A general criminal law principle known as the corpus delicti rule provides that a confession, standing alone, isn't enough for a conviction. With its design of preventing wrongful convictions, the rule implicitly acknowledges the phenomenon of false confessions.
"Corpus delicti" translates to "body of the crime." The phrase refers to the requirement that there be some kind of evidence—apart from the defendant's statements—that establishes that someone committed a crime. In some states, the prosecution can't even present evidence of the defendant's confession (for example, by playing a recording of it) without this kind of corroboration.
Though the corpus delicti rule sounds like a significant protection for criminal defendants, it's relatively easy to satisfy. In general, any evidence that someone committed the crime in question will be enough—the evidence doesn't have to show that the defendant was the one to commit it. And in many places, the corroborating evidence needs only to slightly suggest that the crime was committed.
Some jurisdictions don't follow the corpus delicti rule exactly. Instead, their courts tend to focus not on whether corroborating evidence shows that the crime occurred, but on whether the confession was trustworthy or reliable.
Example: In a state that follows the corpus delicti rule, Bubbles walks into the police station and asks to give a statement. He says that he just robbed someone of money at gunpoint. But Bubbles can't identify whom he robbed or say where the gun is. Nor does he have any money in his possession. The police have nothing to follow up on—they can't come up with any evidence apart from Bubbles's confession indicating that a robbery took place. As a result, Bubbles can't be convicted of robbery.
Example: In a 1987 federal case, a man named Kerley was convicted of failing to register in the armed forces. Kerley had a duty to register in 1980. Instead of registering, he sent several letters to the Selective Service System announcing his refusal to register and opposition to military action. Kerley argued on appeal that his conviction was due solely to the uncorroborated admissions in his letters. The appeals court upheld the conviction, saying that there was enough independent evidence to establish that his admissions were trustworthy. It pointed to the fact that Kerley "proudly volunteered" what amounted to his confession. The court also noted that the Selective Service had twice searched its files and failed to find any proof that Kerley had registered. (United States v. Kerley, 838 F.2d 932, 940 (7th Cir. 1988).)