From our work studying cash-transfer programs across 37 countries, we’ve come to see a consistent logic behind why cash succeeds in some places and falls short in others. Cash transforms health when four particular conditions are met. Most U.S. cash-transfer pilots have lacked them. But one major American policy does come close: the federal food-assistance program SNAP. Its success offers a road map for what effective cash assistance can look like in this country, if we choose to build on it.