The Volokh Conspiracy - Cass Sunstein & Adrian Vermeule for the Death Penalty:

Their essay is available for download here; here's the abstract (paragraph breaks added):

Recent evidence suggests that capital punishment may have a significant deterrent effect, preventing as many as eighteen or more murders for each execution. This evidence greatly unsettles moral objections to the death penalty, because it suggests that a refusal to impose that penalty condemns numerous innocent people to death.

Capital punishment thus presents a life-life tradeoff, and a serious commitment to the sanctity of human life may well compel, rather than forbid, that form of punishment. Moral objections to the death penalty frequently depend on a distinction between acts and omissions, but that distinction is misleading in this context, because government is a special kind of moral agent.

The familiar problems with capital punishment --- potential error, irreversibility, arbitrariness, and racial skew --- do not argue in favor of abolition, because the world of homicide suffers from those same problems in even more acute form. The widespread failure to appreciate the life-life tradeoffs involved in capital punishment may depend on cognitive processes that fail to treat "statistical lives" with the seriousness that they deserve.

I've read the paper, and though I don't entirely agree with all the analysis in it, I think it makes some very important points, and will attract a lot of attention. What most intrigued me, incidentally, was its summary of the recent deterrence studies, which I hadn't known about. (I support the death penalty on retributive grounds, but obviously if it's a powerful deterrent, that would reinforce the retributivists' support and may also bring around many nonretributivists.) Here's their summary of the evidence; for footnotes, please see the paper itself:

For many years, the deterrent effect of capital punishment was sharply disputed. But a great deal of recent evidence strengthens the claim that capital punishment has large deterrent effects. The reason for the shift is that a wave of sophisticated econometric studies have exploited a newly-available form of data, so-called "panel data" that uses all information from a set of units (states or counties) and follows that data over an extended period of time. A leading study used county-level panel data from 3,054 U.S. counties between 1977 and 1996. The authors find that the murder rate is significantly reduced by both death sentences and executions. The most striking finding is that on average, each execution results in 18 fewer murders.

Other econometric studies also find a substantial deterrent effect. In two papers, Paul Zimmerman uses state-level panel data from 1978 onwards to measure the deterrent effect of execution rates and execution methods. He estimates that each execution deters an average of fourteen murders. Using state-level data from 1977 to 1997, Mocan and Gittings find that each execution deters five murders on average. They also find that increases in the murder rate come from removing people from death row and also from commutations in death sentences. Yet another study, based on state-level data from 1997-1999, finds that a death sentence deters 4.5 murders and an execution deters three murders. The same study investigates the question whether executions deter crimes of passion and murders by intimates. The answer is clear: these categories of murder are deterred by capital punishment. The deterrent effect of the death penalty is also found to be a function of the length of waits on death row, with a murder deterred for every 2.75 years of reduction in the period before execution.

In the period between 1972 and 1976, the Supreme Court produced an effective moratorium on capital punishment, and an extensive study exploits that fact to estimate the deterrent effect. Using state-level data from 1960-2000, the authors make before-and-after comparisons, focusing on the murder rate in each state before and after the death penalty was suspended and reinstated. The authors find a substantial deterrent effect. After suspending the death penalty, 91% of states faced an increase in homicides -- and in 67% of states, the rate was decreased after reinstatement of capital punishment.

A recent study offers more refined findings. Disaggregating the data on a state by state basis, Joanna Shepherd finds that the nation-wide deterrent effect of capital punishment is entirely driven by only six states — and that no deterrent effect can be found in the twenty-one other states that have restored capital punishment. What distinguishes the six from the twenty-one? The answer lies in the fact that states showing a deterrent effect are executing more people than states that do not. In fact the data show a "threshold effect": deterrence is found in states that had at least nine executions between 1977 and 1996. In states below that threshold, no deterrence can be found. This finding is intuitively plausible. Unless executions reach a certain level, murderers may act as if the death is so improbable as not to be worthy of concern. Her main lesson is that once the level of executions reaches a certain level, the deterrent effect of capital punishment is substantial.

