between the racial composition of a city's police force and the racial patterns of arrests and crime. Increases in the number … police officers has a less clear-cut impact on crime rates. It appears that own-race policing may be more effective in … reducing property crime, but no systematic differences are observed for violent crime. These results are consistent either with …

Private expenditures on crime reduction have potentially important externalities. Observable measures such as barbed …-wire fences and deadbolt locks may shift crime to those who are unprotected, imposing a negative externality. Unobservable …, any decrease in the aggregate crime rates due to Lojack is an externality from the perspective of the individual Lojack …