These investigators ran a regression over 17 years of the rate of mass shootings in states on the permissiveness of their firearm laws. There are various definitions of mass shootings, but they use a relatively non-restrictive one: four or more people killed in a single incident. Most firearm deaths are actually suicides and most homicides do not result from mass shootings, but these events get a lot of attention. The findings are stark and quite compelling:
Table 1 shows that in fully adjusted models, a 10 unit increase in state gun law permissiveness was associated with a significant 11.5% (95% confidence interval 4.2% to 19.3%, P=0.002) higher rate of mass shootings. A 10% increase in state gun ownership was associated with a significant 35.1% (12.7% to 62.7%, P=0.001) higher rate of mass shootings.The only thing that stops a bad guy with a gun is not letting have a gun in the first place.