Over the last few years, firearm violence rates in US cities have increased. In order to better study this phenomenon, a team including PISC Internal Advisory Board member John MacDonald, PhD, investigated contagious and non-contagious space-time clustering in Chicago, Los Angeles, New York, and Philadelphia shootings from 2016 to 2020. The study found that contagious spread of violence increased in 2020 in Chicago, Los Angeles, and Philadelphia. Spontaneous events characterized most shootings, and the spatial location and concentration of shooting hotspots remained stable across all years. The authors conclude that these findings have large policy implications, such as that increases in the proportion of contagious gun violence may be tackled through short-term, spatially-focused efforts to disrupt contagious spread.