The number of violent crimes in the United States dropped significantly last year, to what appeared to be the lowest rate in nearly 40 years, a development that was considered puzzling partly because it ran counter to the prevailing expectation that crime would increase during a recession.
In all regions, the country appears to be safer. The odds of being murdered or robbed are now less than half of what they were in the early 1990s, when violent crime peaked in the United States. Small towns, especially, are seeing far fewer murders: In cities with populations under 10,000, the number plunged by more than 25 percent last year.
The news was not as positive in New York City, however. After leading a long decline in crime rates, the city saw increases in all four types of violent lawbreaking — murder, rape, robbery and aggravated assault — including a nearly 14 percent rise in murders. But data from the past few months suggest the city’s upward trend may have slowed or stopped.
Criminology experts said they were surprised and impressed by the national numbers, issued on Monday by the Federal Bureau of Investigation and based on data from more than 13,000 law-enforcement agencies. They said the decline nationally in the number of violent crimes, by 5.5 percent, raised the question, at least in some places, of to what extent crime could continue to fall — or at least fall at the same pace as the past two years. Violent crimes fell nearly the same amount in 2009.
“Remarkable,” said James Alan Fox, a criminologist at Northeastern University. “Given the fact that we have had some healthy declines in recent years, I fully expected that the improvement would slow. There is only so much air you can squeeze out of a balloon.”
There was no immediate consensus to explain the drop. But some experts said the figures collided with theories about correlations between crime, unemployment and the number of people in prison.
Take robbery: The nation has endured a devastating economic crisis, but robberies fell 9.5 percent last year, after dropping 8 percent the year before.
“Striking,” said Alfred Blumstein, a professor and a criminologist at the Heinz College at Carnegie Mellon University, because it came “at a time when everyone anticipated it could be going up because of the recession.”
Nationally, murder fell 4.4 percent last year. Forcible rape — which excludes statutory rape and other sex offenses — fell 4.2 percent. Aggravated assault fell 3.6 percent. Property crimes — including burglary, larceny, motor vehicle theft and arson — fell 2.8 percent, after a 4.6 percent drop the year before.
But the gains were uneven. New York saw 536 murders last year — 65 more than in 2009, which was the lowest since 1963.
The number of rapes in the New York City jumped 24.5 percent; robberies, 5.4 percent, and aggravated assaults, 3.2 percent.
New York was the only city with more than a million people besides San Antonio with an increase in the total number of violent crimes — a 4.6 percent jump, to 48,489 — and the only one besides Philadelphia to see a rise in murders.
Some experts cautioned against reading too much into the city’s numbers, noting that New York’s drop in violent crime over the last two decades has far outpaced many places, some of which are only now catching up.
“It’s been so huge, there’s always been this lingering question, how low could it go?” said Michael Jacobson, director of the Vera Institute of Justice, and a former New York City correction and probation commissioner.
There were 2,245 murders in New York in 1990, but the total has been less than 600 for the past nine years.
“One murder is too many, but the 2010 spike has to be viewed in the context of the historic low the year before,” said Paul J. Browne, the New York Police Department’s chief spokesman. He said the department was doing more to encourage victims to report rapes.
Eli Silverman, a professor emeritus at John Jay College of Criminal Justice, said Mr. Browne’s account might only be a “partial explanation” — the other part, he said, was increased scrutiny of the integrity of the department’s crime statistics. When crime rates go up, the police say it is because they are encouraging more victims to come forward, Mr. Silverman said, “but when crime goes down, it’s the work of the police.”
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.
But he said there was no way to know why — at least not yet.
“The only thing that is reassuring being in a room full of crime experts now is that they are as puzzled as I am,” he said.
Joseph Goldstein contributed reporting.