The principal rationale advanced for the exclusionary rule is the deterrence theory, that the rule compels respect for constitutional rights and thus deters police misconduct. However, courts increasingly have found that societal interest in presenting probative evidence to the trier-of-fact far outweighs interests in deterring illegal police behavior. The Supreme Court in Walder v. United States created the impeachment exception, allowing the prosecution to use physical evidence, inadmissible to establish the guilt of the accused, for impeachment purposes. Federal and State courts have generally applied one of four tests to determine criminal responsibility when the accused's sanity is at issue: the narrow M'Naghten rule requiring total impairment, the irresistible impulse test, the Durham v. United States rule that a defendant lacks criminal responsibility when his or her act was the product of mental disease or defect, and the broad American Law Institute standard which considers the individual's entire mental condition. When a defendant raises the insanity defense, allocation of the burden of proof becomes crucial because it determines in part whether insanity negates mens rea -- an essential element of almost every crime. Courts have ruled that using compelled testimony to determine sanity does not create problems of self-incrimination, implicitly showing that insanity and mens rea can coexist. Invalid applications of the insanity-rebuttal exception would include circumstances where sanity was an element of the crime that the prosecution must prove beyond a reasonable doubt and in States that adopt the guilty but insane verdict. No constitutional violations would arise under a bifurcated system which separated the adjudication of guilt and insanity. The paper provides 234 footnotes.