Short Research Article: Independent association between household income and family adversity and bipolar disorder in adolescence and young adulthood - a registry-data study in the Basque Country.

Background: The objective of this study was to estimate separately the excess risk associated with household income and head of household employment status (as an index of family adversity) as socioeconomic risk factors for the incidence of bipolar disorder in children, adolescents, and young adults.

Methods: A real-world data design was applied to identify bipolar disorder in individuals under 30 years of age in the Basque population. Using pharmaceutical copayment categories, we calculated sex-adjusted incidence rates disaggregated by two socioeconomic variables (a) household income and (b) head of household's employment status (as an index of family adversity). The Aalen-Johansen estimator and Poisson-negative binomial regression analyses were used to measure age and sex-adjusted incidence rate ratios (IRR) for household income and head of household's employment status.

Results: The study population comprised 609,281 individuals aged between 1 and 30 years. Of these individuals, 698 (0.11%) had a diagnosis of bipolar disorder. The total cumulative incidence per 1000 person-years was 0.4 at 12 years, 0.9 at 18 years, 2.0 at 24 years, and 3.4 at 30 years. A higher risk of bipolar disorder was associated with being male, living in families classified as having 'no income' (compared with those with high income) and living in families where the heads of the households were not working. Household income and head of household's working status showed an independent and consistent gradient, with intermediate categories such as medium income and head of household having retired from work being associated with intermediate risks (IRR = 1.32 and IRR = 2.52, respectively). All IRRs were statistically significant.

Conclusions: Household income and working status of the head of household (an index of family adversity) are independent factors associated with the risk of bipolar disorder. Clinicians should consider the possibility of family adversity when working with patients with bipolar disorder.