摘要：In this paper we study the causal impact of police on crime by looking at what
happened to crime 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 boroughs as compared to outer London - in fact, police deployment
in central London increased by over 30 percent in the six weeks following the
July 7 bombings. During this time crime fell significantly in central relative
to outer London. Study of the timing of the crime reductions and their
magnitude, the types of crime which were more likely to be affected and a series
of robustness tests looking at possible biases all make us confident that our
research approach identifies a causal impact of police on crime. Implementing an
instrumental variable approach shows an elasticity of crime with respect to
police approximately -0.3, so
