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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."

If legalization opponents are to be believed, these are all the ingredients necessary for an explosion in marijuana problems among the nation's teens. So what happened?

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In looking more closely at the data, Grucza and his colleagues discovered something interesting. They found that the number of adolescents experiencing a broad array of non-drug-related conduct problems — fighting, stealing, arguing with their parents — was declining, too. And so they divided the kids experiencing marijuana-use problems into two groups: those who exhibited marijuana-use disorders alongside other conduct problems and those who had marijuana-use disorders but otherwise experienced no other conduct problems.

They found that the decline in marijuana-use disorders was concentrated almost exclusively in the kids dealing with other problems on top of their pot use: "We observed a decline in the proportion of adolescents who both reported conduct problems and met criteria for marijuana use disorders. In contrast, the proportion of adolescents with marijuana use disorders who did not report conduct problems remained relatively constant."

Researchers know that bad behavior and drug use often go hand in hand among teens. While the causality can go either way — bad behavior causes drug use or vice versa — a reduction in one usually accompanies a reduction in the other. So if teens are becoming better-behaved overall, it stands to reason that drug problems will decrease, too.

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"Other research shows that psychiatric disorders earlier in childhood are strong predictors of marijuana use later on," Grucza said in a statement. "So it's likely that if these disruptive behaviors are recognized earlier in life, we may be able to deliver therapies that will help prevent marijuana problems — and possibly problems with alcohol and other drugs, too."

Again, the research here does not at all show that liberalization of marijuana laws caused these reductions in teenage marijuana abuse. But it does strongly suggest that other factors — such as broader behavioral and mental health trends — are much more likely to drive changes in teen marijuana use than other factors, such as laws or attitudes toward pot.

Grucza's study adds to a growing body of research showing that changes to marijuana policy have had a much smaller effect on teenage drug use than once feared. A paper published in Lancet Psychiatry last year found that passing medical-marijuana laws had no effect on teen marijuana use at the state level. Other large surveys of adolescents, such as the Monitoring the Future Study, find that in recent years teen marijuana use has been flat. State-level federal survey data shows little change in teen marijuana use, even in states that have legalized it for adults.

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As a number of states consider marijuana legalization this fall, opponents are already asking: "But what about the children?" If the research by Grucza and others is any indication, the kids will be just fine.