The U.S. spends trillions every year on a social safety net for people in all steps of life, from childhood to retirement.

A new study from two Harvard University economists, Nathaniel Hendren and Ben Sprung-Keyser, examines 133 U.S. policy changes over the past half-century, including the creation of Medicare, Medicaid expansions, the introduction of food stamps, as well as dozens of state and local programs.

Their...