Compare the crime rate 100 years ago--when the death penalty was widely and swiftly used--to the crime rate now--when the death penalty is used far less.
You're comparing apples to oranges when the only factor that you use to come up with your conclusion, is the frequency of the death peanlty. You're totally leaving out demographics.
There is no accurate way of having a "crime rate" from 100 years ago, because statistics like that weren't kept then.
We have, however, been keeping those
statistics from 1960 forward.
1960: - Overall: 1.8% - Violent: .1% - Murder - .005%
1972 (the year the death penalty was declared unconstitutional): Overall: 3.9% - Violent: .4% - Murder - .008%
1977 (the year the death penalty returns): Overall: 5.0% - Violent: .5% - Murder - .008%
1988 (considered to be the peak year of the crack epidemic): Overall: 5.6% - Violent: .6% - Murder - .008%
2009 (most recent year on the source): 3.4% - Violent: .4% - Murder - .004%
Indeed, the murder rate rose from .008% to .010% between 1977 and the peak year for murder, which was 1991 (the tale end of the worst of the crack epidemic). Under your argument, it should have begun it's decline much earlier. You'll note that the rate had dropped from 1991 (again, .010%) to .005% in 2000 - during the economic boom years of the 1990s.
I would argue that a better economy leads to a lower murder rate - much more so than does the existence of a death penalty or not.
Indeed,
http://davecoop.net/2007.htm
these number bear that out. Poorer states (death penalty or no) have higher murder rates than do wealthier states. Compare
The murder rate is actually lower than it was in 1960 (when the death penalty was pretty prevalent) and is half of what it was in the year the death penalty became legal again and what it was during the peak of the crack epidemic.
Crime is down. But, since the murder rate didn't change at all between the times the death penalty was made illegal or when it was restored, is shows that the death penalty has absolutely no causal relationship to the murder rate.
Again, to declare a deterrent effect, you'd need to prove that the existence of the death penalty has at least a correlation to the murder rate. The numbers don't bear that out either at a state level or at a national level.