THE MOST COMMON argument advanced as justification for capital punishment is deterrence. Deterrence may be defined as the measures taken to prevent criminals, and would-be criminals, from committing crimes in the future. The threat of punishment, it is hoped, will discourage crime.

All criminal laws contain threats of punishment. Certainly, however, not all people are deterred from breaking laws. A threat of punishment may not be effective as a deterrent for any number of reasons. The threat may not be great enough to deter a person from committing a crime. It may not be carried out often enough to be a realistic threat. Or perhaps the rewards for carrying out the crime may be perceived as being so great that the criminal is willing to risk the punishment.

How these risks are perceived also varies from...