The past decade has seen a growing acceptance of cash transfers as a tool for reducing poverty and improving wellbeing, with large-scale national programmes across many low-income and middle-income countries (LMICs) and a wave of guaranteed income pilots across cities in the USA. Yet evidence on the impact of US cash transfer programmes on beneficiaries’ health outcomes has been mixed, and questions about cost and scalability remain unresolved. What has been missing is a clearer theory of when and how cash transfers might improve health outcomes in the USA.