Here is a pretty interesting article by the Wall Street Journal about the reduction in crime. Violent crime is at a 40 year low. They explore many reasons as to why this might be. Among the suggested reasons are:
1. All the criminals are already in jail. Our incarceration rate in this country is at its all time high. These people can't be committing crimes if they are behind bars.
2. People have gotten better at protecting themselves with alarms, lighting, better locks etc. I suppose you could add owning guns to this one.
3. Police have become more efficient. Many cities are focusing on policing problem areas more intensely and that is helping reduce crime.
4. Medical reasons. Doctors have shown that children with a lot of lead in their system are more likely to be aggressive and violent. EPA standards have helped to lower the average amount of lead in American's blood by four-fifths since 1975.
5. Decreasing crack and cocaine use. Between 1992 and 2009 crack use fell by 66%. In that same time frame the average number of 12th graders who had tried cocaine fell from 9.8% to 5.5%.
The reality is likely that no one of these things has caused the decrease. There could also be other factors at play. I would think all of these factors play a role in it and it is many different things all coming together to lower a crime rate.