national Monitoring the Future surveys indicate that perceived harmfulness of marijuana has declined among adolescents since 2007–2009 ( 35), but differences between states with and without MMLs in the perceived harm due to marijuana use have not been tested. Public perceptions of the harms associated with marijuana use have varied considerably over time ( 31, 32) and such variations are consistently associated with changes in the prevalence of marijuana use ( 30, 33, 34). If so, such changes in perceptions might set the stage for subsequent increases in use, since changes in attitudes can be short-term indicators of future behavior change ( 30). One suggested partial mechanism for an association between MML and marijuana use is through changes in the perception of marijuana use MML passage has been postulated to decrease the perception of harm of marijuana use. Large-scale pre-/post-comparisons of marijuana use while taking into account other secular changes and state-level differences face substantial methodological challenges, suggesting that a fruitful approach to understanding the link, or lack of one, between adolescent marijuana use and MML may be to investigate mechanisms that might explain the relationship between MML and changes in marijuana use. Among adults, evidence is mixed for state-level MML effects across a variety of outcomes ( 20– 29). However, most studies that compare adolescents surveyed in states pre- and post-MML passage show no post-MML increase ( 16– 19), save for a recent study demonstrating a potential increase in marijuana initiation ( 20). Studies show that in states with MMLs, adolescents and adults have higher rates of marijuana use than in other states ( 13– 15). In particular, commentators have posited that more permissive marijuana legislation may lead to greater marijuana use among adolescents ( 1– 6), an age group of particular concern because neurobiology develops rapidly during adolescence ( 7– 9), and heavy marijuana use during this critical period is posited to have long-lasting adverse effects ( 10– 12). These changes have stimulated substantial discussion about potential unintended consequences of the laws. In the United States, since 1996, 23 states have legalized medical use of marijuana in some form, and as of 2015, four states have also legalized recreational use for adults. Marijuana use policy is undergoing substantial changes worldwide to include provisions for medical use.
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