Violent death—including homicides and suicides—is a leading cause of mortality and a major contributor to heath disparities in the United States. A total of 67,299 people were killed in violent incidents in 2017, including 19,510 from homicide (1). Risks for homicide are greater for men (2, 3), adolescents and young adults (2), and Black and Hispanic people (4, 5). Homicide is the leading cause of death for black men aged ≤ 44 and is a major contributor to differences in life expectancy between White and Black men (6). Identifying conditions that contribute to the occurrence of homicide is an essential step towards developing effective preventive interventions that reduce the absolute health burden and socio-demographically structured health disparities.
Many decades of theory and empirical evidence identify that neighborhood social environmental conditions are important determinants of violent crime and could therefore be candidates for intervention. For instance, early theories of criminology described human behavior as the result of conscious processes, wherein rational actors weigh prospective costs and benefits of carrying out crime (7). From this perspective, violent crime (assault, homicide, rape, and robbery) can be understood as a solution to an optimization problem (8). The prospective costs of choosing to act violently include injury to oneself, imprisonment, loss of future conventional opportunity; conversely the prospective benefits include settling a grievance, winning access to a resource, enhanced social capital (within some cultural contexts) (9). This explanation of the etiology of individual violent events can be expanded to explain observed distributions of violence within and between populations. The costs of violence typically outweigh the benefits, so violence is a statistically rare event, occurring in only a small proportion of human interactions; but reduced access to social, physical, and economic resources can tip the balance in favor of violence. By definition, access to such resources is more limited for disadvantaged populations and in disadvantaged neighborhoods, and when disadvantaged people conceive such disparities as “losses” they are even more likely to engage in risk-seeking behaviors such as violent crime (10).
An important addendum to rationalist explanations of violent crime is that environmental characteristics, whether at the city, state, or national level, are powerful predictors of violent crime (11–13). There is a growing literature base arguing that indices of relative socioeconomic disadvantage, such as income inequality, are strong predictors of crime (14, 15). Several theories have attempted to explain this relationship between income inequality and violent crime (11, 16). One of the initial hypotheses by Shaw and McKay (1942) stated that concentration of poor economic conditions lead to social disorganization through a breakdown of social cohesion. Social disorganization theory suggests that a person’s residential location is more significant than the person’s characteristics when predicting criminal activity. Moreover, inequality and the concentration of poor economic conditions lead to social disorganization through a breakdown of social cohesion and norms. When these economic inequalities are associated with ascribed characteristics such as race, latent animosities and a situation characterized by social disorganization is created (11). The consequence is socially structured inequalities that result in feelings of “resentment, frustration, hopelessness, and alienation” which the theory suggests leads to widespread social disorganization and violent crime (11).
Empirical studies generally support these theoretical predictions regarding geographic distributions of violent crime. Research identifies that violent crime, including homicide, concentrates in specific neighborhoods (17, 18). For example, 74% of crimes in Boston occurred in 5% of city blocks (17). In Seattle, crime reduction was due mostly to crime declines in a small group of street segments (19). More recently, researchers have demonstrated that the concentration of crime at particular places is stable over time (17, 19). A wide range of physical and social environmental conditions are associated with violent crime incidence in small areas (20, 21). Violence is higher in communities where there are limited economic opportunities; where there are high concentrations of poor and unemployed people; and where there is greater residential instability (22). One study found remediation of abandoned buildings in Philadelphia significantly reduced firearm violence as did vacant lot remediation (23). Abandoned buildings and vacant lots can serve as out-of-sight staging or storage areas for illegal firearms until they are needed (24).
With substantial evidence that economic disadvantage drives crime, a fundamental debate remains on whether it is relative socioeconomic disadvantage (i.e., income inequality) or absolute socioeconomic disadvantage (i.e., income) that is indeed the drive of the relationship. The argument linking income inequality and violent crime differs because income inequality is not an aggregate variable like poverty. The connection between poverty and violent crime is resource deprivation. Moreover, resource deprivation or “material disadvantage” causes frustration which can result in violent aggression (10). Disentangling the contribution of income and income inequality from the multiple potential confounders, including the demographic composition of the resident population, is a methodologically complex problem. Further, a social ecological systems perspective—the dominant theoretical framework that guides much epidemiologic research in neighborhoods and health (25, 26)—suggests that determinants of homicide will also be multi-scale and dynamic, and will reflect fundamental macrosocial causes of structural disadvantage that increase risk for crime and violence. For example, neighborhoods with high rates of poverty and income inequality will be related to distal social and economic policies at county, state, and federal levels. In addition, the strength of state-level measures of poverty and income inequality may affect crime in local areas. The extent to which local and state-level measures of relative and absolute measures of socioeconomic disadvantage are associated with homicide is poorly understood.
The aim of this study was to identify whether income or income inequality at the ZIP-code or state-level contribute to homicide in small areas, operationalized as US ZIP codes. A matched ecological case-control design allowed us to address problems related to various highly correlated confounders, and to examine why some neighborhoods with high concentrations of racial and ethnic minority residents have high homicide rates while other such neighborhoods do not experience homicide.