A new study on the death penalty and deterrence has been making the news lately, as it claims that each execution prevents 18 murders, or something like that. Here in Connecticut, the prosecutor who has sentenced the most to death in this state has publicly acknowledged that the death penalty does not deter.

This post from the Huffington Post was a nice, comprehensive response to this "news":
The Death Penalty Deterrence Myth: No Solid Evidence That Killing Stops
The Killing


Among the many factors in the debate about the death penalty is whether
capital punishment deters violent crime. Although solid research indicates
that there is no valid evidence of such deterrence, recent attention has
been given to a few flawed studies concluding that the death penalty does
deter murder.

A June 10 Associated Press article pointed to statistical studies that
claimed to directly link numbers of executions with numbers of murders
prevented, including a 2003 study from the University of Colorado at
Denver and studies from 2003 and 2006 by researchers at Emory University.
But follow-up studies by top social scientists soundly reject those
conclusions as well as the flawed methodology used to reach them. Jeffrey
Fagan, a professor at Columbia Law School and an expert on statistics,
testified to Congress that the Emory and Denver studies were "fraught with
numerous technical and conceptual errors," and "fail[ed] to reach the
demanding standards of social science."

The truth is that it might be impossible to determine a true statistical
relationship between homicides and executions because the number of
executions is so small compared to the number of homicides. But what we
can say with certainty is that there is no legitimate statistical evidence
of deterrence.

John Donohue, Yale Law School professor and Research Associate at the
National Bureau of Economic Research, and Justin Wolfers, Wharton School
of Business professor and Research Affiliate at the NBER, analyzed the
same data used in the Emory and Denver studies, as well as other studies
by the same researchers and many other nationwide reports. They found that
if anything, executions increase homicides, concluding: "The view that the
death penalty deters is still the product of belief, not evidence ... On
balance, the evidence suggests that the death penalty may increase the
murder rate."

Donohue and Wolfers analyzed data from the 2006 study by the Emory
researchers using non-death penalty states as a control group, a basic
statistical tool used to study causation not used in the Emory study. When
they compared death penalty states with non-death penalty states, they
found no evidence of any effect of executions on murder rates, either up
or down. Donohue and Wolfers also analyzed the data from the 2003 Emory
study that concluded that each execution prevented 18 murders and found
that the reduction or increase in murders was actually more dependent on
other factors used in the study than whether or not the states had the
death penalty. For example, when Donohue and Wolfers slightly redefined
just one of the factors included by the Emory researchers, they found that
each execution caused 18 murders.

Donohue and Wolfers also recomputed data from the Denver study of select
states to account for overall crime trends, a factor not included in the
Denver study, and reached inconclusive results. For two states included in
the Denver study that had abolished the death penalty, Massachusetts and
Rhode Island, Donohue and Wolfers found that the homicides rates actually
fell after capital punishment was ended.

Other studies also refute the deterrence theory. For example, researchers
Lawrence Katz, Steven Levitte and Ellen Shustorovich analyzed state data
between 1950 and 1990 and did not find a correlation between the death
penalty and crime rates. Moreover, one of the Emory researchers, Joanna
Shepherd, published a state study of her own and found that while the
death penalty deterred murder in 6 states, it actually increased murder in
13 states, and had no effect on the murder rate in eight states.

Other statistical analyses show that states with the death penalty do not
have the lowest murder rates in the country. In fact, according to the
Death Penalty Information Center, states without the death penalty have
consistently lower murder rates than states with the death penalty, even
when comparing neighboring states. In addition, while southern states
account for over 80 % of the executions in this country, they have
consistently had the highest murder rate of the nation's four regions.

Comparing American and Canadian statistics is also telling. While Canada
has not had a single execution since 1972 and the United States has
executed over 1,000 people in that time, the homicide rates in the United
States and Canada have closely tracked each other. If anything, Canada's
experience suggests that ending executions leads to a drop in the murder
rate.

As the death penalty debate continues, it will inevitably be filled with
the emotion and passion that have historically and rightly characterized
it. But when it comes to analyzing data and reaching statistical
conclusions that are used to affect our nation's policy and legislation on
a matter as dire as capital punishment, it is critical that the research
use statistically valid methodology. When we come across studies that are
as specific as to tie a number of executions to a number of prevented
murders, a healthy skepticism is in order, especially in the face of
substantial countervailing evidence. This is, after all, a matter of life
and death.

(source: Opinion, Cassy Stubbs, The Huffington Post)