In the wake of the Los Angeles riots more than 20 years ago, Congress created an anti-poverty experiment called Moving to Opportunity. It gave vouchers to help poor families move to better neighborhoods and awarded them on a random basis, so researchers could study the effects.

The results were deeply disappointing. Parents who received the vouchers did not seem to earn more in later years than otherwise similar adults, and children did not seem to do better in school. The program’s apparent failure has haunted social scientists and policy makers, making poverty seem all the more intractable.