We avert our eyes when we meet them on the street: homeless mentally ill people with their hand-scrawled signs, shopping carts, and cardboard boxes. Because of our fear and revulsion or the guilty relief we feel at our own relative good fortune, we fail to see any human connection with them. Still, we're curious. How did they end up on the street? How do they survive such a life? What combination of biological vulnerabilities, childhood traumas, drugs,...