It's difficult to know how often the American justice system gets it wrong: courts can find a person guilty or innocent, based on the best evidence—but that doesn't mean they've always found the truth. Some are optimistic that, most of the time, courts come to the right outcome: U.S. Supreme Court Justice Antonin Scalia once put the rate of error at 0.027 percent of felony convinctions. But, according to new research published in Proceedings of the National Academy of Sciences, the rate of false conviction for people sentenced to death is actually much higher—4.1 percent.
The new study takes into account the reality that only the most serious cases—especially cases where the accused person is sentenced to death—get much attention after the initial trial. "Because of all of the resources spent on capital cases, the researchers reason, it’s likely that many (and perhaps a majority) of innocent defendants on death row will ultimately be exonerated," Virginia Hughes explains at National Geographic.
Based on that reasoning, here's how the researchers calculated the actual false conviction rate, as Hughes explains:
[Law professor Samnuel] Gross and his colleagues collected data on the 7,482 people who were sentenced to death between 1973 — the first year of modern death-penalty laws — and 2004. Of these, 117 were exonerated, or 1.6 percent. But among these, 107 were exonerated while they were still on death row, whereas only 10 were exonerated after their sentence had been reduced to life in prison.
This leads to a bizarre situation. If you’re on death row and your sentence is reduced to life in prison, you’re suddenly much less likely to be exonerated than someone who stays on death row.
In other words, there's two classes of innocent people sentenced to death—those who stay on death row, who are likely to be exonerated, and those whose sentences are downgraded, who are less likely to be proven innocent, even if they are.
The team used a statistical technique called survival analysis to make up for this imbalance, Hughes explains. With all of the subtleties taken into account, their calculations indicated that, they write, "if all death-sentenced defendants remained under sentence of death indefinitely, at least 4.1% would be exonerated."