Indeed, as Dr. Forgas said by e-mail, “I never worked with depressives, and I do not think that the experiments we have done looking at mood effects on cognitive processes in normal populations experiencing minor, everyday mood differences can be readily generalized to depressive cognition.”

Under close scrutiny, the case for depression’s adaptive benefits has problems — big ones. For one thing, the ruminative thinking of depression is often not particularly effective in solving problems. As another patient of mine once said: “I would think the same things over and over and could never decide what to do. It’s not a creative way of thinking.”

More critically, depression can arise without any psychosocial stressor at all, which makes it hard to argue that depression is a response to a difficult situation or problem. Dr. David J. Kupfer, a psychiatrist at the University of Pittsburgh, has found that a major life stressor almost always precedes a first episode of depression, but that episodes recur with milder stressors, or even none at all.

If depression conferred a problem-solving benefit, it should not become a chronic or autonomous condition — which it is for about half the patients.

According to the World Health Organization, depression is the leading cause of disability and the fourth leading contributor to the global burden of disease, projected to reach second place by 2020. There is also strong evidence that it is an independent risk factor for heart disease, and several studies show that prolonged depression is associated with selective and possibly permanent damage to the hippocampus, a region of the brain critical to memory and learning.

Add the fact that 2 percent to 12 percent of depressed people eventually commit suicide, and the “advantages” of depression suddenly don’t look so good.

Why, then, does the notion persist that depression confers special insights and benefits?

I got a clue recently from one depressed patient. He was an educated and articulate young man, unhappy because the world was such an awful place, he said. Because he had so many other symptoms of depression — insomnia, fatigue, low libido and poor self-esteem — I told him that he was clinically depressed and that his Hobbesian worldview was probably a result of depression, not its cause.