David Brooks writes in the NYTimes:
If you want to deter crime, it seems that you’d want to lengthen prison sentences so that criminals would face steeper costs for breaking the law. In fact, a mountain of research shows that increases in prison terms have done nothing to deter crime.
To quote the brilliant statistician John Lott:
A large number of studies indicate that the more certain the punishment, the fewer the crimes committed (for a survey click here.) Arrest rates of criminals are usually the single most important factor in reducing every type of crime. The death penalty may get the most media attention, as it deserves, but everyday police work is really important in making neighborhoods safer. Changes in the arrest rate account for about 16 to 18 percent of the large drop in the murder rate during the 1990s. Conviction rates explain another 12 percent.