Mass shootings occur more often in states where residents own more guns, and murder rates are higher in states with looser concealed carry rules, a new research paper argues.

Almost 40,000 people die annually in the United States from homicides, suicides and accidents involving firearms, and three-quarters of all murders are by gun. Emma E. Fridel of Florida State University found that mass shootings, defined as an act with four or more victims killed, happen an average of 23 times per year, and she evaluated 592 such massacres between 1991 and 2016.

The contributing factors for mass shootings and other gun homicides were different, and therefore require different policy responses, Fridel explained.

“More permissive concealed carry legislation was associated with a 10.8% increase in the firearms homicide incidence rate, yet had no significant effect on mass shootings,” concluded the study, published in Justice Quarterly. “Similarly, household gun ownership was associated with a 53.5% increase in the mass shooting incidence rate, yet has a minimal impact on firearm homicides when accounting for concealed carry legislation.”

Fridel evaluated the effect of several factors on the number of murders, including the property crime rate, divorce rate and percentage of men in a state. States with an “average” level of gun ownership experienced a mass shooting once per 2.4 years. Those with high levels of gun ownership had massacres as often as 1.25 years, compared with once every 4.7 years for states with low ownership.

Fridel noted that two-thirds of mass shootings happen in residences, suggesting the availability of a firearm in the home connects gun ownership with the prevalence of shootings. In addition, an armed third party only stopped one massacre between 2000 to 2013, compared to 21 unarmed interventions.

“Equally important to note is that other factors often cited in the wake of mass shootings, such as access to mental healthcare, do not significantly influence the rate of these crimes,” she wrote.

Although the Gun Violence Archive lists four “mass shootings” in Colorado in 2019, each of those, including the incident at STEM School Highlands Ranch that took the life of one teenager, predominantly resulted in gun injuries that did not kill at least four people. Those crimes would not have been eligible for inclusion in the study.