The circumstances under which Miranda applies are clear: If a suspect makes a statement during "custodial interrogation," it can only be used against him if he was first "read his rights" and waived them [source: FindLaw]. The court's reasoning for the distinction between custodial and noncustodial questioning, as stated in the 1966 ruling, is that "the process of in-custody interrogation ... contains inherently compelling pressures which work to undermine the individual's will to resist and to compel him to speak" [source: U.S. Courts].
The Miranda requirement establishes a way to prove, to a legal standard, that statements made in custody were made voluntarily. The problem is, "in custody" can be slippery. The Miranda decision defines it as the denial of complete "freedom of action," a criterion open to interpretation [source: U.S. Courts].
Sometimes, "in custody" is obvious. Under arrest? Sitting in a locked police car? Wearing handcuffs? In custody. Under any of those conditions, Miranda rights must be read before police start asking questions if they hope to have any of the answers admitted as evidence at trial.
Sometimes it's fuzzy, though. What if police arrive at a subject's home to ask him about a crime that happened down the block, and the subject doesn't feel free to leave (or ask the police to do so)?
Over the years, court decisions have helped to clarify some of the uncertainty. In the preceding example, the subject likely would not be considered in custody, since he wasn't under arrest and therefore could have told the officers to leave at any time; the perception of the subject is not a factor. Traffic stops, too, are noncustodial: If a routine traffic stop yields an un-elicited statement not preceded by Miranda warnings, it may be used in court [source: National Paralegal College].
Let's say, though, a suspect is clearly in custody. Police have arrested a woman for selling drugs, and they question her. She confesses. She wasn't read her rights. What happens?
She's not released. But the prosecutor can't use the confession as evidence at the woman's trial. And if that confession led directly to any other incriminating evidence, and that evidence would likely not have been uncovered by police without information gleaned from the confession, that evidence is out, too [source: NOLO]. It's "fruit of the poisonous tree."
There only one exception to this, and it was invoked under serious public scrutiny in 2013.