Friends' childhood adversity and long-term implications for substance misuse: a prospective Swedish cohort study

Background and aims Although an individual's childhood adversity is predictive of later substance misuse, the effect of adversity within an individual's friendship network has not been established. The current study aims to estimate the strength of the association between exposure to childhood adversity among individuals' friends at the onset of adolescence, relative to individuals' own exposure to childhood adversity, and hospitalization for substance misuse between young adulthood and retirement. Design Prospective cohort study. Setting Stockholm, Sweden. Participants Individuals born in 1953, living in Stockholm in 1963, and who nominated three best friends in the 6th grade school class (n = 7180; females = 3709, males = 3471), followed to 2016. Measurements The outcome was hospitalization with a main or secondary diagnosis attributed to substance misuse, reflected in Swedish inpatient records (ages 19-63 years). Five indicators of childhood adversity (ages 0-12 years) were operationalized into composite measures for individuals and their friends, respectively. Friendships were identified using sociometric data collected in the school class setting (age 13 years). Findings Individuals' own childhood adversity does not predict childhood adversity among friends (P > 0.05). Childhood adversity among friends is independently associated with an increased risk of an individual's later substance misuse [hazard ratio (HR) = 1.17, 95% confidence interval (CI) = 1.09-1.24], independently of an individual's own childhood adversity (HR = 1.47, 95% CI = 1.34-1.61). However, childhood adversity among friends does not moderate the association between individuals' own childhood adversity and later substance misuse. Conclusions Within a birth cohort of individuals born in 1950s Stockholm, Sweden, childhood adversity among an individual's friends appears to predict the individual's substance misuse in later life independently of an individual's own exposure to childhood adversity.