All in all, the recent evidence of a deterrent effect from capital punishment seems impressive. But in studies of this kind, it is hard to control for confounding variables, and a degree of doubt inevitably remains. It remains possible that these findings will be exposed as statistical artifacts or will be found to rest on flawed econometric methods. More broadly, skeptics are likely to question the mechanisms by which capital punishment has a deterrent effect. On the skeptical view, many murderers lack a clear sense of the likelihood and perhaps even the existence of executions in their state; further problems for the deterrence claim are introduced by the fact that capital punishment is imposed infrequently and after long delays. In any case many murders are committed in a passionate state that does not lend itself to an all-things-considered analysis on the part of perpetrators.

As mentioned above, and as we discuss in Part IV, these suppositions are in some tension with existing evidence. But let us suppose that these doubts are reasonable. If so, should current findings be deemed irrelevant for purposes of policy and law? That would be an odd conclusion. In regulation as a whole, it is common to embrace some version of the Precautionary Principle --- the idea that steps should be taken to prevent significant harm even if cause-and-effect relationships remain unclear and even if the risk is not likely to come to fruition. Even if we reject strong versions of the Precautionary Principle, it hardly seems sensible that governments should ignore evidence demonstrating a significant possibility that a certain step will save large numbers of innocent lives.

For capital punishment, critics often seem to assume that evidence on deterrent effects should be ignored if reasonable questions can be raised about it. But as a general rule, this is implausible. In most contexts, the existence of reasonable questions is hardly an adequate reason to ignore evidence of severe harm. If it were, many environmental controls would be in serious jeopardy. We do not mean to suggest that government should commit what many people consider to be, prima facie, a serious moral wrong simply on the basis of speculation that this step will do some good. But a degree of reasonable doubt does not seem sufficient to doom capital punishment, if the evidence suggests that significant deterrence occurs.

In any event, as they say, read the whole thing — and, better yet, also read the studies it cites (something I plan to do shortly).

UPDATE: Reader Dan Markel points to other articles on the deterrence question, which appear to take a contrary view: Richard Berk, New Claims about Executions and General Deterrence: Deja Vu All Over Again?, J. Emp. L. Stud. (forthcoming) (March 11, 2005 draft); Ted Goertzel, Capital punishment and homicide: sociological realities and econometric illusions, Skeptical Inquirer (July-August, 2004).

