Around 300 innocent people were allegedly given death sentences in the United States over a three-decade period, according to a study published in a leading science journal on Monday.
Recent years have seen the exoneration of a dozen defendants who had been sentenced to death, Reuters reported.
However, many more prisoners have probably been 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
Between 1973 and 2004, 7,482 death sentence convictions were examined by Gross and his colleagues in their research.
Due to death penalty issues garnering public attention through the efforts of numerous groups, 177 have been exonerated in recent years, according to Reuters.
It has been estimated that about 4 percent of those sentenced to death were actually innocent, nearly three times the number exonerated during that period, 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 said.
"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," Reuters reported.
"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.
The percentage of false death sentence convictions likely hold true today even though the study focuses on a period ending 10 years ago, Gross said.
