Drugs, Guns and Gangs: An Agent-Based Model of
Homicide
George Tita, Rob Axtell
gtita@uci.edu, raxtell@brook.edu
Using an ABM framework, we examine changes in the level
and pattern of homicide over a 25-year period for Watts, a particularly violent
neighborhood of the City of Los Angeles. The homicide trend for this
neighborhood demonstrates two clear peaks: The first occurs in the
very early 1980s (>120/100,00) and is followed by a trough (approx. 80/100,000)
lasting several years until rates peak again in the 1993 at approximate 110/100,000
residents. Following the 1993 peak, rates have fallen dramatically
bottoming out in 1999 at 30 per 100,000. While the level of homicide
is unique to Watts (4x the L.A. citywide rate), the pattern of change is
not, mirroring changes in the rest of the City of Los Angeles as well as
the U.S.
We focus on Watts because (1) this area saw early adoption of crack cocaine
with a concomitant increase in violence due the increased numbers of individuals
carrying guns, and (2) this neighborhood is home to many notorious urban
street gangs. Our homicide data permits us to specifically examine
the impact of gangs on “gang homicide” as well as overall levels of lethal
violence. While Watts is often considered one of the cultural centers
of African American life in Los Angeles, racial/ethnic succession has been
so remarkable that the largest group is now Latino.
Our agent-based model features crack customers, gangs
and their members who supply crack and own guns, and law enforcement agents.
The interactions between these individuals occur according to simple local
rules: customers buy crack at prevailing prices; gang members adjust their
prices based on market conditions and seek out new customers, occasionally
invading the 'turf' of rival gangs; law enforcement tries to limit the adoption
of crack. Resulting from the model is a homicide time series, which we compare
with the empirical data.