Here is some good news for nearly everyone. Crime is falling. Murder is down. Violent crime generally is down. Property crime is down. In fact, almost every category of criminality that you can think of is declining. Here is the even better news. This is not a blip. The downward trend is now very well established and can be traced back over many years. And here is the funniest bit of this news. No one is really sure why. Those who think of themselves as experts on the causes of crime confess to being nonplussed. The majority of them predicted that a prolonged economic squeeze could only lead to more crime. They are scratching their heads trying to figure out why the opposite is happening.

The figures are striking. Over the last five years, violent crimes dropped by 21%. Public disorder offences fell by 29%, even including the summer riots of two years ago and the tuition fees protests. Crimes involving weapons have diminished by 34% and homicides by 28%. There are a few exceptions. Mugging and pickpocketing are up, most probably the result of displacement because car theft and house burglary have become much more difficult. The number of recorded rapes is also up, but that may be because this crime is now being reported more often. Looked at over a longer time scale, the change is even more dramatic. Your chances of being murdered in Britain are now at their lowest since 1978. Whether measured by police records or public surveys, overall crime is also down to a level last seen three decades ago.

There is an abundance of theories that attempt to explain this. Take your pick from one or more of the following. The reduction of lead in petrol. (Lead in the atmosphere deforms the brain.) The proliferation of CCTV. (Criminals know they are much more likely to be watched.) Car immobilisers and home alarms. (It has got harder to commit a crime.) The steep decline in the cost of consumer goods such as televisions. (A lot of stuff that used to be stolen really isn't worth nicking any more.) The much bigger size of consumer goods such as televisions. (It ain't easy to get a 40-inch plasma screen through a window.) More liberal abortion laws. (The controversial theory advanced by the authors of Freakonomics who argued that making it easier to get an abortion has diminished the number of children born into the underclass.) An ageing population. (Most crime is committed by young men and most of their victims are other young men. Fewer young men means less crime.) Rising numbers of women. (They are much less likely to commit crime than men. The proportion of the population that is female is going up.) A more feminised society. (Men are becoming less larcenous and dangerous under the moderating influence of the more law-abiding sex.)

A more middle-class society. (The middle classes do commit crime, but it is much more likely to be non-violent.) More immigrants. (Immigrants are less likely to commit crime than the indigenous population.). Home computer games. (Give young men an alternative to vandalising, robbing and fighting.) Easy access to hardcore porn on the internet. (Gives young men another alternative to vandalising, robbing and fighting.) Cheap gyms. (Give young men somewhere else to work off their surplus testosterone.) Social media. (It has civilised people and made them more tolerant. If you are reading this online, you can test this proposition by going to the comments section and seeing how civilised that is.)

Some crime is not actually falling; it is changing in ways we have yet properly to discern. (Credit card fraud and other forms of digital crime are under-recorded because people don't report them and the banks don't want to admit to their scale.)

Further theories: David Cameron's creation of a more cohesive society. (A Tory spin doctor offered me that one, laughing at his own ridiculousness as he did so.) The last Labour government's recruitment of record numbers of police officers. (A dud theory, for reasons I will explain later.) A decline in the consumption of alcohol among the young. (Booze makes people, especially youthful males, more violent.) Reductions in the use of illegal drugs. (A lot of crime is drug-related.) Increases in prescriptions of psychiatric drugs. (People who might have turned criminal are now more sedated.) Austerity. (The jobless don't have the money to get tanked up on a Friday night so they aren't on the streets assaulting each other.)

My hunch is that it is unlikely that any one theory provides the complete answer. There is probably something in several of them.

I said this was good news for nearly everyone. So why the qualification? There are a few sections of society for whom this is either a mixed blessing or not really welcome at all. Criminologists are one group for whom this is all rather disorientating as many see the collapse of their cherished models. When Labour was in power, the Home Office's supposed experts on crime composed a study for their then mistress, Jacqui Smith, in which they predicted that the oncoming recession would ineluctably lead to an increase in crime. The document was leaked, much to her embarrassment.

Many academic criminologists also advanced the classic argument that recessions always cause a rise in the crime rate. It looked like a no-brainer: when times are tough, people are angrier and steal more stuff. Yet, as it turns out, these soi-disant experts were completely wrong. It is now six years since the beginning of the crunch and crime has continued to fall. One eminent criminologist recently joked that when he attended seminars with fellow academics he was reassured to find that all of them were as baffled as he was.

Politicians are another set left dazed and confused by the fall in crime. They each have their neatly constructed narratives, whether it be Tony Blair's "tough on crime and tough on the causes of crime" or Michael Howard's "prison works" – the policy of jailing more people for longer that he introduced as home secretary from 1993 and which has been broadly continued by all his successors. For Mr Howard, the most exciting line on the crime graph is the apparent rough correlation between his period in office and the establishment of a downward trend.

Well, we all like to feel vindicated, but is there really justification for Mr Howard to claim to be the father of a more law-abiding society? There might be grounds for it if the fall in crime were confined only to this country and others, such as the United States, which also incarcerate an above-average proportion of their population. But it isn't. Crime is diminishing across the developed world, falling in broadly the same way in conservative countries and in leftish countries. Countries with starkly contrasting social policies and strikingly different penal policies are seeing similar falls in crime. It is dropping in countries that lock up a lot of people and it is also down in countries that put a much smaller proportion of their population behind bars. This strongly suggests that the policy remedies for crime pursued by politicians have had only a marginal influence, if any at all.

And now to the group for whom all this is very disturbing indeed: the police. Crime is their livelihood. Without criminals, we would have no need for people to catch them. Senior officers claim credit for the fall in crime, saying it is the result of their smarter techniques: tactical reforms such as targeted patrols and neighbourhood policing along with changes in focus, such as putting more emphasis on investigating domestic violence. Improved policing probably does deserve some of the credit. But the discomfiting fact that confronts them is that police numbers do not seem to have all that much to do with levels of crime.

The current government has already cut roughly 20% from constabulary budgets. When these cuts were first announced, the coppers, especially their brutish trade union, the Police Federation, screamed blue murder, predicting we would all pay the price because fewer police officers would lead to more dangerous streets. As it turns out, lower police numbers have been accompanied by less crime. I am told by sources at the Home Office that, when they compare the performance of different forces, they are struggling to find any correlation between crime rates and constabulary strength. It would be idiotic to extrapolate this to the conclusion that you could have no police officers at all. But it is very uncomfortable for the police to discover that the number of them that are employed seems to bear so little discernible relation to crime.

After extensive inquiries into the subject, Inspector Rawnsley has to report that he has no firm conclusion about precisely why we are becoming a safer, more peaceful and more law-abiding society. Never mind. Good news is still good news, even when you can't explain it.