Panic on the Streets of London: Police, Crime, and the July 2005 Terror Attacks
利用2005年7月伦敦恐怖袭击导致警力重新部署这一外生冲击，研究发现警力增加10%可使犯罪率下降约3-4%，为警察与犯罪因果关系提供了新证据。
In this paper we study the causal impact of police on crime, looking at what happened to crime and police before and after the terror attacks that hit central London in July 2005. The attacks resulted in a large redeployment of police officers to central London as compared to outer London. During this time, crime fell significantly in central relative to outer London. The instrumental variable approach we use uncovers an elasticity of crime with respect to police of approximately -0.3 to -0.4, so that a 10 percent increase in police activity reduces crime by around 3 to 4 percent.