Violent crime in New York City, the nation’s largest city, has plunged over the past twenty years.

Data from the New York Police Department indicate that while incidences of serious crime has remained roughly unchanged over last year, compared to the early 1990s, the city is an oasis of peace and tranquility.

For example, through the first two weeks of 2011, murder, rape, felonious assault have climbed over the year-ago period, while robbery and burglary have dropped.

However, more striking is the fact that all types of serious crime have plunged compared with data from 1993.

Over that 18-year period, murder has declined by 85 percent, robbery has fallen by 53.6 percent, felonious assault has plummeted by 81.1 percent, burglary is down by 84.5 percent, and grand larceny has sunk by 60.6 percent.

Also, over that period, the population of the city has climbed to 8.4-million from 7.3-million (including the arrival of more than 1-million immigrants during the 1990s).

Such a decline mirrors data witnessed in many other large U.S. cities, however there appears to be no clear answer as to the cause.

In New York City, Mayor Rudolph Giuliani’s “get tough” policy on crime and the hiring of new police officers were widely credited with crime rates starting to fall in the 1990s. However, the last gasps of the crack cocaine epidemic was probably the more likely reason.

Now, we are faced with a still-paralyzing economic crisis (Unemployment in New York City remains in the 9 percent-10 percent level), imminent spending cuts from both city and state government, and layoffs of many civil servants, including police officers.

It will be interesting to see if this period of relatively low crime cane be sustained under such trying conditions.