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Disciplines and deterrence

November 20th, 2007

The NY Times has an interesting piece on statistical studies of the deterrent effect (if any) of the death penalty. For those who want to get straight into fact-free debate, the bottom line is that the evidence is too weak to allow a firm conclusion one way or the other. What’s interesting to me, though is the way in which debates within different disciplines proceed, and the lags in transmission between them. Here I think the NYT story, while excellent in many respects, is quite misleading, presenting a story of deterrence-hypothesis economists facing off against legal critics.

That was pretty much the way things stood in the 1970s, after the publication of Isaac Ehrlich’s study in the American Economic Review claiming that one execution deterred 7 or 8 homicides. Ehrlich used multiple regression analysis (quite difficult and computationally demanding in those days, and correspondingly highly regarded) in an attempt to control for other factors affecting homicide rates and isolate the effect of the death penalty.

Over the next decade, economists learned a lot about the limitations of regression analysis. With limited amounts of data, it’s impossible to avoid mining the data for patterns which are then used to fit the model. And if you try enough specifications on weak data, you can get just about any result you want. A classic exposition of this point was Ed Leamer’s 1983 article “Let’s take the con out of econometrics” which pointed out the fragility of regression analysis on time-series data and picked, as an example, the deterrent effect of the death penalty.

As is the way, there was plenty of back and forth after that, on Leamer’s general claims, his proposal of Extreme Bounds Analysis as a remedy, and on the specific question of the deterrent hypothesis. Still, I don’t imagine any economist who’s been paying even moderate attention for the last twenty-five years would have been surprised by the finding of Justin Wolfers and John Donohue, published in the Stanford Law Review in 2006 that “the death penalty – at least as it has been implemented in the United States – is applied so rarely that the number of homicides that it can plausibly have caused or deterred cannot be reliably disentangled from the large year-to-year changes in the homicide rate caused by other factors. … Sampling from the broader universe of plausible approaches suggests not just ‘reasonable doubt’ about whether there is any deterrent effect of the death penalty, but profound uncertainty – even about its sign”

The other point made in the NYT article is that economists tend to believe that people respond to incentives, and no doubt the favorable initial reception of Ehrlich’s work reflected this. But once economists started thinking about actual magnitudes, the story fell apart. The probability of being executed as a result of committing a homicide is tiny, and pales into insignificance compared to, say, the risks of being a street crack dealer as estimated by Levitt. In fact, I recall, but can’t verify a claim that the mortality rate for prisoners on death row is actually lower than that for prisoners in general.

By contrast with this, I was struck by this passage from Cass Sunstein and Adrian Vermeule also in the Stanford Law Review, and quoted by Wolfers and Donohue

More recent evidence, however, has given new life to Ehrlich’s hypothesis. A wave of sophisticated multiple regression 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 3054 U.S. counties between 1977 and 1996.23 The authors found that the murder rate is significantly reduced by both death sentences and executions. (scare quotes in original)

The reverence with which panel data studies (pretty old hat to economists) are described suggests that not much in the way of critical analysis is going on here. Wolfers and Donohue point out that the study cited here (by Dezhbaksh et al) doesn’t even have fixed year effects, a strategy Wolfers and Donohue describe as “a clear outlier in the literature”. They might have gone on to point out that it’s an outlier because such an omission would lead to immediate rejection at most econ journals.

Like Wolfers and Donohue, I don’t think statistical evidence on the deterrence hypothesis is ever going to get us far. On the other hand, US experience has shown beyond doubt that innocent people are regularly sentenced to death, and it’s virtually certain that some have been executed (the legal system doesn’t provide any real way of establishing this, except in the rare cases when the real killer is convicted subsequently). That’s not the only argument against the death penalty, but it’s sufficient in my view

  1. observa
    November 20th, 2007 at 18:58 | #1

    “Over the next decade, economists learned a lot about the limitations of regression analysis. With limited amounts of data, it’s impossible to avoid mining the data for patterns which are then used to fit the model. And if you try enough specifications on weak data, you can get just about any result you want.”

    Like the economic models those GW one percenters rely so faithfully on?

  2. observa
    November 20th, 2007 at 19:20 | #2

    Speaking of data mining I reckon I could mine some data to show there’s a very weak correlation between Kyoto cap and trade and CO2 emission reductions as well as anyone being murdered by an executed murderer. Not so much mining as tripping over the Pilbara in broad daylight perhaps?

  3. wilful
    November 20th, 2007 at 19:26 | #3

    Your final paragraph is where I stand. The State cannot ever put itself in a position where it kills an innocent man. Proponents of the death penalty can get back to me when they can prove that the system is truly infallible. I’m sure they’ll manage that, oh, sometime around 2500.

  4. rog
    November 20th, 2007 at 20:55 | #4

    I think it is important to remember that in a free society “the State” is representative of the people and when someone is executed the citizens pull the lever. If public opinion is strong enough those remaining states in the US may change the law.

    Life in gaol without hope of parole must be the toughest of sentences

  5. Persse
    November 20th, 2007 at 20:59 | #5

    Put me down as dubious that statistical metrics will assist ethical judgments on this issue.

    Human life is cheap.

    Every day Australia does little to prevent widespread death, from numerous preventable causes, in other countries, we even, without qualm apparently, enter other countries with the express purpose of slaughtering the citizens, in vast numbers.

    We get ourselves in a knot about terminating the life of Joe Blow, who strangled the missus, after she told him she was leaving him for someone less violent.

    I don’t give a stuff about Joe. And I can’t bring the missus back.

    But what I can do is understand how the killing of other people affects me. And it is about me, and others of my fellow citizens who feel as I do.

    I don’t want the taint, and the violence, of killing, to be imposed upon me. By anyone.

