The marijuana policy landscape changed rapidly between 2002 and 2013. During that time, 13 states passed medical-marijuana laws, 10 states relaxed penalties for marijuana use, and Colorado and Washington became the first states to fully legalize recreational pot use.

Opponents of marijuana liberalization warned that these changes would bring devastating consequences, particularly for kids: "But what about the children?" was the common refrain. The president of National Families in Action, an anti-drug group, warned that commercial marijuana would "literally dumb down the precious minds of generations of children." Psychiatrist Christian Thurstone, an outspoken opponent of Colorado's marijuana legalization, argued in 2010 that "the state's relaxed laws have made the drug widely available — and irresistible — to too many adolescents."

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...