Antonia sent me this piece on depression from The New York Times. Absolutely fascinating. It's by Wired contributing editor Jonah Lehrer, who has just written a book called How We Decide. I've only put the first bit on this page, so you'll have to click HERE to see the full article.
(NB - if you're reading this on FaceBook, you'll probably need to go to the original post on my blog to make the links work)

The Victorians had many names for depression, and Charles Darwin used them all. There were his “fits” brought on by “excitements,” “flurries” leading to an “uncomfortable palpitation of the heart” and “air fatigues” that triggered his “head symptoms.” In one particularly pitiful letter, written to a specialist in “psychological medicine,” he confessed to “extreme spasmodic daily and nightly flatulence” and “hysterical crying” whenever Emma, his devoted wife, left him alone.

While there has been endless speculation about Darwin’s mysterious ailment — his symptoms have been attributed to everything from lactose intolerance to Chagas disease — Darwin himself was most troubled by his recurring mental problems. His depression left him “not able to do anything one day out of three,” choking on his “bitter mortification.” He despaired of the weakness of mind that ran in his family. “The ‘race is for the strong,’ ” Darwin wrote. “I shall probably do little more but be content to admire the strides others made in Science.”

Darwin, of course, was wrong; his recurring fits didn’t prevent him from succeeding in science. Instead, the pain may actually have accelerated the pace of his research, allowing him to withdraw from the world and concentrate entirely on his work. His letters are filled with references to the salvation of study, which allowed him to temporarily escape his gloomy moods. “Work is the only thing which makes life endurable to me,” Darwin wrote and later remarked that it was his “sole enjoyment in life.”

For Darwin, depression was a clarifying force, focusing the mind on its most essential problems. In his autobiography, he speculated on the purpose of such misery; his evolutionary theory was shadowed by his own life story. “Pain or suffering of any kind,” he wrote, “if long continued, causes depression and lessens the power of action, yet it is well adapted to make a creature guard itself against any great or sudden evil.” And so sorrow was explained away, because pleasure was not enough. Sometimes, Darwin wrote, it is the sadness that informs as it “leads an animal to pursue that course of action which is most beneficial.” The darkness was a kind of light.

The mystery of depression is not that it exists — the mind, like the flesh, is prone to malfunction. Instead, the paradox of depression has long been its prevalence. While most mental illnesses are extremely rare — schizophrenia, for example, is seen in less than 1 percent of the population — depression is everywhere, as inescapable as the common cold. Every year, approximately 7 percent of us will be afflicted to some degree by the awful mental state that William Styron described as a “gray drizzle of horror . . . a storm of murk.” Obsessed with our pain, we will retreat from everything. We will stop eating, unless we start eating too much. Sex will lose its appeal; sleep will become a frustrating pursuit. We will always be tired, even though we will do less and less. We will think a lot about death.

The persistence of this affliction — and the fact that it seemed to be heritable — posed a serious challenge to Darwin’s new evolutionary theory. If depression was a disorder, then evolution had made a tragic mistake, allowing an illness that impedes reproduction — it leads people to stop having sex and consider suicide — to spread throughout the population. For some unknown reason, the modern human mind is tilted toward sadness and, as we’ve now come to think, needs drugs to rescue itself.
The alternative, of course, is that depression has a secret purpose and our medical interventions are making a bad situation even worse. Like a fever that helps the immune system fight off infection — increased body temperature sends white blood cells into overdrive — depression might be an unpleasant yet adaptive response to affliction. Maybe Darwin was right. We suffer — we suffer terribly — but we don’t suffer in vain.

(THERE'S MORE TO THIS PIECE, SO READ THE REST OF IT HERE)