According to Matthew Friedman, the main differences between places with increases in the murder rate and places with decreases in the murder rate?

Is a theory called Fergusson Effect. One of the most widely discussed and controversial explanations for the increase and decrease in violent crime in some cities. This effect has two possible explanations, The first one is the "de-policing", a movement of removing police proactive efforts due to concerns about public criticism of officer's conduct and associated lawsuits. And the other reason is the empowering of some individuals to engage in behaviors that challenge the legitimacy of law enforcement, because of one interpretation that justice is not being administered reasonably.