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.