The spatial patterning of county homicide rates: An application of exploratory spatial data analysis

The possibility that homicides can spread from one geographic area to another has been entertained for some time by social scientists, yet systematic efforts to demonstrate the existence, or estimate the strength, of such a diffusion process are just beginning. This paper uses exploratory spatial data analysis (ESDA) to examine the distribution of homicides in 78 counties in, or around, the St. Louis metropolitan area for two time periods: a period of relatively stable homicide (1984–1988) and a period of generally increasing homicide (1988–1993). The findings reveal that homicides are distributed nonrandomly, suggestive of posi-tive spatial autocorrelation. Moreover, changes over time in the distribution of homicides suggest the possible diffusion of lethal violence out of one county containing a medium-sized city (Macon County) into two nearby counties (Morgan and Sangamon Counties) located to the west. Although traditional cor-relates of homicide do not account for its nonrandom spatial distribution across counties, we find some evidence that more affluent areas, or those more rural or agricultural areas, serve as barriers against the diffusion of homicides. The pat-terns of spatial distribution revealed through ESDA provide an empirical foun-dation for the specification of multivariate models which can provide formal tests for diffusion processes. KEY WORDS: spatial patterning; homicide; county homicide rates; exploratory spatial data analysis