capital punishment could explain only one twenty-fifth of the drop in homicides in the 1990s. And because the death penalty is rarely given for crimes other than homicide, its deterrent effect can- not account for a speck of decline in other violent crimes. It is extremely unlikely, therefore, that the death penalty, as cur- rently practiced in the United States, exerts any real influence on crime rates. Even many of its onetime supporters have come to this conclusion. "I feel morally and intellectually obligated simply to con- cede that the death penalty experiment has failed," said U.S. Supreme Court Justice Harry A. Blackmun in 1994, nearly twenty years after he had voted for its reinstatement. "I no longer shall tinker with the machinery of death."       So it wasnt capital punishment that drove crime down, nor was it the booming economy. But higher rates of imprisonment did have a lot to do with it. All those criminals didnt march into jail by themselves, of   course. Someone had to investigate the crime, catch the bad guy, and put together the case that would get him convicted. Which naturally leads to a related pair of crime-drop explanations:     Innovative policing strategies Increased number of police     Lets address the second one first. The number of police officers per capita in the United States rose about 14 percent during the 1990s. Does merely increasing the number of police, however, reduce crime? The answer would seem obvious-yes-but proving that answer isnt so easy. Thats because when crime is rising, people clamor for protec- tion, and invariably more money is found for cops. So if you just look at raw correlations between police and crime, you will find that when