An estimated 4.1 percent of all death row inmates are innocent, according to a study published in the April Proceedings of the National Academy of Sciences. Researchers Samuel Gross, Barbara O'Brien, Chen Hu, and Edward H Kennedy, from the University of Michigan Law School, the Michigan State University College of Law, the American College of Radiology Clinical Research Center, and the University of Pennsylvania School of Medicine, respectively, came up with this stunning figure by using the exoneration rate on death row and extending it to inmates whose capital punishment has been replaced by life imprisonment, at which point efforts to exonerate them largely subside. Currently, about 1.7 percent of inmates on death row are formally exonerated.
"Since 1973, nearly 8,500 defendants have been sentenced to death in the United States, and 138 of them have been exonerated," said Gross. "Our study means that more than 200 additional innocent defendants have been sentenced to death in that period. Most of these undiscovered innocent capital defendants have been resentenced to life in prison, and then forgotten."
The study notes that in a concurrence in Kansas v. Marsh, a 2006 Supreme Court decision upholding the death penalty, Justice Antonin Scalia came up with an error rate of .027 percent. He based that number on extending the exoneration rate of a small subgroup of inmates (capital cases) to the wider American prison population, which the study described as a "silly" claim.