"They also gave rise to Martin O’Malley, a city councilman who was elected mayor on an anti-crime platform in 1999. O’Malley set about implementing what was then known as the New York model: zero tolerance for open-air drug markets, data-centric “CompStat” meetings to track crime and hold police commanders accountable and more resources for law enforcement paired with tougher discipline for officers who abused their power. By the time O’Malley, a Democrat, was elected governor of Maryland in 2006, crime rates, including murders, had fallen across the board"

Geez, for how maligned it is, broken window policing seems to work - causally so. In this case, put it in and murder rates go down, take it out and murder rates go up.