During the 2000 presidential campaign, Republican candidate George W. Bush and Democratic candidate Al Gore each expressed strong support for the death penalty. In the third presidential debate, when asked why they backed the death penalty, both candidates answered: “It’s a deterrence.” Without capital punishment, they maintain, murders would increase.

Many U.S. political leaders agree with Bush and Gore. For example, when Republican George E. Pataki became governor of New York in 1995, he reinstated the death penalty there, arguing that “for too long, [murderers] were not subject to swift and certain punishment and, as a result, violent criminal acts were not deterred.” Within just one year, Pataki points out, the restored death penalty led to a dramatic drop in violent crime: assaults went down 22 percent, and murders dropped by nearly one-third. Statistics from other...