Nationally, the drop in violent crime not only calls into question the theory that crime rates are closely correlated with economic hardship, but another argument as well, said Frank E. Zimring, a law professor at the University of California, Berkeley.

As the percentage of people behind bars has decreased in the past few years, violent crime rates have fallen as well. For those who believed that higher incarceration rates inevitably led to less crime, “this would also be the last time to expect a crime decline,” he said.

“The last three years have been a contrarian’s delight — just when you expect the bananas to hit the fan,” said Mr. Zimring, a visiting law professor at New York University and the author of a coming book on the decline in the city’s crime rate.