By 2010, for the first time in history, more than half the world’s population lived in cities. Cities are places of possibility: They are, as E. B. White said of New York, “the visible symbol of aspiration and faith, the white plume saying that the way is up.” But cities also break traditions and fracture families, and they breed psychiatric illness. In a city you are more likely to be depressed, to fall ill with schizophrenia, and to use alcohol and drugs. Poverty and rapid urbanization sharpen these effects.