...How many people are convicted of crimes they did not commit? Last year, a study I co-authored on the issue was published in the Proceedings of the National Academy of Sciences. It shows that 4.1 percent of defendants who are sentenced to death in the United States are later shown to be innocent: 1 in 25.
Death sentences are uniquely well-documented. We don’t know nearly enough about other kinds of criminal cases to estimate the rate of wrongful convictions for those. The rate could be lower than for capital murders, or it could be higher. Of course, in a country with millions of criminal convictions a year and more than 2 million people behind bars, even 1 percent amounts to tens of thousands of tragic errors.