Absolute versus relative socioeconomic disadvantage and homicide: a spatial ecological case–control study of US zip codes

Background: Homicide is a major cause of death and contributes to health disparities in the United States. This burden overwhelmingly affects people from racial and ethnic minority populations as homicide occurs more often in neighborhoods with high proportions of racial and ethnic minority residents. Research has identified that environmental factors contribute to variation in homicide rates between neighborhoods; however, it is not clear why some neighborhoods with high concentrations of racial and ethnic minority residents have high homicide rates while neighborhoods with similar demographic compositions do not. The aim of this study was to assess whether relative socioeconomic disadvantage, (i.e., income inequality), or absolute socioeconomic disadvantage (i.e., income) measured at the ZIP code- and state-levels, is associated with high homicide rates in US ZIP codes, independent of racial and ethnic composition. Methods: This ecological case–control study compared median household income and income inequality in 250 ZIP codes with the highest homicide rate in our sample in 2017 (cases) to 250 ZIP codes that did not experience any homicide deaths in 2017 (controls). Cases were matched to controls 1:1 based on demographic composition. Variables were measured at both the ZIP code- and state-levels. Results: Lower median household income at the ZIP code-level contributed most substantially to the homicide rate. Income inequality at the state-level, however, was additionally significant when controlling for both ZIP code- and state-level factors. Conclusions: Area-based interventions that improve absolute measures of ZIP code socioeconomic disadvantage may reduce gaps in homicide rates.