The association between income and psychotropic drug purchases: individual fixed effects analysis of annual longitudinal data in 2003–2013
Epidemiology, 30:2, 221–229 (2019)
Previous cross-sectional studies show that low income is associated with poor mental health. However, longitudinal research has produced varying results. We assess whether low income is associated with increased psychotropic drug use after accounting for confounding by observed time-varying, and unobserved stable individual differences.
The longitudinal register-based data comprises an 11% nationally representative random sample of Finnish residents aged 30–62 years between the years 2003 and 2013. The analytic sample includes 337,456 individuals (2,825,589 person–years). We estimate the association between annual income and psychotropic purchasing using ordinary-least-squares and fixed effects models, the latter controlling for all unobserved time-invariant individual characteristics.
The annual prevalence of psychotropic purchasing was 15%; 13% among men and 18% among women. Adjusted for age squared, sex and calendar year, the doubling of income decreased the probability of purchases by 4 percentage points (95% confidence interval: 4,4) in the ordinary-least-squares model. We observed no association after further adjusting for observed sociodemographic characteristics and unobserved individual differences in the fixed effects specification.
Following adjustment for an extensive set of confounders, no contemporaneous association between variations in annual individual income and psychotropic drug purchasing was observed. Similar results were obtained irrespective of baseline income level and sex. The results imply that indirect selection based on preexisting individual characteristics plays a major role in explaining the association between variations in income measured over the short term, and psychotropic drug purchases. The association appears largely attributable to unobserved, stable individual characteristics. See video abstract at, http://links.lww.com/EDE/B463.