Summarizing research published in Psychological Review, Paul W. Andrews and J. Anderson Thomson, Jr. write in Scientific American that depression has very real advantages, despite very real costs.
The scientists argue that research in the U.S. and other nations estimates that between 30 to 50 percent of people have met criteria for major depression sometime in their lives, despite the fact that the brain, promoting survival and reproduction via evolution, should have left our brains resistant to such high rates of malfunction -- the way it has to mental disorders, which are fairly rare.
Andrews and Thomson back their claim with science:
One reason to suspect that depression is an adaptation, not a malfunction, comes from research into a molecule in the brain known as the 5HT1A receptor. The 5HT1A receptor binds to serotonin, another brain molecule that is highly implicated in depression and is the target of most current antidepressant medications. Rodents lacking this receptor show fewer depressive symptoms in response to stress, which suggests that it is somehow involved in promoting depression. (Pharmaceutical companies, in fact, are designing the next generation of antidepressant medications to target this receptor.) 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.
That is to say that despite destructive attributes -- lack of concentration, social isolation, lethargy, an inability to enjoy pleasure, and the chance of "severe, lengthy, and even life-threatening bouts of depression" -- depression can actually be useful.
Why? The scientists claim depressed people often think intensely and analytically (they're called "ruminations") about their problems, dwelling on a complex problem, breaking it down into smaller components and considering them methodically, one at a time.
Feeling depressed in the face of a complex problem motivates you to actually solve it, the scientists write.
Analysis requires a lot of uninterrupted thought, and depression coordinates many changes in the body to help people analyze their problems without getting distracted. In a region of the brain known as the ventrolateral prefrontal cortex (VLPFC), neurons must fire continuously for people to avoid being distracted. But this is very energetically demanding for VLPFC neurons, just as a car’s engine eats up fuel when going up a mountain road. Moreover, continuous firing can cause neurons to break down, just as the car’s engine is more likely to break down when stressed. Studies of depression in rats show that the 5HT1A receptor is involved in supplying neurons with the fuel they need to fire, as well as preventing them from breaking down. These important processes allow depressive rumination to continue uninterrupted with minimal neuronal damage, which may explain why the 5HT1A receptor is so evolutionarily important.
In light of that "uninterrupted" mindset, depression makes a lot of sense, the scientists insist: social isolation, the inability to feel pleasure from other activities and loss of appetite all bolster the case that the brain needs to be singularly focused to achieve its goal.
What's more, studies have found that people in depressed states are better at solving social dilemmas -- the kind of problems difficult enough to require analysis, they write.
Consider a woman with young children who discovers her husband is having an affair. Is the wife’s best strategy to ignore it, or force him to choose between her and the other woman, and risk abandonment? Laboratory experiments indicate that depressed people are better at solving social dilemmas by better analysis of the costs and benefits of the different options that they might take.
Or, in other words: depression is nature's way of telling you that your mind is intent on solving a complex social problem with which you are faced.
A disorder, hardly. A refined reaction, indeed.