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Background: Cannabis use is associated with outcomes like income, legal problems, and psychopathology. This finding rests largely on correlational research designs, which rely at best on statistical controls for confounding. Here, we control for unmeasured confounders using a longitudinal study of twins. Method: In a sample of 4,078 American adult twins first assessed decades ago, we used cotwin control mixed effects models to evaluate the effect of lifetime average frequency of cannabis consumption measured on substance use, psychiatric, and psychosocial outcomes. Results: On average, participants had a lifetime cannabis frequency of about one to two times per month, across adolescence and adulthood. As expected, in individual-level analyses, cannabis use was significantly associated with almost all outcomes in the expected directions. However, when comparing each twin to their cotwin, which inherently controls for shared genes and environments, we observed within-pair differences consistent with possible causality in three of the 22 assessed outcomes: cannabis use disorder symptoms (βW-Pooled =.15, SE =.02, p = 1.7 × 10−22), frequency of tobacco use (βW-Pooled =.06, SE =.01, p = 1.2 × 10−5), and illicit drug involvement (βW-Pooled =.06, SE =.02, p = 1.2 × 10−4). Covariate specification curve analyses indicated that within-pair effects on tobacco and illicit drug use, but not cannabis use disorder, attenuated substantially when covarying for lifetime alcohol and tobacco use. Conclusions: The cotwin control results suggest that more frequent cannabis use causes small increases in cannabis use disorder symptoms, approximately 1.3 symptoms when going from a once-a-year use to daily use. For other outcomes, our results are more consistent with familial confounding, at least in this community population of twins. General Scientific Summary: This study suggests that lifetime exposure to cannabis has few persistent effects on mental health and other psychosocial outcomes. The notable exceptions are cannabis use disorder, tobacco frequency, and illicit drug use, for which lifetime cannabis frequency causes small increases. [ABSTRACT FROM AUTHOR]

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 (N = 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). We tested the effect of legalization on outcomes of interest and whether legalization interacts with established vulnerability factors (age, sex, or externalizing psychopathology). Results: In the co-twin control design accounting for earlier cannabis frequency and alcohol use disorder (AUD) symptoms respectively, the twin living in a recreational state used cannabis on average more often (β w = 0.11, p = 1.3 × 10−3), and had fewer AUD symptoms (β w = −0.11, p = 6.7 × 10−3) than their co-twin living in an non-recreational state. Cannabis legalization was associated with no other adverse outcome in the co-twin design, including cannabis use disorder. No risk factor significantly interacted with legalization status to predict any outcome. Conclusions: Recreational legalization was associated with increased cannabis use and decreased AUD symptoms but was not associated with other maladaptations. These effects were maintained within twin pairs discordant for residence. Moreover, vulnerabilities to cannabis use were not exacerbated by the legal cannabis environment. Future research may investigate causal links between cannabis consumption and outcomes. [ABSTRACT FROM AUTHOR]

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 (rgenetic = 0.24) compared with states that did not (rgenetic = 0.78, Pdifference = 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. [ABSTRACT FROM AUTHOR]

Objective: Prior literature indicates that nontraditional attitudes are linked to higher intelligence. However, such attitudes in adolescence often accompany counter‐normative, delinquent‐type behaviors, which are themselves negatively linked with intelligence. This points to the possibility of suppression in the relationship between intelligence and nontraditional attitudes. Method: We analyzed a large community sample of 17 year olds (N = 3330) with data on intelligence, nontraditional attitudes, and a diverse collection of self‐ and teacher‐reported counter‐normative behaviors. Developmental questions for these relationships were examined through cross‐sectional comparisons between the adolescents and their parents as well as longitudinal analysis of the adolescent sample across emerging adulthood. Results: Youth who endorsed nontraditional attitudes had lower school grades, earlier age at first sex, heavier substance use, and were perceived as more oppositional by their teachers. Each of these problem behaviors was inversely related to intelligence. Accordingly, the positive correlation between nontraditional attitudes and intelligence was much weaker in adolescents as compared to their middle‐aged parents. Longitudinal analyses revealed that the association between nontraditional attitudes and intelligence strengthens in early adulthood. Conclusions: Associations between intelligence and sociopolitical attitudes can be obscured even by seemingly distal psychological characteristics. [ABSTRACT FROM AUTHOR]