At any given time, about 5% of Americans report symptoms of moderate or severe depression. Major depressive disorder is so common that at some point in life, one in six Americans (15%) will suffer from it.

The prevalence of depression presents economic problems as well — it's one of the most common causes of disability, it has effects in the workplace, and it's responsible for a host of suicide-related costs. In 2010, the economic burden of depression in the US was estimated to be $210.5 billion.

So why does such a debilitating condition strike so many people?

The traditional understanding is that depression is just a breakdown in the way things are normally supposed to work in the brain — chemical imbalances that can be righted through a combination of medication and behavioral shifts.

But there's also a theory that instead of being purely a disorder, depression might be a specific behavioral strategy that we've evolved, a biological adaptation that serves a purpose. As Matthew Hutson explains in a Nautilus feature on the potential evolutionary roots of depression and suicidal behavior, that purpose might be to make us stop, understand, and deal with an important problem.

Hutson explains that evolutionary psychologist Paul Andrews and psychiatrist J. Anderson Thomson first elaborated on this idea, called the "analytical rumination hypothesis," in an article published in Psychological Review in 2009.

The basic concept is that what we think of as a disorder is actually a way our brains analyze and dwell on a problem in the hopes of coming up with a way to deal with it. The researchers suggest it's possible that a difficult or complex problem triggers a "depressive" reaction in some people that sends them into a sort of analytical mode.

With depression, Hutson writes, "[t]here’s an increase in rumination, the obsessing over the source of one’s pain," along with increased analytical activity in the brain and REM sleep, which helps with memory processing. A major symptom of depression is anhedonia, the inability to take pleasure from normal activities. According to this approach, those activities could be seen as disrupting this "processing" phase.

This hypothesis would account for the fact that the majority of depressive episodes occur after a significant life event like a death or the end of a relationship.

In their 2009 paper, Andrews and Thomson even suggest this idea could explain why depression and anxiety so frequently occur together.

"We hypothesize that depression and anxiety often co-occur because some problems require both analysis (promoted by depressed affect) and vigilance (promoted by anxiety)," they write. Analysis allows someone to understand the inciting factor, vigilance is a way of trying to prevent it from happening again.

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As fascinating as evolutionary explanations are, it's hard to prove they are correct, especially if they haven't yet been reinforced by other research.

The idea that depression might be a biological adaptation rather than a mental disorder is not the main consensus of the mental health community. And even if the hypothesis is correct, it's likely incomplete and doesn't explain all of depression.

Complex problems usually have a number of causes. About 20% of depression cases aren't preceded by a major life event — there may be some cause in the past, but we don't know for sure. We also know that genetics can play some role in a person's susceptibility to depression. As Andrews and Thomson point out, there are different types of depression, some of which may have different causes. They also write that there are alternative evolutionary explanations that could co-exist with their hypothesis.

But if the analytical rumination hypothesis can be further validated, it could have major implications for how we treat this kind of mental illness in the future.

In that case, the approach to treating depression could shift to deal more with the underlying cause, rather than just treating symptoms. One anthropologist told Hutson that if the theory is right, treating depression with antidepressants might be like treating a broken bone with painkillers (instead of providing both painkillers and a cast to help the break heal). By that logic, therapy could be seen as an important part of treatment, since it can help people better understand and cope with whatever caused their depressive episodes.

Traditional antidepressants have been very effective, even life-changing, for some. But those approaches don't work in the long term for everyone, which is why researchers are investigating all kinds of different therapies.

Even if depression is found to be some sort of evolutionary "tool" or adaptation, it clearly doesn't necessarily help people solve the problem that causes it. But new understandings may help us think about new or better forms of treatment — something that's always welcome.