According to the latest edition of Scientific American Mind, new research suggests that depression is not necessarily a a disease or aberration. In many cases, having a depression might increase your chances of survival.

[D]epression should not be thought of as a disorder at all. In an article recently published in Psychological Review, we argue that depression is in fact an adaptation, a state of mind which brings real costs, but also brings real benefits.

The researchers go out of their way to acknowledge that depression is a terrible problem for many people who should seek out help. Nonetheless, they also suggest that the mode of thinking characteristic of many bouts of depression is focused, highly analytical and systematic:

Analysis requires a lot of uninterrupted thought, and depression coordinates many changes in the body to help people analyze their problems without getting distracted . . . [D]epression is nature’s way of telling you that you’ve got complex social problems that the mind is intent on solving. Therapies should try to encourage depressive rumination rather than try to stop it, and they should focus on trying to help people solve the problems that trigger their bouts of depression . . . [D]epression . . . seems . . . like the vertebrate eye—an intricate, highly organized piece of machinery that performs a specific function.

For further reference: I first rand in to this idea that many bouts of depression might be useful in a book called Why We Get Sick, by Randolf Nesse.