Samuel Gross is a law professor at the University of Michigan. His new study suggests that four percent of inmates who were sentenced to death in the United States from 1973 through 2004 were innocent.
"And we conclude that this is a conservative estimate of the rate of innocence among people who are sentenced to death in the United States," Gross said.
"They basically said that if you extrapolate these numbers, the likelihood that four percent of people on death row would be proven innocent," Nick Vilbas with the Innocence Project of Texas
said. "And this is based on the cases that have already been done where exonerations have been proven in death row cases."
Since 1973, not one single person has been officially exonerated after execution. But Gross said it is very likely that at least a small number of those people were innocent, though we do not know who they are and cannot know for sure.
"It's impossible to imagine that we've been through the number of executions we've had in this country, and have somehow systematically managed to avoid executing innocent people every time, but we don't know how many," Gross said.
Gross said the rate of exonerations of people who were sentenced to death is far higher than for any other category of people who were convicted for crimes in the United States.
"And the main reason for that is that everybody is so concerned about the dangers of executing innocent people, that we spend much more time, money and attention on cases of people who might be executed than we do on any other types of cases," Gross said.
"There are thousands of people who are unaccounted for that aren't on death row who are innocent who don't have that scrutiny applied to their cases that need their help," Vilbas said.
Vilbas said innocent people facing other serious sentences are forgotten. He said their chances of exoneration are 90 percent lower than inmates on death row.
"People on death row have that scrutiny applied," Vilbas said. "It's those that aren't on death row that are looking at an effective lifetime sentence of 35, 50, 75 years that need help to prove their innocence. That's the work that we do."
Vilbas said he does not think capital punishment will go away any time soon. He said polls show a majority of Texans agree that someone innocent has probably been put to death, but they still support the death penalty. Gross said he is not sure whether this study will turn any supporter into a death penalty opponent.