Now that the United States has the world’s highest reported rate of incarceration, many criminologists are contemplating another strategy. What if America reverted to the penal policies of the 1980s? What if the prison population shrank drastically? What if money now spent guarding cellblocks was instead used for policing the streets?

In short, what would happen if the rest of the country followed New York City’s example?

As the American prison population has doubled in the past two decades, the city has been a remarkable exception to the trend: the number of its residents in prison has shrunk. Its incarceration rate, once high by national standards, has plunged well below the United States average and has hit another new low, as Mayor Michael R. Bloomberg announced recently. And crime in the city has fallen by more than 75 percent, almost twice as much as in the rest of the country.

Whatever has made New York the safest big city in America, that feat has certainly not been accomplished by locking up more criminals.

“The precise causes of New York’s crime decline will be debated by social scientists until the Sun hits the Earth,” said Michael Jacobson, a criminologist who ran the city’s Correction and Probation Departments during the 1990s and is now the president of the Vera Institute of Justice, a criminal justice research group. “But the 50,000-foot story from New York is that you can drive down crime while decreasing your jail and prison population — and save a huge amount of money in the process.”