exactly alleged cell mate confessions allowed though for the life of me i cant see why they are how anyone on a jury can be stupid enough to believe them.
A confession is a statement against interest. Statements against interest are excluded from the hearsay rule. All confessions are admissible not just jailhouse confessions. How credible the claim is depends on how detailed the claim is and what the person has to gain from lying. Most of the time they say they confessed because they wanted to appear tough so the prisoners would leave them alone. It is up to the trier of fact to decide whether the person did in fact confess and the significance to attribute to such a confession.
Most of the time the details matter the most. if the detials match the evidence in the case that is very bad. If the details don't match that can be indicative of the person having falsely confessed for some reason and in some instances the confession being made up.