As many as 300 people who were sentenced to death in the United States over a three-decade period were likely innocent, according to a study published in a leading science journal on Monday.
Dozens of defendants sentenced to death in recent years have been exonerated before their sentences could be carried out, but many more were probably falsely convicted, said University of Michigan professor Samuel Gross, the study's lead author.
"Our research adds the disturbing news that most innocent defendants who have been sentenced to death have not been exonerated," Gross wrote in the study, published in the Proceedings of the National Academy of Sciences.
However, he stressed that this did not indicate a jump in the number of people believed wrongly executed because some had had their sentences commuted to life imprisonment and others lingered on death row.
In their research, Gross and his colleagues examined the 7,482 U.S. death sentence convictions between 1973 and 2004.
Of those, 117 had been exonerated in recent years, thanks to the efforts of numerous groups and a tide of public attention to issues surrounding the death penalty.
Gross and his co-authors, Barbara O'Brien of Michigan State University, Chen Hu of the American College of Radiology Clinical Research Center in Philadelphia and the University of Pennsylvania's Edward H. Kennedy, estimated that about 4 percent of those sentenced to death were actually innocent, nearly three times the number exonerated during that period.
For their conclusion, the research group used a mathematical formula that included the number of inmates whose sentences were commuted to life imprisonment, the length of time it took for a convicted inmate on death row to be set free, and the number of inmates who were in the end exonerated.
In a twist, once inmates' sentences are commuted to life, they are far less likely to be exonerated, mostly because there are fewer legal resources given to their cases, Gross said.
"If you were never sentenced to death, you never had the benefit - if you call it a benefit - of that process," he said.
Although the study focuses on a period ending 10 years ago, the percentage of false death sentence convictions likely holds true today, Gross said.
The study does not say how many innocent people were likely put to death. It also does not suggest that the rate of false convictions in death sentence cases is the same as in any other conviction category.