Impacts of recreational cannabis legalization on cannabis use: a longitudinal discordant twin study

Abstract

Aims: To estimate the effect of recreational legalization on cannabis use frequency and sources of variance across legal environments.

Design: Longitudinal discordant twin and gene-environment interaction models in twins recruited from birth records and assessed prospectively.

Setting: The United States, including states with different recreational cannabis policies before and after 2014, when recreational cannabis was first legalized.

Participants: Two longitudinal, prospectively assessed samples of American twins aged 24-47 (n = 1425 in legal states, n = 1996 in illegal states), including 111 monozygotic pairs discordant for residence.

Measurements: Current cannabis use frequency (measured continuously and ordinally) was the primary outcome, and the predictor was recreational status of cannabis (legal/illegal) in the participant's state of residence at the time of assessment. Covariates include age, sex and cannabis use frequency prior to 2014.

Findings: Accounting for pre-2014 use, residents of legal states used cannabis more frequently than residents of illegal states (b = 0.21, P = 8.08 × 10^-5 ). Comparing 111 pairs of monozygotic twins discordant for residence confirmed the effect (b = 0.18, P = 0.014). There was inconclusive evidence for genetic influences on cannabis use frequency that were specific to the legal environment [χ² = 2.9 × 10^-9 , degrees of freedom (d.f.) = 1, P > 0.999]. Existing genetic influences were moderated by the legal environment, as the genetic correlation between marijuana use before and after legalization was lower in states that legalized (r_genetic = 0.24) compared with states that did not (r_genetic = 0.78, P_difference = 0.016).

Conclusions: In the United States, there appears to be a ~ 20% average increase in cannabis use frequency attributable to recreational legalization, consistent across increasingly rigorous designs. In addition, the heritability of cannabis use frequency appears to be moderated by legalization.

Keywords: Behavior genetics; causal inference; co-twin control; gene-environment interaction; natural experiment; substance use.