The Houston Chronicle recenly reported on the efforts of social scientists to understand whether the death penalty deters potential murderers.  According to the article, research on the issue has historically produced mixed results:

In 1967, sociologist Thorsten Sellin found no significant impact when he studied murder rates in adjacent states with differing approaches to capital punishment.

The next year, Nobel Prize economist Gary Becker developed a theory supporting the deterrent value of the death penalty, and eight years later one of his students published a study based on national statistics purporting to show that each execution saved eight lives.

The controversy led to a study commissioned by the National Academy of Sciences that found evidence of deterrence to be unconvincing.

More recent studies have reached conclusions all over the map. A national study in 2005 found “profound uncertainty” on the question and even suggested that executions might slightly increase the murder rate (possibly through a cultural “brutalization”). Another study that year suggested that each execution saves 150 lives.

The article discusses a new study, forthcoming in Criminology, by Duke University sociologists Kenneth C. Land and Hui Zheng and Sam Houston State University criminologist Raymond Teske Jr.:

After reviewing earlier studies, these authors came to the conclusion that the death penalty is used too sporadically and inconsistently around the nation for studies on national data to accurately measure its effect on crime.

They decided to focus their study by taking advantage of Texas’ gift to social science, what they call “an orgy of executions in Texas beginning in 1994,” during which time the state provided more than a third of the nation’s executions.

The authors compared this period to an era in which Texas carried out fewer executions from 1980 to 1993, attempting to isolate the effect of the increased use of the death penalty:

They found that many earlier studies had vastly overestimated the effect, but the number of murders did go down in the short-term aftermath of executions.

Based on two different statistical models, they found the effect in the months after each execution to be a reduction of between 0.5 to 2.5 homicides.

That may not sound like much, but as the authors note, “even the estimated .5 deterrent per execution yields an estimated reduction in the expected numbers of monthly homicides of 5 to 10 during the subsequent 12 months, which is substantial.”

Perhaps more interesting are the difficult issues that remain unresolved:

Here’s the mystery:

This study and previous ones show no correlation between the amount of publicity executions receive and their deterrent effect.

“We have no theory on that,” Teske said on Friday. After a few more questions, he said, “I hear your frustration. If I wasn’t working with one of the top guys in the nation, my confidence would be shaken.”

One other mystery: The study shows, as other studies have, more impact on the kinds of murders that don’t qualify for the death penalty than on those that do.