State Firearm Laws and Interstate Firearm Deaths From Homicide and Suicide in the United States: A Cross-sectional Analysis of Data by County.

Importance Firearm laws in one state may be associated with increased firearm death rates from homicide and suicide in neighboring states are uncertain. Objective To determine whether counties located closer to states with lenient firearm policies have higher firearm death rates. Design, Setting, and Participants This cross-sectional study of firearm death rates by county for January 2010 to December 2014 examined data from the US Centers for Disease Control and Prevention for firearm suicide and homicide decedents for 3108 counties in the 48 contiguous states of the United States. Exposures Each county was assigned 2 scores, a state policy score (range, 0-12) based on the strength of its state firearm laws, and an interstate policy score (range, -1.33 to 8.31) based on the sum of population-weighted and distance-decayed policy scores for all other states. Counties were divided into those with low, medium, and high home state and interstate policy scores. Main Outcomes and Measures County-level rates of firearm, nonfirearm, and total homicide and suicide. With multilevel Bayesian spatial Poisson models, we generated incidence rate ratios (IRR) comparing incidence rates between each group of counties and the reference group, counties with high home state and high interstate policy scores. Results Stronger firearm laws in a state were associated with lower firearm suicide rates and lower overall suicide rates regardless of the strength of the other states' laws. Counties with low state scores had the highest rates of firearm suicide. Rates were similar across levels of interstate policy score (low: IRR, 1.34; 95% credible interval [CI], 1.11-1.65; medium: IRR, 1.36, (95% CI, 1.15-1.65; and high: IRR, 1.43; 95% CI, 1.20-1.73). Counties with low state and low or medium interstate policy scores had the highest rates of firearm homicide. Counties with low home state and interstate scores had higher firearm homicide rates (IRR, 1.38; 95% CI, 1.02-1.88) and overall homicide rates (IRR, 1.32; 95% CI, 1.03-1.67). Counties in states with low firearm policy scores had lower rates of firearm homicide only if the interstate firearm policy score was high. Conclusions and Relevance Strong state firearm policies were associated with lower suicide rates regardless of other states' laws. Strong policies were associated with lower homicide rates, and strong interstate policies were also associated with lower homicide rates, where home state policies were permissive. Strengthening state firearm policies may prevent firearm suicide and homicide, with benefits that may extend beyond state lines.