In a country that does not usually look gift horses in the mouth, the news that violent crime was decreasing might be thought likely to be accepted as unambiguously good. But social scientists are assembling a variety of different - and often controversial - theories, which will provide ammunition for political rifts for decades.
One of the latest theories, by a sociology professor at Penn State University, gives credit to the country's ageing baby boomers, whom many had blamed for the rise in crime in the first place. "This enormous, accumulating age shift has now reached the threshold of `critical mass' needed to trigger change in our cultural values and collective conscience," Darrell Steffenmeier told the annual meeting of the American Sociological Association. This had led to a "greater civility" and less emphasis on materialism, he suggested.
Ironically, it was the rise of this generation - with its lack of respect for authority, selfishness and disdain for traditional values such as family and religion - which was previously blamed by many for the increase in criminality.
Mr Steffenmeier also noted that the strong economy had helped - as had low inflation and increasing competition, which meant that television sets were now so cheap it was barely worth stealing them. So has the increase of resources devoted to crime prevention and law enforcement.
Democrats and Republicans have been quick to take credit for the reduction in crime. While President Bill Clinton is keen to point to a greater emphasis on community-based crime prevention schemes and the hiring of more police officers, the Republican Party also claims victory through its efforts to revive the religion and family values eschewed by the baby boomers. In New York, for instance, Mayor Rudolph Giulani's zero tolerance policies will be at the forefront of his attempts to win the state's Senate seat against Hillary Clinton, the First Lady.
Perhaps the strangest theory is that the legalisation of abortion in 1973 helped to tip the scales, by reducing the number of children born into poor, deprived families.
That meant a crime cohort simply dropped out of the demographic curve, according to a highly controversial theory reported in the Chicago Tribune on Sunday. Steven Levitt, a University of Chicago economist, and John Donohue III, a Stanford University law professor, said abortion may account for as much as half of the reduction in crime in the past decade.
Supporters of abortion have argued from some time that it had helped to ensure that there were fewer unwanted children, who were more likely to grow up as criminals. Henry Morgentaler, honorary president of the Humanist Association of Canada, said last year that among the "enormous benefits that legal abortion has brought about is the declining violent crime rate over the last six years in Canada and the United States... Some demographers explain this by the fact that there are fewer young men around, and it is mostly young men who commit crimes."