Suspects in criminal cases across the U.S. can sometimes find themselves with a daunting choice even if they didn't do anything wrong: go to trial and face a potentially long prison sentence or plead guilty in return for lesser punishment.

According to an estimate by the National Registry of Exonerations at the University of Michigan Law School, more than 300 of about 1,900 people who have been exonerated in the U.S. since 1989 pleaded guilty.

Last year, 68 of 157 exonerations were cases in which the defendant pleaded guilty, more than any previous year.

Critics say the numbers reflect an overwhelmed criminal justice system. They say public defenders have more cases than they can handle and courts can save the government money with plea bargains compared with costly trials.