The general trend in crime rates for the past half century has been a general increase from the low levels of 1960 to a peak in 1980, followed by a general decline to fairly low levels today:
It's difficult to find a single factor that would explain this trend. However, there are two factors that, when added together, do have a strong negative correlation with the crime trend, with their sum showing a high levels in 1960, with a general decline until 1980, and a gradual rise after that time. Ignore the first third of this graph since it begins in 1934:
That correlation is evidence that the main contributor to low crime rates today is keeping criminals off the streets and in prison for prolonged periods of time through three strikes laws and such. In the 1940s and 1950s, the same effect seems to have been achieved by sending people to insane asylums from which they were rarely released. Given that the two have roughly the same effect, I think I prefer involuntarily putting people in prison after due process over involuntarily putting people in insane asylums on the say so of a psychiatrist.
Longer term homicide graph showing the decrease in the 1930s:
Comparison of European and U.S. crime rates:
And, it turns out that the same inverse pattern of incarceration versus crime is true in Europe, too. That Europe has followed the same crime patterns is well known, but their incarceration patterns are similar as well. For example, in Germany, mental hospital incarceration rates fell during the period of rising crime, and rose as crime fell:
Article with additional data:
Edited at 2015-01-01 02:53 pm (UTC)