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Association of State-Level Firearm-Related Deaths With Firearm Laws in Neighboring States

Key Points Question How are states’ firearm laws associated with firearm-related deaths in nearby states? Findings In this pooled cross-sectional analysis involving firearm laws and firearm-related deaths from 2000 to 2019 in the 48 contiguous states, a permit requirement for purchasing all firearms had an interstate association with decreased total firearm-related deaths and homicide, whereas the prohibition of firearm possession for individuals who have committed a violent misdemeanor had an interstate association with decreased firearm suicide. Meaning These findings suggest that synergic legislative action to implement firearm laws in proximate states may help prevent firearm-related deaths.


Introduction
Gun violence continues to be a major public health problem in the US. The years 2020 and 2021 have seen extraordinarily high numbers of gun-related fatalities. 1 The response from the political leadership continues to be polarized, with Democrat lawmakers calling for more stringent gun regulations and Republican lawmakers dismissing the need for and the effectiveness of such regulations. On June 23, 2021, the Biden-Harris administration announced its strategy to combat gun violence using funding from the Rescue Act, 2 a critical component of which addresses the flow of firearms used to commit crimes by launching multijurisdictional firearms trafficking strike forces to stop illegal gun trafficking across state lines.
Guns are easily transportable, and there are relatively few barriers to gun migration and gun trafficking from states with weak gun regulations to states with strong gun regulations. For example, crime gun tracing data suggest that, in high-regulation states, crime guns are more likely to have been purchased out of state. [3][4][5] Although actual firearm migration is difficult to measure, there is a growing body of research on interstate differences in gun regulations and within-state firearmrelated violence. [6][7][8][9] A recent study by Liu et al 9 used data on all states from 2000 to 2017 and found that weaker firearm laws in neighboring states were associated with higher within-state firearmrelated deaths and, further, that failing to account for weaker laws in neighboring states led to underestimation of the impact of the state's own laws on within-state firearm-related deaths.
The present study extends the work of Liu et al 9 in 2 critical ways. First, instead of considering the laws in neighboring states only, we used a spatial analysis approach that considers the geographical distance between all contiguous states in the US . Second, instead of aggregating across firearm laws, this study used a more granular approach toward categorizing firearm laws to better inform policy makers on which laws were most strongly associated with firearm-related deaths.

Study Sample
All states of the US were included in this cross-sectional study except Alaska and Hawaii because they are noncontiguous with other US states. The District of Columbia was excluded because it has no relevant entries on state laws in the State Firearm Law Database. The final analysis included the remaining 48 states. The study period was from January 1, 2000, to December 31, 2019. This study was deemed exempt from human participant review by the institutional review board of the University of Alabama at Birmingham. The study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline.

Measures
The total numbers of firearm-related deaths by state from 2000 to 2019 were extracted from the Web-based Injury Statistics Query and Reporting System (WISQARS) of the Centers for Disease Control and Prevention 10 as main outcome variables, including deaths due to all intent, homicide excluding legal intervention, and suicide. WISQARS is a public-access, free online database that provides data on fatal and nonfatal injuries of various causes, violent death, and cost of injuries from a variety of trusted sources.

Outcome Measures
The rates of total firearm-related deaths of each state and year were calculated as the main outcome variable. The rates of firearm-related homicide and suicide were also calculated as secondary outcomes.
Information on state firearm laws was obtained from the State Firearm Law Database 11 developed by Siegel et al. 12 This database tracks the presence of 134 laws in 14 categories across all 50 states from 1991 to 2020. Categories include buyer regulations, dealer regulations, background checks, prohibition of gun purchase and possession, domestic violence-related gun laws, "stand your ground" laws, concealed carry permitting laws, assault weapons regulations, gun trafficking laws, and restrictions on places where guns may be carried. Each of the 134 laws are coded based on their year of implementation as being either present (1) or absent (0) for each state during each year. The deciles of the total number of all firearm laws were used as an index to capture the strictness of states' own firearm regulations.

