Why don't we all get depressed? The short answer is that most of us do - and, paradoxically, there may be good reasons, rooted in our evolutionary past, for this. But depression comes in all degrees of severity, and only a minority of us get clinically depressed: a state which is not only more intense than ordinary everyday gloom and despondency, but less obviously adaptive. In Frontiers, Geoff Watts explores the origins of depression and efforts to find new treatments. The latest research is looking into the brains of those who never get depressed, those who seem to have a natural resilience. Could these hardy individuals hold the key to preventing depression taking hold in the first place?

The notion that milder forms of depression may be helpful emerged a little over a decade ago, prompted by the observation that this state of mind is so relatively common. The claim is part of a more general attempt to explain the kinds of illness we suffer from by reference to our evolutionary history. Natural selection is pretty good at adapting organisms to function effectively in their environments. If depression is a regular feature of our state of mind, so the argument goes, maybe it's serving some useful purpose. It could be a bit like pain: something we don't like, but which has a biological value.

The father of this theory is the American psychologist Randolph Nesse, who believes that mild depression deters you from wasting energy pursuing unattainable goals, and encourages you to disengage from them and turn instead to something else. At first hearing the idea sounds fanciful. But since Nesse put forward the hypothesis, at least one study seems to have confirmed its plausibility.

So much for mild depression; but what of the more severe forms that don't so much prompt sufferers to reconsider their goals as drive them to give up entirely? Why, ask researchers, if mild depression is an adaptation, can it become so destructive so easily? Can this destructive form of depression be understood and prevented?

One helpful clue towards a better means of doing so can be found in the biology of people who experience huge amounts of stress, yet show no signs at all of depression. They have what is known in the trade as "resilience", and a research group in Manchester is trying to understand what it is and why it works. Is it a specific brain process? A variation of brain chemistry, a set of genes or all three in combination with specific life experiences? If something specific in people with resilience can be uncovered and then targeted, might we be able to prevent other people who face major life stress from succumbing to this debilitating disease?

Producer: Rami Tzabar. Show less