Thus began a troubled toddler’s journey from one doctor to another, from one diagnosis to another, involving even more drugs. Autism, bipolar disorder, hyperactivity, insomnia, oppositional defiant disorder. The boy’s daily pill regimen multiplied: the antipsychotic Risperdal, the antidepressant Prozac, two sleeping medicines and one for attention-deficit disorder. All by the time he was 3.

He was sedated, drooling and overweight from the side effects of the antipsychotic medicine. Although his mother, Brandy Warren, had been at her “wit’s end” when she resorted to the drug treatment, she began to worry about Kyle’s altered personality. “All I had was a medicated little boy,” Ms. Warren said. “I didn’t have my son. It’s like, you’d look into his eyes and you would just see just blankness.”

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 author had a good marriage but thought it could be improved. She dragged her husband through all sorts of therapies.