As part of the firearms special issue, authors Ali Rowhani-Rahbar, Duane Alexander Quistberg, Erin R Morgan, Anjum Hajat, Frederick P Rivara have contributed this blog post that articulates with their original article Income Inequality and Firearm Homicide in the United States: A County Level Cohort Study.

The U.S. has one of the highest levels of income inequality among high-income countries in the World. Income inequality, defined as the extent to which income is distributed unevenly among a population, has been on the rise in the U.S. since 1970s. Within the U.S., there is tremendous variability in income inequality levels by state, metropolitan areas, and county. In some counties, for example, the average income of the top 1% is about 45 times greater than that of the bottom 99%. As a result of multiple investigations over the past 40 years, there is a well-established literature about the negative impact of income inequality on several measures of health and well-being at both the individual and societal level.

Violence, as a major health problem in the U.S. and a sensitive indicator of social relations, has been previously studied in relation to income inequality. The evidence indicates that there is a strong relationship between income inequality and the most severe form of violence, namely homicide. Homicide claims the life of 20,000 individuals each year and is the leading cause of death among African–American males aged 15–34 year in the U.S. Firearms play a prominent role as the most common mechanism of homicide in the U.S. Strikingly, the last investigation of the relationship between income inequality and firearm violence was published in 1998. That study’s investigators found that state-level income inequality was strongly correlated with firearm homicide (r=0.76). That relationships held when controlling for poverty and a proxy variable for access to firearms.

Using national data from the Centers for Disease Control and Prevention on firearm homicide among victims aged 14-39 years between 2005 and 2015, we conducted a study to build on the findings of that prior investigation by: [1] examining the association between income inequality and firearm homicide at the county level; [2] adding several other contextual determinants of firearm homicide; and [3] focusing on race/ethnicity-specific patterns. Our goal was to: [1] provide estimates that can be used to further inform firearm violence prevention efforts which go beyond individual level; and [2] help with future theoretical assessment and empirical investigations of other determinants of firearm violence which may need to take income inequality into account.

We used the Gini index as a measure of income inequality which ranges from 0, indicating perfect equality (where everyone receives an equal share of the income), to 1, perfect inequality (where only one individual or group of recipients receives all the income). We examined the association of the Gini index in 1990 and 2000 with firearm homicide rates during 2005-2015 among individuals aged 14-39 years in each county in all races/ethnicities combined, and in each race/ethnicity group separately. Income inequality is unlikely to affect firearm homicide rates immediately; as such, we included those minimum 5-year and 15-year “lag” periods in the analyses. We also examined the aforementioned relationships after accounting for multiple factors that could determine the rates of firearm homicide (e.g., county deprivation level, county social capital).

We found that the Gini Index was associated with firearm homicide rates among all races/ethnicities. Firearm homicide rates were about 60% higher for every 4% greater value of the county-specific Gini index. After accounting for multiple other contextual determinants of firearm homicide, the association persisted among African–Americans. In this group, every 4% greater value of the Gini Index was associated with a 9% higher firearm homicide rate at the county level. These results did not materially change when examining firearm homicide rates during 2005-2015 in relation to the Gini index in 1990 or 2000.

These findings highlight the importance of policies that attempt to shift the underlying societal forces giving rise to firearm violence at the population level. Policies addressing macrosocial forces such as those that reduce the gap between the rich and the poor (e.g., earned income tax credit) may deserve consideration for reducing firearm violence, especially among the subgroups mostly affected by it. Additionally, refinement of socioeconomic deprivation measures to include elements of income inequality (a relative measure of deprivation) beyond poverty (an absolute measure of deprivation) may meaningfully advance public health research and practice.