Analyzing Diffusion and Contagion Effects: The Urban Disorders of the 1960s

Abstract

This study concerns the analysis of diffusion and contagion processes using a lognormal model of overdispersion phenomena. The urban disorders of the past decade are examined and two processes are found to exist in the 1966–67 period. One is a classic diffusion effect in which disorders are precipitated by events which are independent of each other, but lead to outcomes such as numbers of arrests which are proportional to previous disorders. The second process is a contagious one in which disturbances occur as a consequence of smaller cities imitating the behavior of large ones experiencing a disorder. It was found that the explanatory power of the interaction effect between police and black city residents tended to increase as city size increased. Concomitantly, the effects of environmental variables tended to decrease in explanatory power as city size decreased.