Does Death Prevent Death?

Last night, this story came over the Fark wires, showing that California has 700 inmates on Death Row, and has executed only 13 since 1977.

Fark being Fark, inevitably the first guy to make it to the thread said that it was "time to add an express lane", which was variously interpreted as both 'streamline the appeals process so it doesn't take so long' and 'execute them faster; what are we waiting for'.

One of the 'execute them faster' proponents mentioned that the death penalty is meant to be a deterrent.

Sounds like my kind of hypothesis.

What we're going to do today is simple. In practice, the death penalty is nearly exclusively applied to murder cases. The last execution in America for any crime that did not include a murder was on September 4, 1964, when James Coburn was put to death for robbery in Alabama.

Knowing that, the best way to go about investigating the power of the death penalty as a deterrent is to compare murder rates. Specifically, we will compare the murder rate in states with the death penalty to the murder rate in states without.

In addition, as some states have the death penalty but are reluctant to use it, we'll further separate states by the date of their most recent execution. We'll set a cutoff of three years, giving us three total categories:

1. States actively using the death penalty (have executed since April 26, 2007)
Alabama (last executed June 11, 2009)
Arizona (May 23, 2007)
Florida (February 16, 2010)
Georgia (October 20, 2009)
Indiana (December 11, 2009)
Kentucky (November 21, 2008)
Louisiana (January 7, 2010)
Mississippi (July 23, 2008)
Missouri (May 20, 2009)
Ohio (April 20, 2010)
Oklahoma (January 14, 2010)
South Carolina (May 8, 2009)
South Dakota (July 11, 2007)
Tennessee (December 2, 2009)
Texas (April 22, 2010)
Virginia (March 18, 2010)

2. States inactively using the death penalty (have the death penalty, but have not executed since April 26, 2007)
Arkansas (November 28, 2005)
California (January 17, 2006)
Colorado (October 13, 1997)
Connecticut (May 13, 2005)
Delaware (November 4, 2005)
Idaho (January 6, 1994)
Illinois (March 17, 1999)
Maryland (December 5, 2005)
Montana (August 11, 2006)
Nebraska (December 2, 1997)
Nevada (April 26, 2006)
New Mexico (November 6, 2001)
North Carolina (August 18, 2006)
Oregon (May 16, 1997)
Pennsylvania (July 6, 1999)
Utah (October 15, 1999; there's been one in the news that's been scheduled, but it hasn't happened yet, so this is where Utah goes for now)
Washington (August 28, 2001)
Wyoming (January 22, 1992)

3. States without the death penalty, for which there is a moratorium, or have not executed since the 'modern era' of capital punishment began in 1976 with the Supreme Court case Furman v. Georgia)
Alaska
Hawaii
Iowa
Kansas (June 22, 1965)
Maine
Massachusetts
Michigan
Minnesota
New Hampshire (June 14, 1939)
New Jersey
New York
North Dakota
Rhode Island
Vermont
West Virginia
Wisconsin

This gives us 16 states in Group 1, 18 states in Group 2, and 16 states in Group 3, which by lucky happenstance is just about as close to perfect as we're going to get as far as evening up the sizes of the groups.

Step 2 is to compare everybody's murder rates. The most recent information is from 2007 according to census.gov (warning: Excel file), measuring number of murders per 100,000 people. The national average is 5.6.

Group 1 is bolded, Group 2 is italicized, Group 3 is standard. Their group numbers are also placed beside them.

14.2- Louisiana (1)
9.8- Maryland (2)
8.9- Alabama (1)
8.2- New Mexico (2)
8.0- South Carolina (1)
7.5- Georgia (1)
7.5- Nevada (2)
7.4- Arizona (1)
7.1- Mississippi (1)
6.7- Arkansas (2)
6.7- Michigan (3)
6.6- Florida (1)
6.5- Missouri (1)
6.5- North Carolina (2)
6.4- Alaska (3)
6.4- Tennessee (1)
6.2- California (2)
6.1- Oklahoma (1)
5.9- Illinois (2)
5.9- Texas (1)
5.8- Pennsylvania (2)
5.6- Indiana (1)
5.3- Virginia (1)
4.8- Kentucky (1)
4.5- Ohio (1)
--------note this line for later--------
4.4- New Jersey (3)
4.3- Delaware (2)
4.2- New York (3)
3.9- Kansas (3)
3.8- Nebraska (2)
3.5- West Virginia (3)
3.3- Idaho (2)
3.3- Wisconsin (3)
3.1- Colorado (2)
3.1- Wyoming (2)
3.0- Connecticut (2)
2.9- Massachusetts (3)
2.7- Washington (2)
2.2- Minnesota (3)
2.2- Utah (2)
2.1- South Dakota (1)
1.9- North Dakota (3)
1.9- Oregon (2)
1.9- Vermont (3)
1.8- Rhode Island (3)
1.7- Hawaii (3)
1.6- Maine (3)
1.5- Montana (2)
1.2- Iowa (3)
1.1- New Hampshire (3)

Obviously, there's some noise in the data, partially due to the way this was set up- after all, even if you've only executed one person in the last 50 years, if that one happened last week, you're in Group 1. Group 2 is all over the place.

But look at Group 1 and Group 3. Note that demarcation line placed between the 4.5 of Ohio and the 4.4 of New Jersey; it separates the states 25/25. You'll find a very clear separation, with almost all of Group 1 on Ohio's side and almost all of Group 3 on New Jersey's side. And remember, Groups 1 and 3 are of equal size, 16 states apiece.

Among states with at least 4.5 murders per 100,000:
Group 1: 15
Group 2: 8 (including five of the six most recent executions in the group- North Carolina, Nevada, California, Maryland and Arkansas; Montana's below the line)
Group 3: 2 (Michigan and Alaska)

Among states with under 4.5 murders per 100,000:

Group 1: 1 (South Dakota, which among states in Group 1 is the second-closest to slipping into Group 2)
Group 2: 10 (including the five longest waits in the group since their last execution: Wyoming, Idaho, Oregon, Colorado and Nebraska)
Group 3: 14

That's a result. Parse however you wish, but that's a result. The death penalty, from this data, does not appear to be an effective deterrent to the crime it punishes.