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 anoth er has been entertained for some time by social scientists, yet systematic efforts to demonstrate the existence, or estimate the strength, of such a d iffusion process are just beginning. This paper uses exploratory spatial da ta analysis (ESDA) to examine the distribution of homicides in 78 counties in, or around, the St. Louis metropolitan area for two time periods: a peri od of relatively stable homicide (1984-1988) and a period of generally incr easing homicide (1988-1993). The findings reveal that homicides are distrib uted nonrandomly, suggestive of positive spatial autocorrelation. Moreover, changes over time in the distribution of homicides suggest the possible di ffusion of lethal violence out of one county containing a medium-sized city (Macon County) into two-nearby counties (Morgan and Sangamon Counties) loc ated to the west. Although traditional correlates of homicide do not accoun t for its nonrandom spatial distribution across counties, we find some evid ence that more affluent areas, or those more rural or agricultural areas, s erve as barriers against the diffusion of homicides. The patterns of spatia l distribution revealed through ESDA provide an empirical foundation for th e specification of multivariate models which can provide formal tests for d iffusion processes.