Permeable Boundaries: How Neighboring States' Policy Environments Shape Local Mortality Rates in the United States

By analyzing 3,094 U.S. counties from 2015 to 2019, this study examines spatial spillover effects of state policy environments on county-level mortality rates. I developed a weighted spatial difference score that captures relative policy contrasts between neighboring states and geographic proximity to borders. Consistent with prior studies, more liberal state policies are associated with significantly lower mortality rates. After considering spatial spillovers, I found that counties experience cross-border spillovers based on policy differences with neighbors: Those bordering states with relatively more liberal policies showed lower mortality than their own policies predict, while those bordering states with relatively more conservative policies showed higher mortality (11.8 additional deaths per 100,000 per 1 SD of policy contrast). Importantly, conventional models ignoring spatial spillovers underestimated direct policy effects by 24.8%, systematically undervaluing policy interventions' true health returns.