    I oppose judicial killing for selfish reasons.

  6. rog
    November 20th, 2007 at 20:59 | #6

    Using the argument that the convicted may be innocent falls down if the perpetrator was seen in the act and fairly tried with overwhelming evidence. In that instance the death penalty would be seen to be appropriate.

    Better that all death penalties are “hate crimes”

  7. November 21st, 2007 at 06:43 | #7

    i was a supporter of the death penalty as a naive young lad.

    but for reasons canvassed above, i gradually became less certain it was a good idea.

    then one day i realized my nation was engaged in simple assassination and mass murder, in southeast asia.

    that clarified the situation immensely. no government official can be trusted with the power of life and death. not even if the accused confesses, not even if the bishop of sydney swears he was there and saw him do it. even these open and shut cases can be ‘arranged’ by government.

    in a modest way, the haneef case demonstrates the mind-set of government arrogance, determined to have an outcome, determined to be infallible.

    anyone who imagines the afp is anything but a pliable tool of whoever is currently in power, the department of prosecution searches for justice, the ministers of state are not incompetent and often corrupt, is enjoying life in a fairytale.

    the government shouldn’t have the power to do anything without consent of the electorate.

  8. Bring Back CL’s blog
    November 21st, 2007 at 07:30 | #8

    capital punishment for thecrime of deliberately and wantonly murdering another human being is NOT about deterrance.

    It is about the punishment ffitting the crime.

    Deterrance must the most evil concept ever thought up.

  9. Spiros
    November 21st, 2007 at 08:29 | #9

    “It is about the punishment ffitting the crime.”

    It’s just as well as the Birmingham Six, among many others, did not receive a punishment that fitted the crime, they were convicted of committing.

  10. Katz
    November 21st, 2007 at 08:44 | #10

    It is the role of the state to deter.

    It is the role of the state to reform.

    It is the role of the state to punish.

    It is not the role of the state to mete out vengeance.

    If capital punishment does not deter, then there is no legitimate purpose to be served by capital punishment.

    And as JQ has suggested, the evidence of the deterrent effect of capital punishment is thin.

  11. Andy McLennan
    November 21st, 2007 at 09:20 | #11

    The legal system has enormous power over the lives of specific individuals, and a key concept of democratic government is that those who wield that power, and the purposes to which it is put, must command general respect. The stately stone courthouses, quiet oak paneled courtrooms, black robes, and elaborate procedural rules, are all designed with this in mind, so much so that we too easily take their effects for granted. The Founding Fathers had concrete historical recollection of capricious and “creative” punishments that “fit the crime,” and knew from experience how corrosive they can be. That the death penalty is not an effective deterrent, and necessarily results in at least occasional execution of innocents, is sufficient grounds for opposing it, but it is also important that its nature and motivations are directly counter to the principle that the law, and its administration, should be dispassionate.

  12. Bring Back CL’s blog
    November 21st, 2007 at 12:04 | #12

    wrong spiros.

    there was no clear cut evidence in that.

    cpaital punishment should only be used where there is clear cut evidence not on inferences

  13. Spiros
    November 21st, 2007 at 12:43 | #13

    Wrong Homer.

    The evidence was clear cut enough for them to be convicted of a monstrous act of terrorism, which invite the death penalty for sure in today’s climate, if it was allowed.

    It just happened to emerge later on that the evidence was manufactured by the police.

  14. crocodile
    November 21st, 2007 at 12:44 | #14

    Homer,

    What’s clear cut evidence. Is any evidence at all absolutely, 100% irrefutable.

  15. snuh
    November 21st, 2007 at 12:46 | #15

    actually BBCLb there was clear cut evidence to convict in the birmingham 6 trial: the confessions the police had beaten out of those who were tried. no inferences there, they confessed. of course they tried to take back their confessions, but then they would do that, wouldn’t they.

    the trial judge was so convinced of their guilt he expressed regret that the death penalty was no longer an option.

  16. jimbirch
    November 21st, 2007 at 13:07 | #16

    If you ever get a chance, take a look at this 1988 doco:

    http://en.wikipedia.org/wiki/The_Thin_Blue_Line_(documentary)

    It shows how an almost unbelievably wrong death penalty worked, in one case. It might or might not change your mind on the issue, but it’s absolutely fascinating viewing. One of the best docos I’ve ever seen. Mesmerising soundtrack by Phillip Glass.

  17. observa
    November 22nd, 2007 at 00:02 | #17

    OK, I’ll take the point about killing the innocent. What about life meaning life for murder and we put a sturdy noose and chair in each lifer’s spartan cell then? Just to remind them of our mercy of course.

  18. jimbirch
    November 22nd, 2007 at 17:22 | #18

    Would that make you feel better?

  19. Bring Back CL’s blog
    November 22nd, 2007 at 18:23 | #19

    simple ‘confessions’ are not clear cut evidence.

    Now Martin Bryant was clear cut whilst Lindy Chamberlain most certainly wasn’t.

  20. November 22nd, 2007 at 20:06 | #20

    Rog says:

    “Using the argument that the convicted may be innocent falls down if the perpetrator was seen in the act and fairly tried with overwhelming evidence.”

    That is absurd. Cases of mistaken identity happen all the time.

  21. November 23rd, 2007 at 11:19 | #21

    In fact, I recall, but can’t verify a claim that the mortality rate for prisoners on death row is actually lower than that for prisoners in general.

    From page 795 of Donohue & Wolfers:

    Katz, Levitt, and Shustorovich have made this point quite directly, arguing that “the execution rate on death row is only twice the death rate from accidents and violence among all American men� and that the death rate on death row is plausibly lower than the death rate of violent criminals not on death row.

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