Depression is the world's most common mood disorder, and it is spreading like a viral contagion. You can't catch depression in the same way you catch a cold, but the latest research provides overwhelming support that moods spread through social conditions, defining depression as more a social problem than a medical illness. Our social lives directly shape our brain chemistry and powerfully affect the way we think and feel--and our brains can change for the better with healthy social circumstances as much as they can change with medication. Drugs may address some of depression's symptoms, but...