Background Children whose parents misuse alcohol have increased risks of own alcohol misuse in adulthood. Though most attain lower school marks, some still perform well in school, which could be an indicator of resilience with protective potential against negative health outcomes. Accordingly, the aim of this study was to examine the processes of mediation and interaction by school performance regarding the intergenerational transmission of alcohol misuse.

Methods Data were drawn from a prospective Swedish cohort study of children born in 1953 (n=14 608). Associations between parental alcohol misuse (ages 0–19) and participants' own alcohol misuse in adulthood (ages 20–63) were examined by means of Cox regression analysis. Four-way decomposition was used to explore mediation and interaction by school performance in grade 6 (age 13), grade 9 (age 16) and grade 12 (age 19).

Results Mediation and/or interaction by school performance accounted for a substantial proportion of the association between parental alcohol misuse and own alcohol misuse in adulthood (58% for performance in grade 6, 27% for grade 9 and 30% for grade 12). Moreover, interaction effects appeared to be more important for the outcome than mediation.

Conclusion Above-average school performance among children whose parents misused alcohol seems to reflect processes of resilience with the potential to break the intergenerational transmission of alcohol misuse. Four-way decomposition offers a viable approach to disentangle processes of interaction from mediation, representing a promising avenue for future longitudinal research.

Health risk behaviors shape, and are shaped by, the people with whom we interact throughout our lives. The use of substances, including alcohol or narcotics, is one such behavior, yet it is often empirically examined in isolation of other people. As such, it has been repeatedly shown that excessive alcohol consumption, illicit drug use, and the extra-medical use of prescription narcotics are, collectively, a leading cause of preventable injury, severe illness, and premature death. However, by not considering that substance use which leads to harms (hereafter, substance misuse) is inseparable from social contexts, the burden of substance misuse may be greatly underestimated. Drawing on a Swedish birth cohort from 1953, and centered on the concept of interdependent—linked—lives, this thesis encompasses four empirical studies of social relationships and substance misuse. Each study examines the association between substance misuse and friends and parents, siblings, spouses, and offspring, respectively. Study I explored whether childhood adversity in two contexts, within the family of origin and among one’s adolescent friends at age 13, was a potential risk factor for later substance misuse. The study found independent associations between substance misuse and childhood adversity in both contexts. Study II estimated differences in substance misuse between siblings in midlife according to birth order, and showed that substance misuse during this life course period may be better explained by factors within the family of origin. Study III examined the effects of substance misuse on nearly 50 years of marital status transitions. The results suggest that a lifetime history of substance misuse affects marital status transitions, and that substance misuse may increase the risk of marital dissolution. Finally, in investigating changes in parental psychotropic medication use in relation to their offspring’s hospitalization for substance misuse, Study IV demonstrated that such stressful life events as an offspring’s hospitalization for narcotics use may have a detrimental impact on maternal mental health. The findings from the thesis underscore that factors within the families of both origin and destination have the potential to affect, and be affected by, substance misuse throughout the life course. The thesis concludes substance misuse neither starts, nor ends, with the individual. Accordingly, public health efforts could benefit from approaching substance misuse from the perspective of linked lives.

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.

Higher birth order is associated with increased risks of adverse health outcomes attributable to alcohol or narcotics in adolescence, but it remains unclear whether these observed birth order effects are also present in midlife. Drawing on a national Swedish cohort born in 1953 and their siblings, we estimate associations between birth order and alcohol- or narcotics-attributable hospitalization or death with a 25-year follow-up to assess whether birth order differences are observed during this life course period. Health events attributable to alcohol or narcotics use were identified using the Swedish National Patient and Cause of Death registers, respectively. Weapply Cox proportional hazards models to estimate average birth order differences in hazards for alcohol- or narcotics-attributable hospitalization or death between ages 30 and 55. We estimate birth order differences between families, and use two fixed-effects approaches to estimate birth order differences within families and within families of the same type. Bivariate results indicate increased hazards for both outcomes with higher birthorder; however, these results are no longer observed after adjustment for familial background characteristics in all models. Our results thereby show limited evidence for birth order differences in midlife. This study highlights that shared factors within the family of origin may be stronger predictors of adverse health outcomes attributable to substance use among siblings during this life course period. Future research should disentangle the contributions of the social environment within the family of origin for adverse health outcomes attributable to alcohol or narcotics among siblings.

Prior research indicates that parental psychiatric disorders increase their offspring's risk of substance use problems. Though the association is likely bidirectional, the effects of an adult child's substance use on parental mental health remain understudied. We examined parents' psychotropic medication use trajectories by parental sex and educational attainment before and after a child's alcohol- or narcotics-attributable hospitalization. We identified Finnish residents, born 1979-1988, with a first hospitalization for substance use during emerging adulthood (ages 18-29, n = 12,851). Their biological mothers (n = 12,283) and/or fathers (n = 10,765) were followed for the two years before and after the hospitalization. Psychotropic medication use was measured in three-month periods centered around the time of child's hospitalization, and the probability of psychotropic medication use at each time point was assessed using generalized estimating equations logit models. Among mothers, the prevalence of psychotropic medication use increased during the year before, peaked during the 0-3 months after hospitalization, and remained at a similarly elevated level until the end of follow-up. The prevalence among fathers increased gradually and linearly across follow-up, with minimal changes evident either directly before or after the hospitalization. Parents' educational attainment did not modify these trajectories. Our results highlight the importance of considering linked lives when quantifying substance use-attributable harms and underscore the need for future research examining the intergenerational spillover effects of substance use in both directions, particularly in mother-child dyads.