The FBI claim that crime rates in America have been falling for the last 20 years and current rates of robbery and murder are less than half of what they were back then. British figures also show a drop in crime over the same period.
There are many explanations for this and none is universally accepted. Here are a few:
1) The figures are massaged by the Government, ie a burglar breaks into 8 flats in a block or vandalises 15 cars parked in a line and it is reported as one single crime. Offences can also be reclassified, resulting in a drop in certain ones, which can then be applauded.
2) Better policing through increased computerisation, for example constantly updated databases showing which areas of a city currently have high levels of crime leads to more efficient patrolling.
3) Personal wealth has steadily increased on average whilst material possessions are cheaper than ever before. Combine this with the increased difficulty in selling stolen ones due to more common use of security measures such as password protection and invisible marking, along with more easily available credit. It has simply become easier to wave a credit card to buy a shiny gadget than to mug someone.
4) Both Britain and the US have more criminals behind bars than ever before. If they are in jail then they can't commit crimes.
5) New technology such as DNA databases and CCTV along with a general improvement in forensic techniques.
6) Crime rates simply follow a cyclical pattern pretty much like the economy does. When crime is high and we hear about it on the news every night, the general population become more vigilant and take more care to secure their property and possessions, leading to a drop. Then when crime becomes much rarer and we keep hearing that rates are low then we become careless and it starts to rise again.
7) People have more cheap entertainment to occupy themselves. Maybe potential criminals are too busy playing computer games or babbling on Facebook. Why steal a car when you can play Grand Theft Auto?
8) The increasingly short attention span of modern youth means that young muggers are simply not prepared to put in the necessary hours hanging about the streets waiting for a potential victim. Kids nowadays are used to central heating and do not cope well with the cold and wet conditions that are part and parcel of the street hood's life. The declining physical fitness of the young may also prevent them from running away if pursued by an angry OAP.
There's loads of other explanations and you should feel free to add your own.