A fair number of recent papers have shown that an increase in the number of police forces reduces crime, but little is know about the mechanism. It is not entirely clear whether police deters crime by reducing the attractiveness of crimes, or whether the reduction is driven by an incapacitation of criminals (Cook et al.,2011, Levitt and Miles, 2004).
This paper exploits quasi-random redeployment of two police forces within the same Italian city, Milan, providing first evidence of incapacitation. A reduction in the number of police forces that patrol the streets leads to lower clearance rates.