14 December 2017 :

The certainty of apprehension, not the severity of punishment, is more effective as a deterrent. So argues Daniel S. Nagin, one of the nation’s foremost scholars on deterrence and criminal justice policy, in his chapter on Deterrence in the recently released Academy for Justice four-volume study, Reforming Criminal Justice.
Reviewing deterrence scholarship since the 1960s and five leading studies from the past two decades, Dr. Nagin concludes that evidence supporting a deterrent effect from "the certainty of punishment is far more convincing and consistent than for the severity of punishment."
Moreover, he writes, "the certainty of apprehension, and not the severity of the ensuing legal consequence, is the more effective deterrent." Dr. Nagin is the Teresa and H. John Heinz III University Professor of Public Policy and Statistics at Carnegie Mellon’s Heinz College of Information Systems and Public Policy and previously chaired the Committee on Deterrence and the Death Penalty for the National Research Council of the National Academies of Science (NAS). In that capacity, he served as co-editor of the 2012 National Academies report, Deterrence and the Death Penalty. Nagin explains in his Academy for Justice chapter that although "certainty must result in a distasteful consequence" for the punishment to be a deterrent, "the consequences need not be draconian, just sufficiently costly, to deter the prohibited behavior." In making policy judgments about the justification for increasingly severe sanctions, he says, "the deterrent return to increasing an already long sentence appears to be small, possibly zero." The 2012 NAS Committee found that "research to date on the effect of capital punishment on homicide is not informative about whether capital punishment decreases, increases, or has no effect on homicide rates" and recommended that those deterrent studies "not be used to inform deliberations requiring judgments about the effect of the death penalty on homicide."
A February 2015 study by the Brennan Center for Justice of the dramatic drop in crime in the U.S. in the 1990s and 2000s found that the death penalty had no effect on the decline in crime.