Purpose: The U.S. federal Earned Income Tax Credit (EITC) is often considered the most effective antipoverty program for families in the U.S., leading to health benefits for mothers, infants and children. State EITC supplements to the federal credits can significantly enhance the magnitude of this intervention. In this paper we advance EITC and health research by: 1) describing the diffusion of state EITC policies over 40 years, 2) presenting patterns in important EITC policy dimensions across space and time, and 3) disseminating a reliable and valid data set to advance future research by policy analysts and scientists. Methods: We used current state-of-the-art public health law research methods to systematically collect, conduct textual legal analysis and numerically code all EITC legislative changes from 1980 through 2020 in the 50 states and Washington, D.C. Results: First, the pattern of diffusion across states and time shows initial introductions during the 1990s in the Midwest, then spreading to the Northeast, with more recent expansions in the West and South. Second, differences by state and time of important policy dimensions are evident, including size of credit and refundability. Third, state EITC benefits vary considerably by household structure. Conclusion: Continued research on health outcomes is warranted to capture the full range of potential beneficial effects of EITCs on family and child wellbeing. Collaboration between lawyers and policy analysts with epidemiologists and economists points the way for high-quality empirical studies of the many other dimensions of policy and law likely affecting the social determinants of health.