"...study after study has shown that the death penalty deters murders. Some studies show really dramatic effects, with each execution of a murderer deterring as many as 18 or more murders. That's according to Emory University professors, who found as well that delaying execution also leads to further murders. Most studies have concluded that some number of murders between three and 18 are prevented for every application of capital punishment."
A dozen research papers in the last six years support Thompson's assertion that capital punishment prevents murders, according to the Associated Press. Three studies get the most attention: a 2006 report by two economists and statistical papers in 2003 and 2006 from the Emory researchers.
The Law & Order star accurately quotes the Emory study, but his generalizations about "most studies" ignore significant contrary research.
A handful of studies directly challenge pro-deterrence findings by highlighting various missteps in methodology. "These studies fail to reach the demanding standards of social science to make such strong claims, standards such as replication and basic comparisons with other scenarios," said Jeffery Fagan of the Columbia Law School during testimony before a New York legislative panel.
Most notable is a paper by two National Bureau of Economic Research fellows who concluded "that execution policy drives little of the year-to-year variation in homicide rates. As to whether executions rise or lower the homicide rate, we remain profoundly uncertain."