I think it is instinctive to confess to something we didn't do when repeatedly accused of it. I believe in evolutionary psychology and in the past giving in to an accusation was probably in a persons best interest. Since an accusation of something was generally an equivalent to a conviction, someone who gave in could express remorse and ask for leniency. Look at the Salem witch trials. People who confessed were treated much better than those who insisted that they were innocent. Give me a day in a room with your average meth addict and I bet I could get him to confess to any nearby unsolved murder through trickery and psychological intimidation.