	[Eugene Volokh, March 23, 2005 at 4:20pm] Trackbacks Cass Sunstein & Adrian Vermeule for the Death Penalty: Their essay is available for download here; here's the abstract (paragraph breaks added): Recent evidence suggests that capital punishment may have a significant deterrent effect, preventing as many as eighteen or more murders for each execution. This evidence greatly unsettles moral objections to the death penalty, because it suggests that a refusal to impose that penalty condemns numerous innocent people to death. Capital punishment thus presents a life-life tradeoff, and a serious commitment to the sanctity of human life may well compel, rather than forbid, that form of punishment. Moral objections to the death penalty frequently depend on a distinction between acts and omissions, but that distinction is misleading in this context, because government is a special kind of moral agent. The familiar problems with capital punishment --- potential error, irreversibility, arbitrariness, and racial skew --- do not argue in favor of abolition, because the world of homicide suffers from those same problems in even more acute form. The widespread failure to appreciate the life-life tradeoffs involved in capital punishment may depend on cognitive processes that fail to treat "statistical lives" with the seriousness that they deserve. I've read the paper, and though I don't entirely agree with all the analysis in it, I think it makes some very important points, and will attract a lot of attention. What most intrigued me, incidentally, was its summary of the recent deterrence studies, which I hadn't known about. (I support the death penalty on retributive grounds, but obviously if it's a powerful deterrent, that would reinforce the retributivists' support and may also bring around many nonretributivists.) Here's their summary of the evidence; for footnotes, please see the paper itself: For many years, the deterrent effect of capital punishment was sharply disputed. But a great deal of recent evidence strengthens the claim that capital punishment has large deterrent effects. The reason for the shift is that a wave of sophisticated econometric studies have exploited a newly-available form of data, so-called "panel data" that uses all information from a set of units (states or counties) and follows that data over an extended period of time. A leading study used county-level panel data from 3,054 U.S. counties between 1977 and 1996. The authors find that the murder rate is significantly reduced by both death sentences and executions. The most striking finding is that on average, each execution results in 18 fewer murders. Other econometric studies also find a substantial deterrent effect. In two papers, Paul Zimmerman uses state-level panel data from 1978 onwards to measure the deterrent effect of execution rates and execution methods. He estimates that each execution deters an average of fourteen murders. Using state-level data from 1977 to 1997, Mocan and Gittings find that each execution deters five murders on average. They also find that increases in the murder rate come from removing people from death row and also from commutations in death sentences. Yet another study, based on state-level data from 1997-1999, finds that a death sentence deters 4.5 murders and an execution deters three murders. The same study investigates the question whether executions deter crimes of passion and murders by intimates. The answer is clear: these categories of murder are deterred by capital punishment. The deterrent effect of the death penalty is also found to be a function of the length of waits on death row, with a murder deterred for every 2.75 years of reduction in the period before execution. In the period between 1972 and 1976, the Supreme Court produced an effective moratorium on capital punishment, and an extensive study exploits that fact to estimate the deterrent effect. Using state-level data from 1960-2000, the authors make before-and-after comparisons, focusing on the murder rate in each state before and after the death penalty was suspended and reinstated. The authors find a substantial deterrent effect. After suspending the death penalty, 91% of states faced an increase in homicides -- and in 67% of states, the rate was decreased after reinstatement of capital punishment. A recent study offers more refined findings. Disaggregating the data on a state by state basis, Joanna Shepherd finds that the nation-wide deterrent effect of capital punishment is entirely driven by only six states — and that no deterrent effect can be found in the twenty-one other states that have restored capital punishment. What distinguishes the six from the twenty-one? The answer lies in the fact that states showing a deterrent effect are executing more people than states that do not. In fact the data show a "threshold effect": deterrence is found in states that had at least nine executions between 1977 and 1996. In states below that threshold, no deterrence can be found. This finding is intuitively plausible. Unless executions reach a certain level, murderers may act as if the death is so improbable as not to be worthy of concern. Her main lesson is that once the level of executions reaches a certain level, the deterrent effect of capital punishment is substantial. All in all, the recent evidence of a deterrent effect from capital punishment seems impressive. But in studies of this kind, it is hard to control for confounding variables, and a degree of doubt inevitably remains. It remains possible that these findings will be exposed as statistical artifacts or will be found to rest on flawed econometric methods. More broadly, skeptics are likely to question the mechanisms by which capital punishment has a deterrent effect. On the skeptical view, many murderers lack a clear sense of the likelihood and perhaps even the existence of executions in their state; further problems for the deterrence claim are introduced by the fact that capital punishment is imposed infrequently and after long delays. In any case many murders are committed in a passionate state that does not lend itself to an all-things-considered analysis on the part of perpetrators. As mentioned above, and as we discuss in Part IV, these suppositions are in some tension with existing evidence. But let us suppose that these doubts are reasonable. If so, should current findings be deemed irrelevant for purposes of policy and law? That would be an odd conclusion. In regulation as a whole, it is common to embrace some version of the Precautionary Principle --- the idea that steps should be taken to prevent significant harm even if cause-and-effect relationships remain unclear and even if the risk is not likely to come to fruition. Even if we reject strong versions of the Precautionary Principle, it hardly seems sensible that governments should ignore evidence demonstrating a significant possibility that a certain step will save large numbers of innocent lives. For capital punishment, critics often seem to assume that evidence on deterrent effects should be ignored if reasonable questions can be raised about it. But as a general rule, this is implausible. In most contexts, the existence of reasonable questions is hardly an adequate reason to ignore evidence of severe harm. If it were, many environmental controls would be in serious jeopardy. We do not mean to suggest that government should commit what many people consider to be, prima facie, a serious moral wrong simply on the basis of speculation that this step will do some good. But a degree of reasonable doubt does not seem sufficient to doom capital punishment, if the evidence suggests that significant deterrence occurs. In any event, as they say, read the whole thing — and, better yet, also read the studies it cites (something I plan to do shortly). UPDATE: Reader Dan Markel points to other articles on the deterrence question, which appear to take a contrary view: Richard Berk, New Claims about Executions and General Deterrence: Deja Vu All Over Again?, J. Emp. L. Stud. (forthcoming) (March 11, 2005 draft); Ted Goertzel, Capital punishment and homicide: sociological realities and econometric illusions, Skeptical Inquirer (July-August, 2004).