The death-penalty deterrence question resists controlled experiments because society cannot randomize executions.

Evidence

The issue concerning deterrence is not whether the threat of punishment by death deters homicides and other serious crimes but whether it does so more effectively than life imprisonment. It is the alleged unique deterrent influence of the death penalty that is disputed. Advocates of the death penalty contend that executions save lives by deterring homicides. Oppositionists argue that there is no credible evidence that it does so. Resolving this dispute empirically is not as straightforward as it may appear. Society is not at liberty to conduct randomized experiments in which half of the condemned prisoners in one region are executed and the other half are assigned to life imprisonment in a matched region. Researchers must therefore try to tease out the unique influence of the death penalty from among many naturally co-occurring influences that can affect homicide rates. — Albert Bandura, Moral Disengagement, 1944

Categorization

• corroborated: 2 independent passages support this claim — Albert Bandura, Moral Disengagement: How People Do Harm…, 1944