Firearm Regulations
For this study, the following categories of laws were identified based on previous literature as having potential association with interstate movement of firearms and firearm-related mortality 6,7,9,[13][14][15][16][17] : (1) requiring universal background checks at the point of purchase for all firearms (background check laws); (2) requiring background checks for handgun sale at gun shows (gun show laws); (3) requiring a license or permit to purchase all firearms (permit laws); (4) requiring a state dealer license for sale of handguns (licensed dealer laws); (5) requiring all private sellers and licensed dealers to keep and retain records of handgun sales (record-keeping laws); (6) prohibiting any person from purchasing a handgun on behalf of another person (straw purchase laws); (7) prohibiting firearm possession for people who have committed a violent misdemeanor (violence prohibition laws); (8) requiring people to relinquish their firearms after they become prohibited from possessing them (relinquishment laws); and (9) providing authorities with discretion in deciding whether to grant a concealed carry permit, or the law bans all concealed weapons (may-issue laws).

Other Covariates
We controlled for additional state-level and potentially time-varying covariates that may be associated with the number of violent deaths based on previous literature. 7,9,13,18 These covariates Wildlife-Associated Recreation, 22 were included as indicators of household gun ownership. 13,23,24 As a measure of the general state sentiment toward firearm regulation, the vote share differences between the Republican and Democratic presidential candidates in each presidential election year within the study period 25 were included, and extrapolated for years between presidential elections.

Statistical Analysis
To investigate the association between states' firearm laws and other states' firearm-related deaths (interstate association), we built a spatial autoregressive linear model (spatial Durbin model), accounting for geographically correlated dependent and independent variables 26 : y nt = ρWy nt + X nt β + P nt γ + WP nt θ + μ n + τ t + ⑀ nt where y denotes the rate of firearm-related deaths; ρ, the spatial autoregressive coefficient; W, the spatial weight matrix; X, the state-level variables, including the deciles of the total number of firearm laws and other state-level covariates; P, the laws of interest; μ, the state fixed effect; and τ, the year fixed effect. In this spatial Durbin model, coefficients corresponding to the independent variables are difficult to interpret and, moreover, do not directly correspond to obvious measures of association.
To resolve this, an overall association via a total effect is then calculated 26,27 that compares what would happen in the (hypothetical) scenario in which all those 48 contiguous states have that particular law with the (also hypothetical) scenario in which no state has that law. Furthermore, this total effect can be decomposed into within-state (direct effect) and interstate (indirect effect or spillover effect) associations. 27 Details are shown in eMethods and eFigure 1 in the Supplement.

JAMA Network Open | Public Health
To account for the distance decay of the association between firearm laws and outcome, the inverse distance squared weight matrix was used for the primary analysis, because the contiguity matrix may underestimate the association. First, a basic model without any of the 9 laws of interest was built (ie, equation 1) with P = 0. Then each law of interest was added to the basic model one at a time, and we selected laws that resulted both in a decreased Akaike information criterion and P < .10 for the likelihood ratio test between the model including this law and the basic model for the final model. Then we built a pairwise correlation matrix for the selected laws to check for the potential collinearity, and highly collinear variables were removed from the final model. The final model was also used for analyzing the secondary outcomes.
We performed the following sensitivity analyses: (1) using state random effects in addition to year and census division 28 fixed effects, (2) using a contiguity matrix instead of an inverse distance squared matrix, and (3)

