TWO decades ago, Washington, D.C. had the highest murder rate in America. Now the drive-by shootings that claim the lives of innocent teenagers are infrequent enough to shock, and make the newspapers.

Criminologists and sociologists have spent years grappling to explain the dramatic slide in violent and other serious crime in the US capital, but it's not unique to Washington.

The latest FBI figures show that murder, rape, robberies and other serious crimes have fallen to a 48-year low across the country.

In Washington last year, 131 people were murdered, the lowest number in half a century. Two decades ago, there were 482 homicides in the city amid turf wars among drug gangs and crack-driven violent robberies.

It's a pattern replicated across the country. In 2009, New York City had the lowest number of murders since detailed FBI records began in 1963. There was a small increase last year but even so the total of 536 homicide victims was still well below the 2,245 murdered in 1990 when Times Square was infamous for peep shows and drug pushers, not the Disney Store.

Twenty years ago, the murder rate for the whole US was 9.8 per 100,000 people. It has fallen by nearly half, although it is still twice the rate in France. It's not just murder. Robberies were down nearly 10 per cent last year and eight per cent the year before.

There are a score of explanations offered by sociologists for collapsing crime figures, from theories that it is tied to legalisation of abortion or reduction of lead in fuel to the closing of mental institutions.

One theory has it that better medical treatment has reduced the number of murders by saving the lives of assault victims who would otherwise have died. But that doesn't explain why overall violent crime is also down.

Anti-gun activists note that the cities with two of the sharpest falls in murder rates, New York and Washington, have enacted strict gun control laws by US standards. Yet Houston, where some regard it as criminal not to own a gun, has also seen a sharp drop in homicides.

One of the most widely accepted explanations is also one of the most politically and socially sensitive — that the imposition of sharply stiffer prison sentences since the early 1980s, which has resulted in the US having the highest rate of incarceration in the developed world, has kept large numbers of criminals off the streets.

The US imprisons 2.3 million of its citizens, a number that has risen dramatically since the 1980s when state legislatures began greatly increasing sentences out of fear of the surging crime rate.

“We now incarcerate four times as many people as we did 20 years ago,” said John Roman, director of the District of Columbia Crime Policy Institute, who has spent years studying crime trends in the city and the US. “Just by sheer size you've removed a lot of potential offenders from the street. I don't think that's very popular in many circles but it's very hard to argue with.”

Roman says that in parallel with an ever-expanding jail population was the peak and collapse of the crack cocaine epidemic in major cities. He said that the crack epidemic burned itself out, largely because a new generation saw the effect of the drug on older users and were discouraged. Sociologists credit a couple of other important factors in falling rates of some crimes. It is considerably more difficult than 30 years ago to steal a new car given all the electronic security, and houses are better protected.

An explanation favoured by some politicians and police officers traces back to New York's 'zero tolerance' strategy in the early 1990s, which followed the theory that arrests for minor crimes deter major ones, and that most serious crimes are committed repeatedly by a small number of hardcore criminals.

Roman is sceptical, saying the strategy went hand in hand with a large increase in the police force which led to more people being arrested for crimes in general. Also, detaining people for minor crimes, such as jumping the turnstiles at New York subway stations, led to a significant number of wanted criminals being nabbed. So the real effect was not so much to deter as to lock up.

There is no shortage of other theories.

One has it that the lead poisoning through paint and petrol of a generation raised in 1960s and 1970s caused violent behaviour as they entered their teens.

The economist Steven Levitt has argued that the 1973 supreme court ruling legalising abortion reduced the number of criminals by reducing the number of unwanted babies. There are even those who believe the election of Barack Obama has inspired young black men to steer away from a life of crime, although that only works for the past two years. — The Guardian, London