Everyone gets depressed from time to time. Right now, one in ten American adults struggles with depression. This prevalence has lead some scientists to ponder why we sometimes get depressed, and whether it perhaps served some evolutionary function in our distant past. Two psychiatrists noted a correlation between depression and resistance to infection, and they propose that the same genes that push us toward depression also possibly helped our ancestors stave off infectious disease.

Co-author Andrew Miller of Emory University explains the theory:

"Most of the genetic variations that have been linked to depression turn out to affect the function of the immune system. This led us to rethink why depression seems to stay embedded in the genome."

Many researchers have noticed that people with depression seem to have higher levels on inflammation, a sure sign of an overactive immune system. The authors of the recent paper note that factors associated with depression, like avoiding others and general inactivity, could have helped our ancestors avoid disease, by pushing them away from other sick people. Additionally, the authors suspect that the link between stress and depression may be a byproduct of the immune system priming itself in anticipation of a wound or illness.

The persistence of depression through human history certainly raises questions, and we know that the immune system and psychology are closely related. Though social bonds were incredibly important for our ancestors' survival, in the case of infectious disease, isolation could have saved lives. Now, with the benefit of antibiotics and other treatments, depression is less of a side-effect and more of a problem in itself. By treating the body and the mind together, we can get a better perspective on what makes us feel the way we feel, and even understand why we might be feeling blue.

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