I’ve written some long posts deconstructing the argument that the death penalty deters murderers from committing homicides they would otherwise commit (here and here, for example).
Here are Betsey Stevenson and Justin Wolfers, summing up all the statistical problems facing those who want to claim … well … anything at all about the death penalty and deterrence:
The debate over the death penalty offers a vivid illustration of a tragic flaw in the market of ideas: Strong beliefs attract a lot more attention, and can have a lot more influence, than the truth.
In recent years, five U.S. states have eliminated capital punishment, and several others are currently reconsidering their policies. Advocates of the death penalty insist the moves will lead to more murders. They point to a number of studies conducted over the past couple of decades that purport to find clear evidence supporting their view. Experts happily serve up unequivocal congressional testimony, and feed their analyses to lobby groups.
The reality, unsatisfying and inconvenient as it may be, is that we simply don’t know how capital punishment affects the homicide rate. That’s the conclusion of the National Academy of Sciences, which typically plays the role of impartial arbiter in these social-science debates. Their expert panel recently concluded that existing research “is not informative about whether capital punishment decreases, increases, or has no effect on homicide rates,” and that such studies “should not influence policy judgments about capital punishment.”
Taken together, the various problems of measuring the relationship between crime and punishment yield what the National Academy panel calls “model uncertainty.” In English, that means there are many seemingly plausible ways of looking at the evidence that yield dramatically different answers. The true effect could be big or small, positive or negative. We just can’t estimate it with any certainty.
What the panel didn’t say — but we think they should have — is that model uncertainty facilitates abuse in the midst of a politically charged debate. With so many plausible approaches, there’s nothing to stop researchers fishing for a flashy result that may further their careers or ideological objectives.