Research suggests that income inequality may be a threat to population health, including both physical and mental health outcomes. Mechanisms by which income inequality is hypothesized to deleteriously affect health include psychosocial stress, such as through frustration induced by a heightened sense of relative deprivation, as well as reduced social cohesion and its sequelae, such as increased crime (Adjaye-Gbewonyo & Kawachi, 2012; Cifuentes et al., 2008; Kawachi & Subramanian, 2014; Pabayo, Kawachi, & Gilman, 2015). Ecological analyses among high-income countries indicate that income inequality may be more strongly correlated with mental illness (correlations of 0.73) than other health outcomes such as life expectancy, obesity, infant mortality, and homicide (Pickett & Wilkinson, 2015). However, much of the evidence linking income inequality to mental health outcomes fails to adequately address several important questions that might improve causal inference, including confounding and temporal order.

Major depression represents a growing segment of the global burden of disease. Major depressive disorder rose from the 15th leading cause of global disability-adjusted life years to the 11th leading cause between 1990 and 2010, and depressive disorders contribute more to years lived with disability than other mental and behavioral conditions (Murray et al., 2013). In addition, while depression results in major morbidity in its own right, including risk of suicide, it has also been linked to greater risk of physical ailments such as cardiovascular disease (Hare, Toukhsati, Johansson, & Jaarsma, 2014; Rumsfeld & Ho, 2005).