Study: More Innocents on Death Row Than Previously Thought

In his 2006 dissent in a Kansas death penalty case, U.S. Supreme Court Justice Antonin Scalia argued that the incidence of wrongful conviction is, in essence, vastly overblown.

"Like other human institutions, courts and juries are not perfect. One cannot have a system of criminal punishment without accepting the possibility that someone will be punished mistakenly," he wrote. "But with regard to the punishment of death in the current American system, that possibility has been reduced to an insignificant minimum."

New research published today in the Proceedings of the National Academy of Sciences suggests that "insignificant minimum" is far from insignificant: at least 4.1% of all defendants sentenced to death in the U.S. are innocent, according to the peer-reviewed study by University of Michigan School of Law professor Samuel Gross in collaboration with Michigan State University law professor Barbara O'Brien, and biostatisticians Chen Hu and Edward H. Kennedy. The researchers utilized "survivor analysis" – a statistical approach generally used to determine the efficacy of medical treatments – in order to reach what Gross describes as the most accurate estimate to date of the number of folks wrongly sentenced to death across the nation. Necessarily among that group, the authors note, are those who have been wrongly executed.

The rate of exoneration among those sentenced to death sentences in the U.S. is higher than for any other category of conviction, reads the study – death sentences account for "less than one-tenth of 1% of prison sentences in the United States, but they accounted for about 12% of known exonerations of innocent defendants from 1989 through early 2012, a disproportion of more than 130 to 1." That rate is so high, the researchers argue, precisely because the cases of inmates facing imminent execution are subjected to multiple layers of review. Starting with a discrete number of death sentences imposed between 1973 and 2004 (7,482) and exonerations that came from case reviews begun during that time period, while the defendant's were still under "threat of execution" (107) provided the researchers the framework to conclude that at least 4.1% of defendants are erroneously sentenced to death.

Indeed, Gross and colleagues write that this number does not represent the number of wrongful convictions within the larger criminal justice system. The rate of false conviction is often described "as a 'dark figure' – an important measure of the performance of the criminal justice system that is not merely unknown but unknowable," reads the report. That is in part because so many criminal convictions are never scrutinized – including many of the approximately 95% of all criminal cases that are resolved by plea bargain. And nearly 39% of all death cases imposed within the study period ended with a defendant being removed from death row, but not exonerated (think, for example, of juvenile defendants removed from death rows after the Supremes concluded in 2005 that the execution of juveniles is constitutionally barred), have also escaped the more stringent review and are thus not included among the 4.1%, the report concludes.

"This estimate can't be generalized to other crimes," said Gross. "My view is that it makes you concerned about [wrongful convictions for] other serious violent crimes." That percentage "could be higher, could be lower. We don't know and we can't say," he said. "It is not [an] insignificant [question], and it is something to be concerned about."