Loopholes in federal and state law make it easy for dangerous people to obtain firearms and commit crimes. Previous research by Everytown for Gun Safety found that more than one in three perpetrators of mass shootings—incidents with four or more firearm homicide fatalities—had a prior history of felonies, domestic violence, or mental illness that prohibited him from buying or possessing guns.
This analysis offers further evidence that common-sense public safety laws may help prevent dangerous people from obtaining guns and save lives: controlling for population, in states that require background checks for all handgun sales, there were 52 percent fewer mass shootings between January 2009 and July 2015. Notably, states that require criminal background checks on all handgun sales experienced 63 percent fewer mass shootings committed by people prohibited from possessing firearms and 64 percent fewer domestic violence mass shootings.
The FBI defines “mass shooting” as any incident where at least four people were murdered with a gun. Everytown has documented these incidents comprehensively since January 2009 and identified 133 incidents that met this definition by July 2015. A prior analysis of these shootings showed that, similar to gun violence generally, nearly 40 percent were perpetrated by assailants who were prohibited by federal law from possessing guns, and which more effective enforcement of existing gun laws might therefore have prevented.Everytown for Gun Safety, Analysis of Recent Mass Shootings, August 2015, available at http://every.tw/1WIarom. There was also a noteworthy connection between mass shootings and domestic or family violence: in 57 percent of the shootings, the perpetrator killed a current or former intimate partner or family member.
A nationwide analysis of mass shootings may nevertheless mask significant differences between patterns in individual states. In particular, there is a growing body of evidence that state laws requiring background checks for all handgun sales deter criminals and domestic abusers from accessing firearms. To test whether such laws were associated with a different frequency of mass shootings, Everytown compared the frequency of mass shootings in states that required background checks for unlicensed handgun sales to states that did not, controlling for the number of residents living in both groups of states each year.
At the beginning of the period of observation, 15 states and the District of Columbia required background checks on all handgun sales; by the end of the period, Colorado and Washington State had extended background check requirements to all gun sales.Since then, Oregon has passed a similar requirement. To account for these legislative changes during the period of observation, the number of residents living in a state each year was converted into “person-years” lived there, and assigned to either the group of states requiring background checks for all handgun sales or the group without, based on the law of the state during that year. For example, Colorado, which passed a background check law in 2013, accounted for 20,384,693 person-years without a background check requirement on all handgun sales between January 1, 2009 and June 30, 2013, and the state accounted for 13,752,207 person-years with a background check requirement between July 1, 2013 and July 31, 2015.Because 2015 population estimates had not been released at the time of analysis, and the period of observation extends over only seven months of 2015, person-years in 2015 were estimated to be seven-twelfths of the 2014 population.
The frequency of mass shootings in each group of states was calculated, controlling for the number of person-years during the period of observation. Separate analyses were conducted to assess the frequency of particular subgroups of mass shootings: those committed by prohibited people and those committed against intimate partners or family.
Between January 1, 2009 and July 31, 2015, there were 37 mass shootings in states where background checks were required for all handgun sales and 96 mass shootings in states where they were not. Controlling for population, there were 52 percent fewer mass shootings in states that require background checks for all handgun sales than in states that do not.
The difference was more pronounced among shootings committed by prohibited people. During the period of observation, there were 44 mass shootings committed by assailants known to be prohibited from possessing firearms—10 in states that require background checks for all handgun sales and 34 in states that do not. Controlling for population, there were 63 percent fewer mass shootings committed by people prohibited from possessing firearms in states that require background checks for all handgun sales than in those that do not.Everytown for Gun Safety, Analysis of Recent Mass Shootings, August 2015, available at http://every.tw/1WIarom.
There was also a more pronounced difference in mass shootings committed against domestic partners or family members. Everytown identified 76 such incidents — 17 in states that require background checks for all handgun sales and 59 in states that did not. Controlling for population, there were 64 percent fewer domestic violence mass shootings in states that require background checks for all handgun sales.
This analysis finds a pronounced association between state laws requiring criminal background checks for all handgun sales and the frequency of mass shootings, an association that is particularly pronounced for mass shootings committed by prohibited people and in the context of domestic or family violence.
Correlation does not equal causation. Due in part to longstanding restrictions on access to crime gun trace data, in the majority of cases Everytown was unable to determine how individual mass shooters obtained their firearms.
But the results of this analysis are consistent with data from other sources, which show that states that require background checks on all handgun sales have lower levels of gun violence. Specifically, in states with background checks for all handguns, 46 percent fewer women are shot to death by their intimate partners,Everytown for Gun Safety, State Background Check Requirements and Rates of Domestic Violence Homicide, January 2015, available at http://every.tw/1Aj9HZj. 48 percent fewer law enforcement officers are killed with handguns,Everytown for Gun Safety, State Background Check Requirements and Rates of Homicide Against Law Enforcement Officers, January 2015, available at http://every.tw/1FpRqkh. and there is 48 percent less gun trafficking.Daniel W. Webster, Jon S. Vernick, and Maria T. Bulzacchelli, “Effects of State-Level Firearm Seller Accountability Policies on Firearm Trafficking,” Journal of Urban Health 86, no. 4 (July 2009): doi:10.1007/s11524-009-9351-x.
Click here to see a full appendix of the data used to compile this fact sheet.