The purpose of the Miranda warning is to protect against admitting coerced statements at trial. The US Supreme Court has held that “[i]t does not follow from Miranda that evidence inadmissible against an accused in the prosecution's case in chief is barred for all purposes.” See Harris v New York, 401 US 222, 224 (1971).
In the Harris case, the court held that unwarned confessions obtained in custodial interrogation were admissible to impeach a defendant's testimony. The absence of warnings, the Court held, did not taint the statement's use for impeachment provided that “the trustworthiness of the evidence satisfies legal standards.” See Harris, 401 US at 224);
Similarly, in New York v Quarles, 467 US 649 (1984), the Court made an exception to Miranda's general bar on the use of unwarned statements in the government's case in chief. The court in Quarles held that voluntary unwarned statements made by a defendant during custodial interrogation, but obtained in order to protect the public safety, were admissible in the trial against the defendant.
Neither the impeachment rule nor the public safety rule can be reconciled with the position that Miranda means that unwarned statements made in custodial interrogation are inherently “compelled” under the Fifth Amendment.
Consequently, you must be aware that statements that are the product of government compulsion are not admissible for all purposes in a criminal trial. However, as the Harris and Quarles cases point out, if you make voluntary statements made not in conformity with the Miranda procedures during custodial interrogation are under certain circumstances your statements may be admissible against you at trial.