In the 1990s, John DiIulio, a conservative American academic, argued that a new breed of "superpredators" — "kids that have absolutely no respect for human life and no sense of the future" — would terrorize Americans almost indefinitely. He was not alone. Experts were convinced that crime would keep rising.

DiIulio later recanted, and it is clear the pessimists were wrong. Even as he wrote, America's crime wave was breaking. Its cities have become vastly safer, and the rest of the developed world has followed.

From Japan to Estonia, property and people are now safer than at almost any time since the 1970s.

Confounding expectations, the recession has not interrupted the downward trend. Even as America furiously debates the shooting of Trayvon Martin, new data show that the homicide rate for young Americans is at a 30-year low.

Some crimes have all but died out. Last year there were just 69 armed robberies of banks, building societies and post offices in England and Wales, compared with 500 a year in the 1990s.

In 1990, some 147,000 cars were stolen in New York. Last year, fewer than 10,000 were.

Cherished social theories have been discarded. Conservatives who insisted that the decline of the traditional nuclear family and growing ethnic diversity would unleash an unstoppable crime wave have been proved wrong. Young people are increasingly likely to have been brought up by one parent and to have played a lot of computer games. Yet they are far better behaved than previous generations.

Left-wingers who argued that crime could never be curbed unless inequality was reduced look just as silly.

There is no single cause of the decline; rather, several have coincided. Western societies are growing older, and most crimes are committed by young men. Policing has improved greatly in recent decades, especially in big cities such as New York and London, with forces using computers to analyze the incidence of crime; in some parts of Manhattan, this helped to reduce the robbery rate by more than 95 percent. The epidemics of crack cocaine and heroin appear to have burned out.

The biggest factor may be simply that security measures have improved. Car immobilizers have killed joy riding; bulletproof screens, security guards and marked money have all but done the same for bank robbery. Alarms and DNA databases have increased the chance a burglar will be caught. At the same time, the rewards for burglary have fallen because electronic gizmos are so cheap. Even small shops now invest in closed-circuit cameras and security tags. Some crimes now look very risky — and that matters because, as every survey of criminals shows, the main deterrent to crime is the fear of being caught.

Many conservatives will think this list omits the main reason crime has declined: the far harsher prison sentences introduced on both sides of the Atlantic over the past two decades. One in every 100 American adults is now in prison. This has obviously had some effect — a young man in prison cannot steal your car. But if tough prison sentences were the cause, crime would not be falling in the Netherlands and Germany, which have reduced their prison populations. New York's prison population has fallen by a quarter since 1999, yet its crime rate has dropped faster than that of many other cities.

Harsh punishments, and in particular long mandatory sentences for certain crimes, increasingly look counterproductive. American prisons are full of old men, many of whom are well past their criminal years, and with nonviolent drug users, who would be better off in treatment. In California, the pioneer of mandatory sentencing, more than a fifth of prisoners are older than 50. To keep each one inside costs taxpayers $47,000 a year (about the same as a place at Stanford University). And because prison stresses punishment rather than rehabilitation, most of what remains of the crime problem is really a recidivism issue.

Politicians seem to have grasped this. In America, the number of new mandatory sentences enacted by Congress has fallen. Even in the Republican South, governors such as Rick Perry and Bobby Jindal have adopted policies favoring treatment over imprisonment for drug users. Britain has stopped adding to its prison population. But more could be done to support people when they come out of prison, and to help addicts. In the Netherlands and Switzerland, hard-drug addiction is being reduced by treatment rather than by punishment. American addicts, by contrast, often get little more than counseling.

Policing can be sharpened, too — and, in an era of austerity, will have to be. Now that officers are not rushed off their feet responding to car thefts and burglaries, they can focus on prevention. Predictive policing, which employs data to try to anticipate crime, is particularly promising.

Better-trained police officers could focus on new crimes. Traditional measures tend not to include financial crimes such as credit-card fraud or tax evasion. Since these are seldom properly recorded, they have not contributed to the great fall in crime. Unlike rapes and murders, they do not excite public fear.

But as policing adapts to the technological age, it is as well to remember that criminals are doing so, too.