Cannabis Legalization Increased Use but Was Not Linked to Broad Psychiatric or Social Harm
A long-term study of adult twins in Colorado and Minnesota found that recreational cannabis legalization was associated with more frequent cannabis use, but not with a broad increase in substance-use problems, psychiatric symptoms or social difficulties.
The study, published in Psychological Medicine in 2023, analyzed data from 4,043 participants drawn from two longitudinal twin cohorts. Participants were first assessed during adolescence and were between 24 and 49 years old when researchers conducted the post-legalization assessment, between 2018 and 2021. About 40% were living in states with recreational cannabis laws at the time.
The researchers compared twins who lived in different policy environments, including 240 twin pairs in which one sibling lived in a recreationally legal state and the other lived in a state without recreational legalization. This co-twin design helped account for factors twins often share, including genetics, early family environment and aspects of socioeconomic background. The project is part of the Colorado and Minnesota Twin Study of Marijuana Legalization.
After accounting for earlier cannabis use and alcohol-use-disorder symptoms, the twin living in a recreationally legal state reported using cannabis more frequently than their co-twin. Legalization was not associated with cannabis-use disorder, tobacco problems, use of other drugs or most of the other outcomes examined.
The researchers also observed fewer recent alcohol-use-disorder symptoms among twins living in legal states. However, that result did not meet the study’s stricter statistical threshold for multiple comparisons, so it should be interpreted cautiously rather than as evidence that legalization reduces alcohol problems.
The analysis covered a wide range of psychiatric and psychosocial measures, including externalizing behavior, legal problems, relationship functioning, workplace behavior, civic engagement, financial distress and cognitive ability. Apart from the differences in cannabis frequency—and the tentative alcohol finding—the researchers found little evidence that recreational legalization substantially changed these outcomes.
These results do not show that cannabis is risk-free, nor do they establish that legalization has identical effects in every state or population. The sample was drawn primarily from Colorado and Minnesota, and the study measured policy exposure through participants’ residence rather than randomly assigning people to legal and illegal states. The findings also reflect the policy environments and data available through 2021, before Minnesota’s recreational market opened.
Overall, the study offers evidence against the claim that recreational legalization automatically produces widespread psychiatric or substance-use harm. Its clearest finding was narrower: legalization was linked to increased cannabis use, while the researchers found no comparable broad increase in the other adverse outcomes they measured.
