A prospective co-twin control analysis of internalizing and externalizing pathways for alcohol, nicotine, and cannabis use problems from adolescence through adulthood.

BACKGROUND: Alcohol, nicotine, and cannabis use problems have a poorer prognosis when they co-occur with internalizing distress (INT). One hypothesis to account for the association between INT and alcohol and substance use problems (SUP) is that people engage in heavy substance use to reduce aversive affective states characteristic of INT (e.g. affective reinforcement). Whether INT operates as a cause or a shared liability with SUP (e.g. shared genetic and environmental risk) over and above an alternative cause (e.g. externalizing problems, EXT) remains unclear.

METHODS: We tested INT as a developmental pathway for SUP by applying a prospective, co-twin control approach to the Minnesota Twin and Family Study sample (n = 1881 twin pairs). A multilevel modeling framework was used to estimate the between-twin pair effect and the within-twin pair effect of INT over and above EXT on separate SUP outcomes (i.e. alcohol, nicotine, and cannabis use problems) from adolescence (age 14) to young adulthood (age 29).

RESULTS: We failed to detect any non-familial causal effects of INT on SUPs. Instead, their associations were due to common genetic and shared environmental influences. In contrast, EXT had consistent non-familial causal influences on alcohol and cannabis use in adolescence and young adulthood.

CONCLUSIONS: The link between INT and SUPs is primarily due to shared, familial influences while EXT likely serves as a unique determinant, with elevated EXT increasing selection into situations that pose SUP risk (e.g. peer environments that normalize and sustain heavy use) over and above common genetic influences.