At least one in 25 people sentenced to death in the United States is innocent.
So finds a damning study published Monday in leading scientific journal Proceedings of the National Academy of Sciences. The authors, hailing from Pennsylvania and Michigan, employ a statistical technique called "survival analysis" to arrive at their conclusions.
According to what researchers call a "conservative estimate," between 1973 and 2004, at least 4.1 percent of people in the United States handed death sentences were innocent.
Yet, during this same time period, only 1.6 percent of them had been exonerated. The remaining innocent were either executed or had their sentences commuted to life in prison.
Death sentences are sometimes reversed in situations where the "accuracy of the defendants’ convictions is in doubt," note the researchers. However, those defendants are often re-sentenced to life in prison. Once removed from death row, their cases no longer receive the same scrutiny of review, causing many of the innocent to languish in prison.
“Since 1973, nearly 8,500 defendants have been sentenced to death in the United States, and 138 of them have been exonerated," said Professor Samuel Gross of the University of Michigan Law School, one of the authors of the study. "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 re-sentenced to life in prison, and then forgotten.”
The figure of 4.1 percent, while conservative, is higher than estimates by previous researchers and casts doubt on the previous claim by conservative U.S. Supreme Court Justice Antonin Scalia that criminal conviction rates have an error rate of .027 percent.