An amazing piece on the research being done into depressive mental disorders. According to the article the brain is performing a specific set of functions during the state of depression to make it possible to solve complex problems. From the article:

When scientists have compared the composition of the functional part rat 5HT1A receptor to that of humans, it is 99 percent similar, which suggests that it is so important that natural selection has preserved it. The ability to “turn on” depression would seem to be important, then, not an accident.