"Does Death Penalty Save Lives? A New Debate"

This is easy for me. It doesn't matter whether the research on the issue is valid or not. I'm against the death penalty. Period.:

Does Death Penalty Save Lives? A New Debate, by Adam Liptak, NY Times: ...According to roughly a dozen recent studies, executions save lives. For each inmate put to death, the studies say, 3 to 18 murders are prevented.  The effect is most pronounced, according to some studies, in Texas and other states that execute condemned inmates relatively often and relatively quickly. The studies, performed by economists in the past decade, ... say that murder rates tend to fall as executions rise. ...

The studies have been the subject of sharp criticism, much of it from legal scholars who say that the theories of economists do not apply to the violent world of crime and punishment. Critics of the studies say they are based on faulty premises, insufficient data and flawed methodologies.

The death penalty “is applied so rarely that the number of homicides it can plausibly have caused or deterred cannot reliably be disentangled from the large year-to-year changes in the homicide rate caused by other factors,” John J. Donohue III, a law professor at Yale with a doctorate in economics, and Justin Wolfers, an economist at the University of Pennsylvania, wrote... “The existing evidence for deterrence,” they concluded, “is surprisingly fragile.”

Gary Becker, who won the Nobel Prize in economics in 1992 ..., said the current empirical evidence was “certainly not decisive” because “we just don’t get enough variation to be confident we have isolated a deterrent effect.” But, Mr. Becker added, “the evidence of a variety of types — not simply the quantitative evidence — has been enough to convince me that capital punishment does deter and is worth using...”

The ... studies have started to reshape the debate over capital punishment and to influence prominent legal scholars. ... To a large extent, the participants in the debate talk past one another because they work in different disciplines.

“You have two parallel universes — economists and others,” said Franklin E. Zimring, a law professor at the University of California, Berkeley... To economists, it is obvious that if the cost of an activity rises, the amount of the activity will drop. “To say anything else is to brand yourself an imbecile,” said Professor Wolfers, an author of the Stanford Law Review article criticizing the death penalty studies.

To many economists, then, it follows inexorably that there will be fewer murders as the likelihood of execution rises.

“I am definitely against the death penalty on lots of different grounds,” said Joanna M. Shepherd, a law professor at Emory with a doctorate in economics who wrote or contributed to several studies. “But I do believe that people respond to incentives.”

But not everyone agrees that potential murderers know enough or can think clearly enough to make rational calculations. And the chances of being caught, convicted, sentenced to death and executed are in any event quite remote. Only about one in 300 homicides results in an execution. ...

Critics say the larger factors are impossible to disentangle from whatever effects executions may have. They add that the new studies’ conclusions are skewed by data from a few anomalous jurisdictions, notably Texas, and by a failure to distinguish among various kinds of homicide.

There is also a classic economics question lurking in the background, Professor Wolfers said. “Capital punishment is very expensive,” he said, “so if you choose to spend money on capital punishment you are choosing not to spend it somewhere else, like policing.”

A single capital litigation can cost more than $1 million. It is at least possible that devoting that money to crime prevention would prevent more murders than whatever number, if any, an execution would deter.

The recent studies are, some independent observers say, of good quality, given the limitations of the available data. “These are sophisticated econometricians who know how to do multiple regression analysis at a pretty high level,” Professor Weisberg of Stanford said. The economics studies are, moreover, typically published in peer-reviewed journals, while critiques tend to appear in law reviews edited by students.

The available data is nevertheless thin, mostly because there are so few executions. ... “It seems unlikely,” Professor Donohue and Professor Wolfers concluded in their Stanford article, “that any study based only on recent U.S. data can find a reliable link between homicide and execution rates.”

The two professors offered one particularly compelling comparison. Canada has executed no one since 1962. Yet the murder rates in the United States and Canada have moved in close parallel since then, including before, during and after the four-year death penalty moratorium in the United States in the 1970s. ...

Professor Wolfers said the answer to the question of whether the death penalty deterred was “not unknowable in the abstract,” given enough data. “If I was allowed 1,000 executions and 1,000 exonerations, and I was allowed to do it in a random, focused way,” he said, “I could probably give you an answer.”

Update: PGL says in comments that "this type of research was labeled junk science by Dr. Jeffrey Fagan of Columbia Law School."  See PGL's post at EconoSpeak for more.