IT IS HUMAN nature to feel a sense of urgency about figuring out exactly what happened and punishing someone when a violent crime has been committed against an innocent person. The founders of the United States government knew that when they built protections for ordinary citizens into the Constitution. Anyone accused of a crime, for instance, is guaranteed the right to an attorney—one who specializes in defending the accused. The defense attorney does not need to prove innocence. The defense attorney's job is solely to place a reasonable doubt in the minds of the jurors that prevents them from finding a suspect guilty. But sometimes, even with guaranteed rights and Constitutional safeguards, innocent people are still convicted and sent to prison—even sentenced to die.

When DNA appeared on the criminal justice scene, it gave defense attorneys a new...