Phillips, D. P., & Hensley, J. E. (1984). When violence is rewarded or punished: The impact of mass media stories on homicide. Journal of Communication, 34 (3), 101-116.

Abstract:  Reviews 8 studies indicating that some mass media stories in which violence is rewarded or unpunished are followed by increases in suicides, auto accidents, plane accidents, and murders. In all these studies, the more publicity given to the story, the greater the increase in mortality thereafter. These studies strongly suggest that some mass media stories trigger imitative increases in fatal violence. In the present study, 140,000 US daily homicides were examined before and after prizefights, innocent verdicts, life sentences, death sentences, and executions. Data indicate that the number of homicides significantly decreased for no more than 4 days after the broadcasting of a punishment story. (47 ref) (PsycINFO Database Record (c) 2004 APA, all rights reserved)