Province-Level Income Inequality and Health Outcomes in Canadian Adolescents

Countries with greater disparity between the rich and the poor—or greater income inequality—have been shown to have worse population health (see Wilkinson & Pickett, 2006 for a summary). These findings have formed the basis of the income inequality hypothesis: A more unequal distribution of income in society, over and above societal average income, has an adverse effect on the health of the individuals in that society (Wilkinson & Pickett, 2007). To test the hypothesis that income inequality has a contextual effect on health, Subramanian and Kawachi (2004) have argued that multilevel consideration of individual income and societal/community income inequality, and their effects on individual health, is essential. In a meta-analysis of 28 multilevel studies, Kondo et al. (2009) found that income inequality had a “modest” adverse effect on adult self-rated health and mortality. Adolescence is a period of transition from childhood to adulthood. During this time, one’s socioeconomic status (SES) also shifts from being determined by one’s parents or family toward being primarily self-determined. Existing evidence suggests that graded associations between SES and health (or socioeconomic gradients in health), which are well-established in adulthood and childhood (Braveman, Cubbin, Egerter, Williams, & Pamuk, 2010), may be present inconsistently during adolescence (Chen, Martin, & Matthews, 2006; Goodman, 1999; West, 1997). Similarly, associations between income inequality and health may be different in adolescence compared with adulthood. Two main mechanisms have been proposed to explain the link between income inequality and health, both of which may differentially affect adolescent versus adult health. The social cohesion pathway suggests that income inequality leads to low social capital and stressful social comparison, which affect health through psychological processes and associated physiological changes (Wilkinson, 1997a, b; Wilkinson & Pickett, 2009). Social comparison and social cohesion may be particularly relevant to health during adolescence, owing to the importance of peer relations during this time (West, 1997). Psychosocial processes are also critical during this developmental period, and mental disorders are the most common health problems (Gore et al., 2011). The policy pathway suggests that the adverse influence of income inequality may operate through social and health policies, such as health care, welfare spending, child care, tax policy, and unemployment compensation (Subramanian & Kawachi, 2004). Policies and spending related to education and mental health care may be particularly important during adolescence. To date, only a handful of studies have examined associations between income inequality and adolescent health outcomes with multilevel study designs. Using data from the Health Behaviour in School-aged Children study, greater income inequality at the country level has been shown to be related to poorer adolescent self-related health (although results did not control for country mean income; Torsheim, Currie, Boyce, & Samdal, 2006), drinking alcohol in young adolescents (with no effect in older adolescents; Elgar, Roberts, Parry-Langdon, & Boyce, 2005), and a steeper within-country socioeconomic gradient in adolescent life satisfaction (but no main effect of income inequality on life satisfaction; Levin et al., 2011). In the United States, greater state-level income inequality was linked to higher adolescent obesity prevalence (Singh, Kogan, & van Dyck, 2008) and lower physical activity levels (Singh, Kogan, Siahpush, & van Dyck, 2009), although these findings did not control for state mean income. State-level income inequality was inversely correlated with birth-control usage but was not significant in multivariate analyses (Crosby, Holtgrave, DiClemente, Wingood, & Gayle, 2003). Finally, higher municipal-level income inequality was associated with worse oral health in Brazilian adolescents (Celeste, Nadanovsky, Ponce de Leon, & Fritzell, 2009). Results from these studies suggest that associations between income inequality and adolescent health may vary by health outcomes, such that income inequality may have a stronger effect on certain health outcomes. Moreover, associations between SES and adolescent health have been shown to vary by health outcome (Goodman, 1999). Examination of multiple health outcomes within a single sample would help to elucidate how income inequality may be differentially linked to health outcomes. To date, no studies have examined associations between income inequality and mental health outcomes, a critical domain of adolescent health. Moreover, some of the previous findings did not statistically control for country/state mean income, which limits their interpretability, as this is a potential confounder of the influence of country/state income inequality. It also remains unclear whether the geographical scale (e.g., country, state, city) of income inequality comparison matters for adolescent health. Existing evidence in adults, as reported in the meta-analysis by Kondo et al. (2009), suggests that stronger associations between income inequality and self-rated health exist for between-country versus within-country comparisons. Moreover, meta-analytic findings suggest that within-country associations may emerge in highly unequal societies only. Ross et al. (2005) found that within-country city-level income inequality was linked to mortality in highly unequal countries (United States, United Kingdom) but not in more equal countries (Canada, Sweden, Australia). To date, within-country adolescent comparisons are limited to U.S. states (Crosby et al., 2003; Singh et al., 2008, 2009) and Brazilian municipalities (Celeste et al., 2009). There is a need for more within-country comparisons outside of the United States, particularly in more equal countries, like Canada. In terms of income inequality, Canada is more equal than the United States, United Kingdom, Italy, Australia, and Japan, and less equal than Switzerland, Ireland, France, Sweden, Denmark, and other peer countries (Conference Board of Canada, 2013). Canada is made up of 10 provinces and 3 territories. Each province is responsible for its funding and delivery of health, social, and educational services. Canadian provinces also vary widely in terms of level of taxation. For instance, the highest provincial taxation rate in Alberta is 10%, while it is 21% in Nova Scotia. As such, Canadian provinces differ in terms of level of income inequality as well as in the provision of programs and services. Therefore, examination of the scale of income inequality at the province level was thought to be the most appropriate within-country comparison for the Canadian context. To our knowledge, no previous studies have tested for the effects of income inequality on adolescent health within Canada. Further understanding of how geographical scale and the inequality level of the country affects within-country effects may help to elucidate the mechanisms by which income inequality may influence health. The aim of the current study was to test the effects of provincial income inequality, independent of province mean income and family SES, across a number of health outcomes in Canadian adolescents. Therefore, using a within-country design, we tested a contextual main effect of province-level income inequality on individual health outcomes in adolescents, while controlling for province mean income, household income, and parental education. We expected that greater provincial income inequality would be associated with worse adolescent health outcomes. We also tested the interaction between province-level income inequality and family SES on adolescent health. We expected stronger associations between family SES and adolescent health (or steeper socioeconomic gradients in health) in more unequal provinces.