The theory of deterrence is based on the idea that the threat of punishment must be severe enough to counter the benefits or pleasures that the criminal would receive from the crime. In addition, the punishment must be administered swiftly so that potential criminals will see a clear causeand- effect relationship between the crime and the punishment. When punishment deters potential criminals from committing crimes, it is called “general deterrence.” Another kind of deterrence, “specific deterrence,” refers to the inability of convicted criminals to commit further crimes as a result of their punishment. There is no doubt that capital punishment serves as a specific deterrent: The executed criminal will never kill again. However, experts and others have long debated whether capital punishment is a more effective general deterrent than life in prison.

Social scientists have examined the general deterrent effect of capital punishment since the early twentieth century. Early studies, including those by Thorsten Sellin, took two approaches: Some studies compared homicide rates in states with and without capital punishment; others compared homicide rates for states before or after the reintroduction or abolition of capital punishment. Researchers found that murder rates in neighboring states with and without the death penalty were not significantly different. They also found that homicide rates in states did not increase after the abolishment of the death penalty or decrease after the reinstatement of the sanction. More recent comparative studies have come to the same conclusion, supporting Sellin’s contention in 1967 that “the presence of the death penalty in law and practice has no discernible effect as a deterrent to murder.”

In the mid-1970s, these results were countered by Isaac Ehrlich, a statistician who, after looking at national homicide rates between 1930 and 1970, estimated that each execution deterred between seven and eight homicides. Many researchers have tried to duplicate Ehrlich’s results, but most of them have been unsuccessful. It has proven extremely difficult to demonstrate a relationship between executions and crime rates nationwide because of the large number of sociodemographic, legal, and historical variables. Criminologist Frank Zimring has suggested, for example, that the omission of key variables in Ehrlich’s studies, including the increased availability of guns and the decline in time served in prison for homicide, calls the results into question. Typically, death penalty opponents claim that Ehrlich’s results have been proved invalid, while proponents assert that the results are inconclusive. In the end, social science has been unable to either conclusively support or disprove the theory that capital punishment deters crime.