A 2011 report from the Centers for Disease Control gives some of the most solid evidence yet that depression has less to do with "genetic predisposition" and other speculative causal factors than with real-world problems. The accompanying graphic (below) from CDC's report shows that income level correlates very strongly, in an inverse fashion, with depression.
The numbers show that if you are living below the poverty level ($11,490/yr if you're single) and you're between the ages of 20 and 64, you're at roughly five times greater risk of depression than if you're making 400% or more of poverty level income ($45,960/yr, if you're single).
|Latest (2011) CDC data on the demographics of depression. Taken
The numbers also show that you're at much greater risk of depression if you're 45 to 64 years old than if you're under 45 or over 64, particularly if you're earning less than twice the poverty level. This suggests that the stressors of middle age are qualitatively different than the stressors of old age or of young adulthood.
The main takeaway: Money may not buy happiness, but it sure as hell buys freedom from depression.
The policy implications are clear. We now have strong evidence that a reliable and effective way to reduce the incidence of depression (and concomitant medical spending) in the U.S. is to reduce poverty and increase income levels generally.