The Compulsory Process Clause had very little history in Supreme Court adjudication before it was incorporated against the states by the 14th Amendment’s Due Process Clause in Washington v. Texas (1967). This case struck down a Texas statute that barred anyone who was implicated in the same crime as the defendant from being called as a witness for the defendant (although prosecutors could call them as witnesses). The Court held that the 6th Amendment “right to offer the testimony of witnesses, and to compel their attendance, if necessary, is in plain terms the right to present a defense, the right to present the defendant’s version of the facts as well as the prosecution’s to the jury so it may decide where the truth lies.” Texas had argued that its statute was necessary because witnesses who were implicated in the same crime as the accused were untrustworthy. The Court decided that it was a violation of the 6th Amendment for any state to enforce “arbitrary rules that prevent whole categories of defense witnesses from testifying.”
Certain exceptions to compulsory testimony have always been recognized. No witness can be forced to give testimony that incriminates himself, for instance, and usually a person is given immunity from testifying against his or her spouse. Certain professions, such as priests and doctors, can claim a confidentiality privilege, but this privilege is not extended to all professions. In one early Pennsylvania case, Holmes v. Comegys (1789), a commercial agent tried to argue that it would be a breach of confidence to testify against his client. The judge responded that “we are too fond of getting at the truth, to permit him to excuse himself from declaring it” on such flimsy grounds.