mean, as some criminologists have argued, that crime will fall if crim- inals are released from prison. To show causality, we need a scenario in which more police are hired for reasons completely unrelated to rising crime. If, for instance, police were randomly sprinkled in some cities and not in others, we could look to see whether crime declines in the cities where the police happen to land. As it turns out, that exact scenario is often created by vote-hungry politicians. In the months leading up to Election Day, incumbent mayors routinely try to lock up the law-and-order vote by hiring more police-even when the crime rate is standing still. So by comparing the crime rate in one set of cities that have recently had an election (and which therefore hired extra police) with another set of cities that had no election (and therefore no extra police), its possible to tease out the effect of the extra police on crime. The answer: yes indeed, ad- ditional police substantially lower the crime rate.     Again, it may help to look backward and see why crime had risen so much in the first place. From 1960 to 1985, the number of police officers fell more than 50 percent relative to the number of crimes. In some cases, hiring additional police was considered a violation of the eras liberal aesthetic; in others, it was simply considered too expen- sive. This 50 percent decline in police translated into a roughly equal decline in the probability that a given criminal would be caught. Cou- pled with the above-cited leniency in the other half of the criminal justice system, the courtrooms, this decrease in policing created a strong positive incentive for criminals. By the 1990s, philosophies-and necessities-had changed. The policing trend was put in reverse, with wide-scale hiring in cities across the country. Not only did all those police act as a deterrent, but they also provided the manpower to imprison criminals who might have otherwise gone uncaught. The hiring of additional police ac-