Spillovers and effect attenuation in firearm policy research in the United States

In the United States, firearm-related deaths and injuries are a major public health issue. Because of limited federal action, state policies are particularly important, and their evaluation informs the actions of other policymakers. The movement of firearms across state and local borders, however, can undermine the effectiveness of these policies and have statistical consequences for their empirical evaluation. This movement causes spillover and bypass effects of policies, wherein interventions affect nearby control states and the lack of intervention in nearby states reduces the effectiveness in the intervention states. While some causal inference methods exist to account for spillover effects and reduce bias, these do not necessarily align well with the data available for firearm research or with the most policy-relevant estimands. Integrated data infrastructure and new methods are necessary for a better understanding of the effects these policies would have if widely adopted. In the meantime, appropriately understanding and interpreting effect estimates from quasi-experimental analyses is crucial for ensuring that effective policies are not dismissed due to these statistical challenges.