Although encouraged to apply to colleges, Brianna Pelletier, a mathematically-gifted high school senior with cystic fibrosis, dwells on her mortality and the unfairness of life.

"Which is more dangerous, a gun or a swimming pool? What do schoolteachers and sumo wrestlers have in common? Why do drug dealers still live with their moms? How much do parents really matter? How did the legalization of abortion affect the rate of violent crime? These may not sound like typical questions for an economist to ask. But Steven D. Levitt is not a typical economist. He is a much-heralded scholar who studies the riddles of everyday life--from...