Why do so many people suffer from depression? Research in the U.S. and other countries estimates that between 30 to 50 percent of people have met current psychiatric diagnostic criteria for major depressive disorder sometime in their lives. This staggeringly high prevalence—compared with other mental disorders that affect only around 1 to 2 percent of the population, such as schizophrenia and obsessive-compulsive disorder—seems to pose an evolutionary paradox. The brain plays crucial roles in promoting survival and reproduction, so the pressures of evolution should have left our brains resistant to such high rates of malfunction. Mental disorders are generally rare—why isn’t depression?

This paradox could be resolved if depression were a problem of growing old or a result of our modern lifestyles. Aging cannot explain depression, however, because people are most likely to experience their first bout in adolescence and young adulthood. So perhaps depression is like obesity—a problem that arises because modern conditions are so different from those in which our ancestors lived. But this explanation is not satisfactory, either. The symptoms of depression have been found in every culture that has been carefully examined, including societies such as the Ache of Paraguay and the !Kung of southern Africa—societies in which people are thought to live in environments similar to those that prevailed in our evolutionary past.

There is another possibility: perhaps in most instances, depression should not be thought of as a disorder at all. We believe that depression is in fact an adaptation: a state of mind that brings real costs but that also brings real benefits. During depression, the mind becomes more analytical and focused—a useful response for solving the complex problems that probably triggered the depression in the first place. If mental health professionals consider depression in this light, they will be able to better relieve the pain and suffering that accompanies it, while helping patients to work toward a real resolution of their problems.

Not a Disorder
Psychiatry has struggled with defining mental disorder throughout its history. Is our modern understanding correct? Current diagnostic criteria require the presence of “clinically significant distress or impairment” for a psychological condition to be considered a mental disorder. But is this enough to ensure that a trait is a disorder?

Consider the fact that people with fever would seem to experience significant distress and impairment. They are impaired in their ability to work and think, and they often feel considerable aches and pain. But these symptoms are not reason enough to ensure that fever is a disorder. Fever is, of course, an evolved response to infection—it coordinates immune responses. It directs infection-fighting cells to tissues that are most likely to be infected, and it staggers the production of chemicals that are necessary to the immune response but could cause tissue damage if produced at the same time.

This sophisticated coordination is strong evidence that fever is an adaptation: a trait that has been shaped over evolutionary time by natural selection to perform a useful function. Indeed, various studies involving humans and nonhuman animals have shown that suppressing fever with aspirin or other medications tends to prolong the infection, and fever increases the chances of surviving a serious infection. When applied to fever, the “distress and impairment” criteria that psychiatry uses lead to erroneous conclusions about disorder—fever is not a result of the body malfunctioning; it is just the opposite.

Distress and impairment are also normally present in depression. Depression is a painful emotional condition, and depressed people often have trouble performing everyday activities. They cannot concentrate on their work, they tend to socially isolate themselves, they are lethargic, and they often lose the ability to take pleasure from such activities such as eating and sex. But this does not necessarily mean that an episode of depression is a mental disorder, any more than fever’s painful symptoms mean that it is a disorder.

Even if psychiatry’s definition of mental disorder is faulty, however, we need further grounds to suspect that a state of mind as debilitating as depression is an adaptation rather than a malfunction. One reason to believe depression is useful 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 the 5HT1A receptor is somehow involved in promoting depression. When scientists compared the composition of the functional part of the rat 5HT1A receptor to that of humans, they found it is 99 percent similar, which suggests it is so vital that natural selection has preserved it through the millions of years since our common ancestor lived. The ability to “turn on” depression would seem to be important, then, rather than an evolutionary accident or the result of a malfunctioning brain.

Focused Thought
So what could be so useful about depression? Depressed people often think intensely about their problems. These thoughts are called ruminations; they are persistent, and depressed people have difficulty thinking about anything else. Numerous studies have shown that this thinking style is often highly analytical. Depressed people dwell on a complex problem, breaking it down into smaller components, which are considered one at a time.

This analytical style of thought can be very productive. Each component is not as difficult by itself, so the problem becomes more tractable. Indeed, when you are faced with a difficult problem, such as a math problem, feeling depressed is often a useful response that may help you analyze and solve it. For instance, in some of our research, we have found evidence that people who get more depressed while they are working on complex problems in an intelligence test tend to score higher on the test.

