On one of the lifeless, uniform streets of America's capital, a bulky former crack cocaine dealer who goes by the nom de guerre of Tiny laments the passing of the old Washington DC.

"Back then they called it the murder capital of the world. These few blocks here were the murder capital of the murder capital of the world, and right here's where I did my business. Made a lot of money too," he says, hovering on a corner in the mostly black Trinidad neighbourhood a few blocks north of that largely white citadel, the Capitol. "Even sold it down by the White House. Could do anything back then. We owned this city. Now it's like everywhere else. One giant coffee shop."

Tiny long ago moved on to the more legal if less lucrative, and certainly less adrenaline-pumping, enterprise of parcel delivery, which is why he is reluctant to give a name other than the one he used to be known by on the streets.

Two decades ago, Washington DC 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% last year and 8% 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 and swifter 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 in Texas, 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 80s when state legislatures began greatly increasing prison 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."

James Wilson, author of Thinking About Crime, backed that view in the Wall Street Journal recently.

"Imprisonment's crime-reduction effect helps to explain why the burglary, car-theft and robbery rates are lower in the US than in England. The difference results not from the willingness to send convicted offenders to prison, which is about the same in both countries, but in how long America keeps them behind bars. For the same offence, you will spend more time in prison here than in England," he wrote.

But Wilson adds that cannot be the sole explanation, as Canada has experienced roughly the same decline in crime without the same lengthening of prison sentences.

Roman says that in parallel with an ever-expanding jail population was the peak and collapse of the crack cocaine epidemic in major cities.

"If you look at the crime statistics over time you see one big rise throughout the 70s and then an additional big rise at the end of the 80s. That big rise at the end of the 80s was almost entirely due to the crack epidemic and users committing crimes to support their habits and due to violence within gangs and drug trafficking networks," he said.

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.

"As fewer and fewer new users entered the crack market it really started to dry up those networks and shrink them," he said.

On the streets of Washington, Tiny thinks there is something to both theories.

"There's a lot of guys who were from around here in jail. If you're black and you do crack you go to jail for a long time. There's guys who were selling here with me in the 80s who are still locked up," he said. "But I went out of business because nobody wanted to buy anymore. Crack got a bad name on the streets."

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 size of 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 60s and 70s caused violent behaviour as they entered their teens.

Steven Levitt, co-author of the bestselling book Freakonomics, has argued that the 1973 supreme court ruling legalising abortion reduced the number of criminals by reducing the number of unwanted babies born to single mothers who would raise youths prone to crime.

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 and falls flat when trying to explain the past two decades.

Some theories have had to be binned outright.

A favourite of some sociologists was that economic hard times result in increased robbery and associated violence on the grounds that crime is a rational act prompted by particular circumstances. Yet, through the past three years of financial depression in the US, crime rates have not only continued to fall but the drop has accelerated in many cities.

With growing support for the view that high rates of imprisonment and lengthy sentences are a leading factor in reducing crime, the debate is now shifting to whether that is an argument for maintaining a policy that critics say is disproportionately applied to black men and causes other social damage, including taking fathers away from their children for much of their upbringing.

Roman thinks the policy may have served its purpose and should be changed.

"If you look at the homicide statistics from major cities in 1990 they're absolutely appalling. I think the reaction of the legislatures at the time was to say three strikes and you're out, mandatory minimums, you have to actually serve the time. They were responding to what was in front of them which was an epidemic of violence and I think to some extent they were right," he said.

"You can make the case that mass incarceration hastened the end of the crime wave. You would have a much more difficult time making the case that a continuation of that mass incarceration is necessary. The benefit from preventing crime, since crime rates are so much lower, is a lot smaller than it used to be and the costs continue to go up. We're investing more and more in prison and getting a smaller and smaller return."

But the public may not share that view. A recent poll showed most Americans feel crime is still getting worse.