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Unlike many of the topics covered in this book, death penalty litigation involves a wide variety of empirical issues. The Eighth Amendment of the U.S. Constitution provides that "Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishment inflicted." But what is a "cruel and unusual punishment?" It could be a punishment that is morally unacceptable to the American people, like cutting off noses or hands. Following the other clauses of the amendment, it could be a punishment that is excessive, in that a lesser penalty would achieve the same ends. For example, if a death sentence served no penological purpose that was not served just as well by life imprisonment, the death penalty might be seen as excessive. Or the death penalty could be cruel and unusual in practice if it is rarely imposed and if the decision to take or spare the criminal's life is unprincipled. We would like to believe that the people who are executed are those who have committed the most monstrous crimes. If instead there is no rational means of differentiating the few who are sentenced to die from other killers, except perhaps for impermissible criteria like race or poverty, that could be cruel and unusual. All of these definitions have been proposed, and each has produced various lines of empirical research that have played a role in death penalty litigation.

The constitutionality of the death penalty, and of the procedures that are used to impose it, are questions of law. Although much of the research involves juries and the public at large, on these issues the ultimate fact finders and evaluators of the research are judges, usually Supreme Court justices, and it is their "common knowledge" and their ability to evaluate the research that matters. The evidence comes to them summarized in briefs written by the parties or by "friends of the court" - amici curiae. These amici may include professional organizations such as the American Psychological Association, or ad hoc groups of social scientists. The justices may evaluate the quality of the research, but initially they also decide whether the research is even relevant to the constitutional question before them, as they understand it.

It would be impossible in a chapter of this length to provide a detailed description of all the varieties of empirical research related to the death penalty, the ways in which they have been presented to the courts, and the courts' responses to them. Instead, this chapter provides a general framework for classifying and understanding the kinds of empirical questions that have been raised and their constitutional implications, the kinds of data that have been presented to the courts, and the courts' response.

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