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 constant firing is very energetically demanding for VLPFC neurons, just as going up a mountain road causes a car’s engine to eat up fuel. Moreover, continuous firing can cause neurons to break down, just as the car’s engine is more likely to break down when overtaxed. 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 necessary processes allow depressive rumination to continue uninterrupted with minimal neuronal damage, which may explain why the 5HT1A receptor is so evolutionarily important.

Many other symptoms of depression make sense in light of the idea that analysis must be uninterrupted. The desire for social isolation, for instance, helps the depressed person avoid situations that would require thinking about other things. Similarly, the inability to derive pleasure from sex or other activities prevents the depressed person from engaging in activities that could distract him or her from the problem. Even the loss of appetite often seen in depression could be viewed as promoting analysis because chewing and other oral activity interferes with the brain’s ability to process information.

Depressive rumination is so resistant to distraction that depressed people often score lower than nondepressed people on many cognitive tasks, including tests of intelligence and reading comprehension. Abundant evidence indicates that they score lower because they are thinking about other things, which interferes with their ability to focus on the cognitive exercises that psychologists give them. Depressed people simply have trouble thinking about anything other than the problems that triggered their depression.

Social Solutions
Is there any evidence that all this rumination does any good? Most clinicians and researchers believe that depressive rumination is harmful. If this hypothesis were true, then strategies for avoiding or disrupting rumination should lead to a quicker resolution of episodes. But this prediction is not borne out by the evidence. People who try to avoid their ruminations, distracting themselves or escaping through alcohol or drugs, tend to have longer bouts of depression. Interventions that encourage rumination, however, such as expressive writing, promote a quicker resolution of depression.

Another suggestive line of evidence comes from various studies that have found that people in depressed mood states are better at solving social dilemmas—conflicts of interest with a partner on whom one is dependent for cooperation or help, such as a mate or a parent. These complex situations seem to be precisely the kind of problems challenging enough to require focused analysis and consequential enough to drive the evolution of such a costly state of mind.

Consider a woman with young children who discovers her husband is having an affair. Is the wife’s best strategy to ignore it, or should she force him to choose between her and the other woman—and risk abandonment? Social dilemmas require careful thought and political skill, and laboratory experiments indicate that depressed people are better at solving social dilemmas by better analyzing the costs and benefits of the different options available. Research also suggests that social dilemmas are a natural trigger for depression—people who are in conflict with a cooperative partner are at high risk for depression.

When one considers all this evidence—depression being triggered by complex social problems, uninterruptible rumination helping depressed people to solve those very problems, the 5HT1A receptor’s ancient ability to turn on depression and the receptor’s involvement in ensuring that rumination continues uninterrupted—depression seems unlikely to be a disorder in which the brain is operating in a haphazard way. Instead depression seems like fever—an intricate, though painful, organized piece of our biology that performs a specific function. As we argue in much greater detail in our article in the July 2009 issue of Psychological Review, the hypothesis that depression is an adaptation is supported by evidence from many levels: genes, neurotransmitters and their receptors, neurophysiology, neuroanatomy, pharmacology, cognition, behavior, and the efficacy of treatments.

Depression undoubtedly exists as a disorder, but similar to schizophrenia and obsessive-compulsive disorder, the true rate of the disorder is probably closer to 1 to 2 percent of the population than to 30 percent. The overdiagnosis of depression may occur because sometimes people are reluctant to talk about the problem that triggered their depression. The issues at hand may be embarrassing, sensitive or painful. Some people believe they must soldier on and ignore their troubles, or they may simply have difficulty putting their complex internal struggles into words. Under such circumstances, the therapist or researcher may be more likely to believe that the depressive episode is not a normal response to life’s problems but is instead the result of a malfunctioning brain.

But depression is nature’s way of telling you that you have complex social problems that the mind is intent on solving. Therapies should try to encourage depressive rumination rather than trying to stop it, and they should focus on trying to help people solve the dilemmas that trigger their bouts of depression. In instances when a patient resists discussing his or her troubles or ruminations, the therapist should try to identify and dismantle those barriers. Recognizing depression’s true purpose will help millions of sufferers discover the root of their painful emotions and work through their problems in a fruitful way.