What constitutes a promise of leniency that destroys the voluntariness of a subsequent confession?

Questions Presented Larry Hayes gave detectives a false confession, which the state later used to help convict him at trial. His case presents two unresolved questions regarding the parameters of permissible police conduct during a custodial interrogation: First, what constitutes a promise of leniency that destroys the voluntariness of a subsequent confession? Second, given what we now know about the pressure of custodial interrogations and the rate at which false confessions occur during them, should a Miranda waiver be a “strong indicator” of a subsequent confession’s voluntariness? i