How do socioeconomic health inequalities develop?

The main aim of this dissertation is to gain more insight into the interplay of two processes that may contribute to the emergence of socioeconomic health inequalities in adolescence and young adulthood: social causation and health-related selection. Social causation emphasizes the role of the socioeconomic background of the family and of differences within the educational setting that underlie the development of health inequalities. Health-related selection refers to processes whereby health problems influence adolescents’ and young adults’ educational trajectories (direct health-related selection), or whereby individual differences already present in childhood (e.g., in genetics or cognitive skills, such as effortful control and IQ) predict later educational attainment as well as health characteristics or behaviours (indirect health-related selection). In addition, this dissertation examined whether associations between parental socioeconomic status (SES) and adolescent health behaviours differ between countries with low and high levels of social mobility. Country-level social mobility, like other social and cultural characteristics at the societal level, could influence the strength of the mechanisms that contribute to the development of health inequalities. The results highlight the importance of both social causation and health-related selection in the development of socioeconomic health inequalities in adolescence and young adulthood. Direct health-related selection was mainly found regarding attention problems, and relatedly ADHD symptoms, which consistently predicted decreases in education over the whole course of adolescence and young adulthood. In addition, we found that associations between ADHD symptoms and lower education were not mediated by key features of adolescents’ social context (i.e., family functioning, social support by teachers, and social support by classmates). Furthermore, we did not find differences in these associations depending on the level of family functioning and social support. We did not find much evidence of indirect health-related selection related to cognitive skills in childhood (i.e., IQ and effortful control), which mainly played a role as predictors of adolescents’ educational trajectories but were mostly not directly associated with health characteristics. Analyses using polygenic scores (PGSs) were suggestive of some indirect health-related selection related to genetic factors when considering educational inequalities in smoking. Regarding inequalities in substance use, social causation processes appeared to be present. Early adolescents in the lower educational trajectories increased their drinking behaviour more strongly three years later. In young adulthood, the influence of the educational context was in the opposite direction, increasing the drinking behaviour of those in the higher educational trajectories. Conversely, following a lower educational trajectory was consistently associated with higher risks of smoking in adolescence and young adulthood, including after taking into account individual differences in effortful control, IQ, and genetics. Lastly, higher levels of social mobility at the national level predicted larger inequalities by parental SES in physical activity, but not in any of the other health behaviours studied (healthy and unhealthy foods consumed, having breakfast regularly, and smoking). This dissertation sheds new light on the complex interplay of individual characteristics and the social context at home, at school, and at the national level which underlies the development of socioeconomic health inequalities in adolescence and young adulthood.