The provision in Amendment V that no person shall be compelled in any criminal case to be a witness against himself, which is found also in many, though not all, of the state constitutions, is an announcement of a general rule of evidence long recognized in common-law courts as applicable in civil as well as criminal cases. The object of this rule of evidence is to protect the witness against being compelled in any judicial proceeding to disclose facts which would tend to subject him to a criminal prosecution. Under the civil-law system as administered in France and some other European countries, one who is put on trial for a crime is subjected to an inquiry into his whole life and conduct, without regard to its relevancy to the particular crime with which he is charged; and in such countries physical torture was formerly permitted for the purpose of securing confessions of guilt. But such proceedings were not recognized as justifiable by the common law as it prevailed in England at the time that the American colonies became independent, and are not permitted in any of the states of the Union. Some of the rules resulting from the recognition of the principle that a witness cannot be required to give self-criminating testimony are the following: Admissions of guilt made outside of court cannot be proven against one accused of crime unless they are voluntarily made. The accused cannot be required to testify in a criminal case. In no case either civil or criminal can a witness be compelled to give testimony tending to show that he has been guilty of a crime, nor to produce books and papers having such tendency.

As to some crimes it is found so difficult to secure the evidence of persons not implicated that statutes have been passed in various states providing that as to certain classes of crimes persons implicated therein may be required to testify against others, with the provision that their testimony shall not afterwards be used against themselves in prosecutions for the same crime; but to these statutes it has been objected that they subject the witness to the ignominy of disclosing his criminal conduct, and to the danger that after his connection with the crime has been discovered his guilt thereof may be proven by other evidence to which his enforced disclosure has furnished a clue; and it is thought that such statutes do not adequately protect the person required to testify unless it is provided further that he shall not subsequently be held accountable in a criminal prosecution for any crime committed by him in any way connected with the transaction with reference to which he is compelled to testify (Brown v. Walker).