Consider Moving to Opportunity, an experiment in the 1990s that gave families housing assistance, in some cases contingent on their moving to less poor neighborhoods. Initial evidence from the randomized trial was disappointing, finding little or no improvements in test scores for children or earnings for adults. A new paper by the Harvard economists Raj Chetty, Nathaniel Hendren and Lawrence F. Katz, however, followed the children for another decade. It found that traditional rental vouchers had increased their earnings as adults by 15 percent, and experimental vouchers, which required people to move to less poor neighborhoods, by 31 percent. The additional tax revenue from these higher earnings was enough to repay the program’s cost.