American charitable giving veers from the hyperbolically generous to the hyperbolically stingy. On some days, no one has a quarter to spare; in times of disaster, Americans will put their lives on hold to build houses for those displaced by hurricanes. The crucial question of who gives and why they do it lies at the heart of American Generosity.

Patricia Snell Herzog and Heather E. Price, sociologists who focus on philanthropy, draw on findings from the groundbreaking Science of Generosity initiative, which combines a nationally representative survey of adult Americans with...