In upholding the revised capital punishment statutes of several states back in 1976, the U.S. Supreme Court recognized two main purposes of the death penalty: 1.) deterrence (to stop people from committing capital offenses out of fear of being executed) and 2.) retribution (punishment that is proportionate to, and warranted by, the crime). A new report from the National Research Council finds that while the subject of deterrence has been exhaustively studied and debated over the last 35 years, essentially none of this research can be considered useful or informative in determining whether the death penalty has any impact on homicide rates. Even more significantly, the committee of distinguished scholars that published the report recommended that “these [deterrence] studies should not be used to inform judgments about the effect of the death penalty on homicide, and should not serve as a basis for policy decisions about capital punishment.”
Also, check out this editorial in the New York Times: “The Myth of Deterrence,” April 28, 2012.