Do supporters of the death penalty in Maryland have secret evidence proving that it works as a deterrent?

It's always cause for celebration when the barbaric practice of capital punishment is abolished somewhere, and yesterday, Maryland became the first state south of the Mason Dixon line to do so. Governor Martin O'Malley was persistent on this issue and he deserves credit for seeing it through. There are, it should be noted, five death row inmates in Maryland to whom this legislation does not apply, and O'Malley is set to decide on their fate. Let us hope he does the right thing and commutes their sentences.

The abolitionist cause has seen a string of recent successes; Maryland is the sixth state in as many years to vote for repeal. Slowly, on a state-by-state basis, progress is being made. Executions in the United States are now tightly concentrated in a few reactionary states, with Texas obviously reigning supreme as the most ruthless. That will be the white whale for abolitionists. One can at least envision a time, though, when the U.S. leaves the ranks of states like Saudi Arabia, Iran, and China, and joins virtually the entire democratic world in opposition to the state-sponsored killing of citizens.

Reading Brian Witte's report for the Associated Press on Maryland's repeal, though, I was struck by one sentence (presented in full context):
Supporters of capital punishment said the governor was taking away an important tool to protect the public. Del. Neil Parrott, a Washington County Republican, criticized the governor for moving ahead with banning the death penalty during the same session as he pushed for a gun-control bill to restrict firearms access to law-abiding citizens. Parrott said he is considering launching a petition drive to put the death penalty ban on the ballot for voters to decide in 2014.
Now, whether or not capital punishment "protects the public" is an empirical question. These unnamed death penalty supporters are making a factual assertion that a given population is less safe once the threat of capital punishment is rescinded. Fortunately, our federalist system provides a fair amount of evidence on this question. Remember, it is the unnamed supporters who are making a factual assertion, so the burden of proof is on them. Is there any evidence that affirmatively supports their view?

Let's take a look at murder rates, because the statement is, fundamentally, about deterrence. This is, of course, the most common practical argument in favor of state executions, that it deters potential killers from killing. Personally, I don't even think this is prima facie plausible; it's a stretch to think that someone who is planning a murder will write up a pros-and-cons list, and call it off on the grounds that he might be executed, as opposed to being locked in a cage for the rest of his life. Besides, Maryland had only executed five people since 1976, and none since 2005, so it strains rationality to think that execution was some sort of ever-present threat in the minds of criminals or potential criminals.

As it turns out, murder rates in non-death penalty states have been lower than those in death penalty states every year since 1990, and the difference has steadily increased over the years. Before you dismiss this as an apples and oranges comparison, involving states with dramatically different cultures and populations, you should know that the trend holds even for neighboring states. 

We can also consult the experts. A 2009 survey found that 88% of the nation's leading criminologists do not believe the death penalty acts as a deterrent to homicide. Are these unnamed supporters of the death penalty in possession of information denied to leading criminologists? If so, they have an obligation to release it, so that we can all reevaluate our views on this matter.

As for law enforcement, in a 2008 poll, 500 police chiefs ranked the death penalty last when asked to name one area as "most important for reducing violent crime." Just 2% thought "insufficient use of the death penalty" was a major impediment. Again, if the opponents of Maryland's legislation know something we don't about the the deterrent effects of the death penalty, they should share it with law enforcement immediately so that police chiefs can join their cause.

Because the article did not specify in what sense the death penalty in Maryland was allegedly "an important tool to protect the public," it's difficult to offer a thorough argument in opposition. It's incumbent on Brian Witte, as a reporter, to require evidence for this view, instead of just allowing supporters of the death penalty to lie with impunity in his report. Again, this is an empirical question. These unnamed supporters are either right about this or they are not. Witte should not have just mindlessly parroted this view without even mentioning whether it's supported by any actual evidence. As far as I know, it's not, which means that supporters of state-sponsored killing were freely allowed to propagandize in an Associated Press report, without being challenged.

I have contacted Witte through Twitter and asked if the people he cited provided him with any evidence supporting their view. If he responds, I'll post it here.