COMMENTARY

For several decades, we have observed socioeconomic inequalities in major depression. The meta-analysis by Lorant et al integrates a diverse, and somewhat inconsistent, group of studies. In the aggregate, the odds of depression were nearly double among people of low socioeconomic status (SES) compared with high SES. This suggests an association of considerable magnitude between low SES and depression. Although social and economic policies could be considered as one avenue for preventing depression,¹ we must be careful not to draw causal inferences from correlational data. People may be depressed because of circumstances surrounding their economic status. Alternatively people could have low SES in part due to persistent depression, or SES may be a marker for other variables that have some association with persistent depression. Some complex combination of these possibilities is likely.

Lorant et al reported a larger effect of SES on the chronicity of depression than on initial depression onset, concluding that enhanced secondary prevention efforts are needed among low SES individuals with a history of depression. In light of the highly persistent nature of major depressive disorder, these findings equally argue for improved primary prevention strategies. Preventing or delaying the initial onset of depression may confer a greater degree of protection against depressive episodes over the life course than secondary prevention efforts initiated after depression is established.²

A limitation of the meta-analysis is the small number of studies available on differences in the effects of SES on initial depression onset and on subsequent depressive episodes. Of the 5 studies of depression onset, only 2 were true incidence studies (studies that investigated the occurrence of depression over time among individuals without any prior history of depression).^3–4 As the remaining 3 studies did not exclude people in remission, some cases analysed as incident cases were actually recurrent cases.^5–7

Of the 4 studies of persistence, 3 focused on continuing depressive symptoms among depressed individuals.^{4, 6, 8} The remaining study included both depressed and non-depressed participants. Therefore it did not distinguish risks for the initial onset of depression from those for episode recurrence.⁹ The results of this meta-analysis are therefore less conclusive regarding the differential effects of SES on the processes of depression onset and persistence than they are regarding the overall association between SES and prevalent depression.

The association between SES and depression is likely to endure over the life course. Adults exposed to socioeconomic disadvantage as children have a higher risk of developing depression and a higher rate of recurrent episodes than adults from higher SES backgrounds.^10–11 In order to clarify the relation between SES and depression further, measurements of SES and depressive symptomatology are needed at multiple stages of the life course. Similarly, policy responses to socioeconomic inequalities in depression need to address the mental health consequences of low SES beginning early in childhood and continuing into adulthood.¹²