Fat Doctor asked a question which is analogous to a pop fly to my infield. I'm calling for the ball. The question was: Why do people confess to crimes they didn't commit?
Here's an even better question: Why do people confess to crimes at all?
First, let's cover a little history regarding the making of confessions. In the Middle Ages the law was divided into secular and ecclesiastical law. Church law or ecclesiastical law placed a heavy emphasis on confession as a means of absolution and to restore the offender into church society. In the Twelfth Century it became a requirement of the Roman Catholic church. Later in the Fifteenth Century, with the onset of the Spanish Inquisition, confession became a means of enforcing dogma and to punish deviation from church leadership. (Altogether now all you Monty Python fans: "No one expects the Spanish Inquisition!") On the secular side of things, in 1215 the Magna Carta established the beginnings of the modern judicial system with a requirement for trial by one's peers and the use of judges. In modern times this eventually morphed into modern criminal procedure which included a defendant's right to enter a plea and protection against self-incrimination. Miranda warnings came about as a way of preventing self-incrimination.
(Trust me, this history becomes relevant eventually.)
Now, a little overview of interrogation techniques.
When judicial procedure was first adopted in the Middle Ages the defendant was required to enter a plea to a criminal charge. The plea had to be entered before any prosecution could take place. Not being totally stupid, the criminals did the most logical thing to avoid punishment: they would stay silent and refuse to speak a plea. The courts dealt with this by putting the defendant on the ground and laying stones on his chest until he either entered a plea or died. This is the origin of the modern colloquialism "to press for an answer". There were other interrogation techniques which are best left to the imagination.
In Colonial America the Bill of Rights forbid such things in the Eighth Constitutional Amendment against cruel and unusual punishment. (Note that not everyone was in favor of the Eighth Amendment. When it was being debated the senator from Virginia declared that "brigands deserve to be whipped. Or perhaps to have their ears cut off.") So physical cruelty was out when it came to extracting information from defendants. In modern times police are legally allowed to use just about any means short of physical abuse to interrogate suspects and extract information. Interrogation has become a science in and of itself, to the extent that an Amazon.com book search on the term 'police interrogation' will net you over two thousand results. Eesh. (Interestingly, a search on the term 'civil rights' will net you over 69,000 results. The balance is still in the right direction.)
The best practical man-on-the-street description of a police interrogation I've ever read was in the book Homicide by David Simon, based on actual observed homicide interrogations. This is how he describes the role of the police investigator:
"He becomes a salesman, a huckster as thieving and silver-tongued as any man who ever moved used cars or aluminum siding---more so, in fact, when you consider that he's selling long prison terms to customers who have no genuine need for the product."
(That man can write. You really have to read this book. He's now a writer for the HBO series The Wire, based on the book of the same name written with former homicide detective Ed Burns.)
Investigators, like psychiatrists, learn to establish rapport. They learn to conduct and control an interview. They learn how to collect information and then present it back to the defendant in a way that elicits more information. And they learn tricks---how to lie to a defendant to convince him that it's in his best interests to cooperate, or that his co-defendant has already 'turned' and is ratting him out, or that by confessing others will be protected from full prosecution.
While the investigators are trained and skilled, the defendants are...well...not. You know all those 'dumb criminal' stories you read or hear about in the news? Defendants really are like that. On average, they're not too bright. They use bad judgement. They may be juveniles or mentally ill or in substance withdrawal at the time of the interrogation. These impairments can lower resistance to interrogation. They might have personality traits that make them susceptible to certain interrogation techniques: someone with a need for admiration will be interrogated by using flattery; a dependent person with a desire to please will want the detective to be happy with him. Antisocial folks may enjoy bragging about their crimes---like the serial killer who confesses to a few extra murders just to raise his personal body count. Finally, some criminals really do experience remorse. Confession, in the true historical and religious sense of the term, is a relief to these folks. (See? The history part was relevant.)
So this is why people confess. Phew, I've typed myself out.
Now, for those of you who feel compelled to act on what I've presented here I am providing a link for your own personal online confession site. Trust me, it will go easier for you if you tell everything.