Sapolsky (who spends half the year studying baboons in the wild and the other half in the lab) talks about severe depression first from the angle of brain chemistry and then from the emotional angle.
I realized that severe depression could be viewed as an inability to heal-- a lot of people get knocked out for a while from bad events, but they come out of it on their own. What's more, there are people who have cyclical severe depression-- they go into and out of severe depression on a yearly or a year-and-a-half cycle. If someone has had four or five episodes of severe depression caused by bad events, their risk of falling into severe depression goes up sharply.
I can't swear that no one is studying spontaneous recovery from depression, but I'm pretty sure that if they are, it hasn't hit the popular press. I'm pretty sure that understanding the process of recovery would be an important clue to preventing depression.
Subject thanks to tamnonlinear, who posted about this Sapolsky lecture yesterday, but I couldn't get it to work and went looking for other lectures. Today it seems to be working. It's tempting to blame AVG, but that's probably unfair.
It was unfair-- the lecture just had a longish introduction.