Deep End

Sendhil Mullainathan had studied poverty for years, and something haunted him in nearly every study. Born into a small rural village in India, the Harvard behavioral economist and winner of the MacArthur Fellowship—commonly known as a “genius grant”—was inherently skeptical of the narrative that the poor somehow deserve their lot due to a lack of discipline.

I am seated cross-legged on a brothel floor on a hot April afternoon. The door is ajar. Just beyond it, a disheveled man in a...