A novel gun law strength index and influential state gun laws for firearm homicide in the United States, 1999–2018.

Although some existing indices quantified the mixture strength of state gun laws, methodological supports are still lacking. The study aimed to build a new state gun law strength (GLS) index, evaluate the contribution of all state gun laws to the new index, and assess the association between the new index and firearm homicide mortality. This is a population-based ecological study. We condensed 134 state gun laws into a mixture GLS index at the state level to quantify each state's overall GLS by using the weighted quantile sum regression in each year from 1999 to 2018. The weights of state gun laws in the GLS index revealed their influence on GLS. The model also estimated the association between the GLS index and state-level firearm homicide mortality. From 1999 to 2018, 15 of 134 (11.19%) state gun laws significantly contributed to the GLS index for firearm homicide every year, five of which related to the minimum age of possessing firearms. The most influential gun law was "no possession of handguns until age 21." The GLS index was significantly and negatively associated with firearm homicide mortality; however, the association gradually diminished over time. The GLS index has methodological support and can take different gun violence outcomes into account. Future research can adopt the GLS index to conduct additional gun violence research or apply the modeling approach to build new GLS indexes for other gun violence outcomes. [ABSTRACT FROM AUTHOR]