Depression is a leading cause of disability in high- and middle-income countries and is of increasing relative burden in low-income countries. The Global Burden of Disease study illustrates how depression is increasing as a proportion of all the disabilities resulting from illness. This is because we know how to prevent other major causes of disability such as cardiovascular disease and infection and so their incidence is on the decline. Meanwhile, there is evidence that rates of depression are rising slightly. In order to have an impact on this major public health burden, we will need to devise preventative strategies to reduce the incidence. As depression is a continuum, much of the disability is experienced by the larger numbers of those with mild and moderate levels of depression who might not seek treatment for themselves. Therefore, effective preventative strategies applied to the whole population will have more widespread benefits than interventions simply targeted towards those at high risk. In order to develop preventative interventions, we need to know what causes depression.

Epidemiology tends to be primarily concerned with causes outside the individual or with genetic causes.