From the Economist, The curious case of the fall in crime:
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 analyse the incidence of crime; in some parts of Manhattan this helped to reduce the robbery rate by over 95%. The epidemics of crack cocaine and heroin appear to have burnt out.
The biggest factor may be simply that security measures have improved. Car immobilisers have killed joyriding; bulletproof screens, security guards and marked money have all but done 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 CCTV 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.
Liberals have predicted higher crime as a result of increasing inequality. Conservatives have predicted higher crime as a results of social decay. Both have been wrong. It is worth considering that much of our social policy fails because we are unable to predict trends and causes. Solutions often are discovered without the heavy hand of government.