The Effect of Prison Population Size on Crime Rates: Evidence from Prison Overcrowding Litigation
利用美国各州监狱过度拥挤诉讼作为工具变量，分离监狱人口与犯罪率之间的相互影响，发现减少一名囚犯每年会增加15起一类犯罪，且犯罪减少的社会效益至少等于边际囚犯的监禁成本。
Simultaneity between prisoner populations and crime rates makes it difficult to isolate the causal effect of changes in prison populations on crime. To break that simultaneity, this paper uses prison overcrowding litigation in a state as an instrument for changes in the prison population. The resulting elasticities are two to three times greater than those of previous studies. A one-prisoner reduction is associated with an increase of fifteen Index I crimes per year. While calculations of the costs of crime are inherently uncertain, it appears that the social benefits associated with crime reduction equal or exceed the social costs of incarceration for the marginal prisoner.