"The crucial variable is not the presence of firearms...

[russ1943]

17. Gangs account for around 13% THIRTEEN PERCENT of homicides.

Last edited Fri Aug 7, 2015, 04:50 PM - Edit history (2)

Gang-related homicides typically accounted for around 13 percent of all homicides annually…In a typical year in the so-called “gang capitals” of Chicago and Los Angeles, around half of all homicides are gang-related; these two cities alone accounted for approximately one in four gang homicides recorded in the NYGS from 2011 to 2012. https://www.nationalgangcenter.gov/Survey-Analysis/Measuring-the-Extent-of-Gang-Problems.
13% is considerably less than a primary cause IMO.

The National Gang Center (NGC) is a project jointly funded by the U.S. Department of Justice’s (DOJ) Office of Juvenile Justice and Delinquency Prevention (OJJDP) and the Bureau of Justice Assistance (BJA). The NGC program works to further the mission of DOJ by providing national leadership, information, training, and technical assistance that target gangs and street gang members of all ages. Serving researchers, policymakers, and practitioners nationally, NGC activities contribute to reductions in gang-related crime and violence and gang activity by juveniles and adults.

http://www.nationalgangcenter.gov/Survey-Analysis/Measuring-the-Extent-of-Gang-Problems#homicidesnumber