Several regression models of strain theory reveal a positive correlation between income inequality and mass shootings as well as an interaction effect of higher median household income and higher income inequality with higher incidents of mass shootings. Strain theory posits that communities which experience relatively high levels of economic deprivation and a lack of opportunity for social mobility create hostile environments conducive to high rates of violent crime. Several previous studies have found a significant relationship between income inequality, class salience and violent crime without specifically focusing on mass shootings, operationally defined as shootings involving four or more victims and excluding gang and drug related shootings. The particular model in this study found that a one standard deviation increase in income inequality (using the post-tax Gini Coefficient) and a 0.53 increase in household income predicted a 0.46 increase in incidents of mass shootings. Demographic controls such as population density, median population age, and proportion of minority population were also positively associated with incidents of mass shootings.
Using this assessment, we can see that the availability and ease of acquiring firearms is not the sole risk factor for mass shootings, and thus should not be the be all end all of the discussion. In order to properly address the mass shooting epidemic we really need to address widening disparities in social class, economic deprivation, and the policies that create it. Much of income inequality stems from the government's fiscal and land use policies: regressive taxes at state and local level (e.g. sales taxes, property taxes and court fines) and rents outpacing wages, making the poor poorer, due to artificial and natural scarcity in housing stock and a lack of diversity of housing options (due to exclusionary zoning). Combine these state and local policies with low interest rates, which being inverse to stock and bond prices make the rich richer, and you have a recipe for growing income inequality. Of course, remedying the causes of growing income inequality would not completely prevent mass shootings, but it would eliminate environments that foster these incidents.