Recreational cannabis legalization has had limited effects on a wide range of adult psychiatric and psychosocial outcomes.

Background: The causal impacts of recreational cannabis legalization are not well understood due to the number of potential confounds. We sought to quantify possible causal effects of recreational cannabis legalization on substance use, substance use disorder, and psychosocial functioning, and whether vulnerable individuals are more susceptible to the effects of cannabis legalization than others.

Methods: We used a longitudinal, co-twin control design in 4043 twins ( = 240 pairs discordant on residence), first assessed in adolescence and now age 24-49, currently residing in states with different cannabis policies (40% resided in a recreationally legal state).