Results
Baseline variables are summarized in Table 1. We identified 662 883 firearm-related deaths of all intents, which translated to a rate of 10.80 firearm-related deaths per 100 000 population during the study period. For this period, the median number of implemented firearm laws was 15 (range, 1-111). The distribution of how many of the 9 laws of interest were implemented for each state and the rates of firearm-related deaths of all intents in that state compared with the national median are shown for the first and last year of the study period in the Figure. The effect sizes of within-state, interstate, and overall associations between each law and total      Index of strictness of firearm regulation c −0.07 (−0.14 to 0.00) .06 0.01 (−0.01 to 0.03) .23 −0.06 (−0.12 to 0.00) .07 a Effect sizes are shown as per 100 000 population.
b Models were adjusted for the following state-level time-varying variables: population size, proportion 65 years or older, race and ethnicity, unemployment rate, poverty rate, proportion 25 years or older without a high school diploma, the rates of crime against property, the percentage of state's population holding a hunting license, number of licensed gun dealers per 100 000 residents, and the vote share difference between the Republican and Democratic candidates in the presidential election (linearly interpolated), in addition to state and year fixed effect.
c Expressed as the decile of the total number of state firearm laws.

Discussion
In this cross-sectional study, we used data from 2000 to 2019 to investigate the association between firearm laws and firearm-related deaths both within state and interstate. Our findings suggest that having a permit requirement for purchasing all firearms was associated with a decrease of any firearm-related death and firearm-related homicide within state, interstate, and overall. The prohibition of firearm possession for individuals who have committed a violent misdemeanor had within-state and overall associations with decreased total firearm-related death rates. This law also had within-state, interstate, and overall associations with decreased firearm-related suicide rates.
Firearm laws vary substantially across states in the US. A rich literature has established associations between stricter state firearm laws and reduced firearm-related violence within those states. 17,[32][33][34] States with strict laws also host fewer firearm-manufacturing establishments than states with relatively permissive laws. 35  used county-level firearm-related homicide data to evaluate the spillover effects of different categories of firearm laws. They found that the benefits of within-state firearm laws in reducing firearm-related homicide were not independent of firearm laws in nearby states, which is supported by our study findings.
Our study adds to this literature by evaluating specific firearm laws individually rather than as an aggregated index. Particularly, our findings suggest that permit-to-purchase laws, which research finds to be protective against within-state firearm-related deaths, 17 are also protective for neighboring states, suggesting that more restrictions on the eligibility of obtaining a gun also discourage out-of-state persons from buying firearms from those states. Further, the laws prohibiting firearm possession for people who have committed a violent misdemeanor were associated with decreased firearm-related suicide rates within state, interstate, and overall and with decreased total firearm-related death rates both within state and overall. Prior studies have found that safe storage laws 37 and alcohol regulation 38 were associated with within-state declines in firearm suicides. At the same time, other studies 39,40 have established correlations between committing acts of violence and inflicting self-harm, 39,40 and our findings imply that regulations preventing firearm possession among those who commit violence in turn also reduce extreme self-harm in both within-state and nearby state populations. We speculate that spillover protective effects occur because the presence of these laws disincentivizes gun traffickers from acquiring guns in these states to send to neighboring states and disincentivizes individuals in neighboring states from crossing state borders and purchasing a weapon.
In models that included laws individually with other state covariates, may-issue and recordkeeping laws also showed associations with total firearm-related deaths. However, collinearity issues (eTable 3 in the Supplement) and a tendency among states to concurrently implement regulations regarding gun show, background check, and record-keeping laws (eTables 8 and 9 in the Supplement) limited our ability to include them in the final model. Notably, some of the firearm laws that were intended to reduce firearm-related deaths did not show any conclusive association. This could be either a true nonassociation or an association that could not be detected by our method owing to limited sample size, rarity of states that have such laws, and insufficient time variability of those laws during our study period.

Conclusions
In this pooled cross-sectional analysis of firearm laws and firearm-related deaths from 2000 to 2019, we found that permit-to-purchase laws were associated with decreased firearm-related death rates both within state and interstate. The presence of interstate association between firearm laws and firearm-related deaths suggests that policy initiatives to reduce gun trafficking-such as those adopted by the Biden-Harris administration-are an important component of eliminating firearm violence. It also underlines the importance of synergic legislative action to implement laws such as permit requirements in proximate states as an effective approach to reduce firearm-related deaths.