That's the title of a column at Bloomberg News View, written by Betsey Stevenson and Justin Wolfers on the recent deterrence study published by the National Reserach Council. Stevenson and Wolfers are professors at the University of Pennsylvania’s Wharton School. Here's an extended excerpt from the beginning of the essay. There are graphs at the link.
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.”
The panel’s conclusions largely echo those from research conducted by one of us (Justin Wolfers) jointly with Stanford University law professor John Donohue. That research replicated and probed the leading studies, finding that even minor changes in how the analyses were conducted dramatically altered the conclusions. As a result, there’s “not just ‘reasonable doubt’ about whether there is any deterrent effect of the death penalty, but profound uncertainty,” the authors wrote. Indeed, “we remain unsure even of whether” the effects “are positive or negative.”
How could the confident claims of those earlier researchers end up being so wrong? Let’s start by exploring why it’s so difficult to give a precise answer to those interested in the effects of the death penalty.
As big a deal as capital punishment may seem, it’s actually quite rare. Since the death penalty was reinstated in the U.S. in 1976, there have been about 670,000 homicides and only 1,296 executions, a rate of about one execution per 500 murders. This makes the task of discerning its specific impact very difficult.
To complicate things further, the homicide rate fluctuates enormously for reasons unrelated to capital punishment. So the correlation between capital punishment and homicide rates can be positive or negative, depending on the specific sample of states or countries analyzed, the sample period chosen, and which other determinants are accounted for.
Even if the correlation between capital punishment and murder rates could be reliably estimated, that wouldn’t be enough to prove causation. For instance, more vigorous capital punishment probably occurs at the same time as other reforms to sentencing, prisons and policing. Unless these variables are measured accurately -- and our existing criminal-justice statistics do not provide adequate measures -- it is impossible to disentangle which reforms are driving the homicide rate.
The Bloomberg column is noted at the Wall Street Journal Real Time Economics blog.
Betsey Stevenson and Justin Wolfers look at the problems of measuring the effectiveness of the death penalty. “As big a deal as capital punishment may seem, it’s actually quite rare. Since the death penalty was reinstated in the U.S. in 1976, there have been about 670,000 homicides and only 1,296 executions, a rate of about one execution per 500 murders. This makes the task of discerning its specific impact very difficult. To complicate things further, the homicide rate fluctuates enormously for reasons unrelated to capital punishment.
Earlier coverage of the NRC deterrence study begins at the link.