Holding her infant patients, Dr. Mona Hanna-Attisha felt a deep sense of frustration. “I’m doing everything I’m supposed to do as a pediatrician,” she told me, describing counseling her patients’ parents about vaccines, a healthy diet, safe sleeping, and car seats. But Hanna-Attisha practices in Flint, the poorest city in Michigan and one in which more than half of children grow up in poverty. That poverty means her patients are more likely to miss milestones and fail to thrive, and more likely to grow up to have heart disease and diabetes, or to experience psychological distress. She felt like she was only ever applying a “Band-Aid,” she said. Poverty’s “a really big problem. I can’t fix that.” Except it turns out that she can. Over the past half decade, four dozen cities have piloted cash-transfer programs, giving no-strings-attached money to residents. Hanna-Attisha watched with shock as Congress did basically the same thing, passing an enormous, albeit temporary, child allowance as part of a pandemic-relief package, giving large monthly grants to parents in the last six …