However much Americans may disagree about the morality of capital punishment, no one wants to see an innocent person executed.

And yet, far too often, people end up on death row after being convicted of horrific crimes they did not commit. The lucky ones are exonerated while they are still alive — a macabre club that has grown to include 152 members since 1973.

The rest remain locked up for life in closet-size cells. Some die there of natural causes; in at least two documented cases, inmates who were almost certainly innocent were put to death.

How many more innocent people have met the same fate, or are awaiting it? That may never be known. But over the past 42 years, someone on death row has been exonerated, on average, every three months. According to one study, at least 4 percent of all death-row inmates in the United States have been wrongfully convicted. That is far more than often enough to conclude that the death penalty — besides being cruel, immoral, and ineffective at reducing crime — is so riddled with error that no civilized nation should tolerate its use.