Thursday, July 19, 2007

Death Penalty And Deterrence: The Scientific Evidence

The recent round of deterrence studies published over the last six years by economists that declared that an execution prevents between 3 and 18 homicides has been shown to be flawed.

read more digg story

The crux of this article is both encouraging and timely. The AP story a few weeks ago proclaimed the results while implying that they were both recent and widely accepted. In actuality, the research had been available for a number of years and had been reviewed and discredited prior to the AP publication.

Many newspapers ran articles and editorials that simply regurgitated the purported findings. Death penalty supporters pointed gleefully at this literature and claimed justification for their support of capital punishment. Fred Thompson, presidential candidate in waiting, proclaimed "For decades, the self-proclaimed smart kids have been telling us that the death penalty just doesn't work. The people with the top jobs in academia and the news business have scoffed at the American people's insistence that executions prevent murder" (click here to view the entire statement).

The Stanford Law Review article referenced in the article contains a thoughtful and through review of deterrence studies. The authors' basic conclusion is similar to the findings of the 1978 National Academy of Sciences report - the evidence to support claims of deterrence is unconvincing. This batch of studies appear to suffer from the same problems that plagued Ehrlich in 1975.

For the sake of clarity, I would like to point out three issues. First, discussions of deterrence need to point out that there are two categories of deterrence - specific and general. Fred Thompson, among others, seem to confuse specific deterrence with incapacitation with regard to the death penalty. Obviously an executed person cannot commit further offenses. But that outcome is incapacitation; you have to be alive in order to be deterred. What we really need to state clearly is that the expectation is that the death penalty (not sure if it is the sentence or the execution) deters other individuals from committing murder.

Second, there may be some deterrence derived from capital punishment. As Donohue and Wolfers observe, we may never prove deterrence with the current data. It is difficult to measure what does not occur, as the economists have discovered. Deterrence is very appealing intuitively, but the reality is that thankfully very few of us actually need deterring!

Last, I encourage you to review the work of Ruth Peterson and William Bailey. The conclusion of the majority of comparative deterrence research, which is a far better question than simply asking does the death penalty deter, is that no statistically significant difference exists between capital punishment and life imprisonment in terms of deterrence and capital homicide.

For retributivists like van den Haag, deterrence is not central to their support (according to a 2006 Gallup poll, 64% of respondents did not think the death penalty was a deterrent). For utilitarians, the issue is important. Life imprisonment costs less, is reversible, and is no more or no less an effective deterrent than capital punishment.

I also encourage you to listen to my podcast (Episode 3 - Deterrence, Brutalization, and Future Dangerousness) if you would like to learn more about this topic.


PersianCowboy said...

Great post on the AP article. I was looking for a good response and analysis.

dudleysharp said...

It is important to note that Donahue and Wolfers criticism of the studies finding for deterrence has been responded to.

(2006) "This analysis shows that attempts to make the deterrence effect disappear are  ineffective." (p 16)
---  Existence of the death penalty, in law, has a statistically significant impact on reducing murders. (p 23)
---  Execution rates show significant impact in reducing murders. (p 13 & 23)
---  Death row commutations, and other removals, increase murders. (p13 & 23)
--- The criticism of our studies is flawed and does not effect the strength of the measured deterrent effect.
"The Impact of Incentives On Human Behavior: Can we Make It Disappear? The Case of the Death Penalty",  Naci H. Mocan, R. Kaj Grittings, NBER Working Paper, 10/06, www(dot)

(2006) " . . . (Donohue and Wolfers' "D&W") criticisms of Zimmerman's analysis are misrepresentative, moot or unsupportable in terms of the analyses they perform."  "It is shown that Zimmerman's published empirical results, or the conclusions drawn from them, are not in any way refuted by D&W's critique." (pg 3)  "This later estimate suggests that each execution deters 14 murders on average . . .". (pg 7) "It is shown that D&W made a number of serious misinterpretations in their review of Zimmerman's study and that none of the analyses put forward by D&W (which ostensibly refute Zimmerman's original results and conclusions) hold up under scrutiny. (pg8) " . . . D&W do not even report Zimmerman's "preferred"  results correctly, and then proceed by carrying on this error throughout the remainder of their critique."(pg8) "Of course, (D&W's) omission tends to create a strong impression that Zimmerman's analysis 'purports to find reliable relationships between executions and homicides', when his actual conclusions regarding the deterrent effect of capital punishment are far more agnostic." (pg10) " . . . D&W's method of interpreting their results is not consistent with that proscribed by the received econometric literature on randomized testing . . .".  "As such, D&W's interpretation of their randomized test in itself does not (and cannot) reasonably lead one to conclude that Zimmerman's estimates suggesting a deterrent effect of capital punishment are spurious." (pg12) " . . . D&W do not appear to have interpreted their randomization test in any meaningful fashion." (pg14) " . . . the state clustering correction employed by D&W may not be producing statistically meaningful results." (pg16) "And while D&W once lamented that recent econometric studies purporting to demonstrate a deterrent effect of capital punishment yield 'heat rather than light', as shown herein, their criticisms of Zimmerman (2004) tend to yield 'smoke rather than fire'."(pg26)
Zimmerman, Paul R., "On the Uses and 'Abuses' of Empirical Evidence in
the Death Penalty Debate" (November 2006).  ssrn(dot)com/abstract=948424