The Impact of State Firearm Laws on Homicide and Suicide Deaths in the USA, 1991-2016: a Panel Study.

Background: Firearm injuries are a major cause of mortality in the USA. Few recent studies have simultaneously examined the impact of multiple state gun laws to determine their independent association with homicide and suicide rates.
Objective: To examine the relationship between state firearm laws and overall homicide and suicide rates at the state level across all 50 states over a 26-year period.
Design: Using a panel design, we analyzed the relationship between 10 state firearm laws and total, age-adjusted homicide and suicide rates from 1991 to 2016 in a difference-in-differences, fixed effects, multivariable regression model. There were 1222 observations for homicide analyses and 1300 observations for suicide analyses.
Participants: Populations of all US states.
Main Measures: The outcome measures were the annual age-adjusted rates of homicide and suicide in each state during the period 1991-2016. We controlled for a wide range of state-level factors.
Key Results: Universal background checks were associated with a 14.9% (95% CI, 5.2-23.6%) reduction in overall homicide rates, violent misdemeanor laws were associated with a 18.1% (95% CI, 8.1-27.1%) reduction in homicide, and "shall issue" laws were associated with a 9.0% (95% CI, 1.1-17.4%) increase in homicide. These laws were significantly associated only with firearm-related homicide rates, not non-firearm-related homicide rates. None of the other laws examined were consistently related to overall homicide or suicide rates.
Conclusions: We found a relationship between the enactment of two types of state firearm laws and reductions in homicide over time. However, further research is necessary to determine whether these associations are causal ones.