New York City, like Chicago, has restrictive gun laws. Disproving the Rahm Emanuel theory of the problem, guns purchased elsewhere can enter New York as easily as they do Chicago. Then why is New York’s rate of gun-related assaults and homicides so much lower than Chicago’s?

The answer is the logical corollary of laws that make it hard for citizens to defend themselves—namely, a need for police to take active, relentless, effective measures to disarm criminals.

Such has been the function of the stop-and-frisk activities of the New York Police Department, originally under its famed Street Crimes Unit, which developed an uncanny expertise at identifying those likely to be carrying illegal weapons. The value of stop-and-frisk wasn’t just the arrests made and guns seized, but the incentive to New York’s criminal population to leave its guns at home.