Under the common law, a party to a court case was disqualified from testifying at trial. The rule was based on the belief that the testimony of an “interested” witness would be self-serving and false. Because a criminal defendant was interested in the outcome of his trial, he was not competent under the common law to testify in his own behalf ―that is, he could not call himself to testify as a witness and the defendant could not call others accused of the same crime.
In 1865 Maine was the only state which allowed a prisoner to testify in his own behalf in a trial for murder. Even Federal Courts and tribunals followed the common law; e.g. the Lincoln assassination defendants were not allowed to testify in their own criminal prosecution, nor could they call any other defendant to testify on their behalf.
While that may seem like ancient history to you, consider that in 1967, two Texas statutes provided at that persons charged or convicted as co-participants in the same crime could not testify for one another, although there was no bar to their testifying for the State. In Washington v. Texas, 388 U.S. 14, 87 S. Ct. 1920, 18 L. Ed. 2d 1019 (1967), those statutes were stricken as a violation of the Sixth Amendment. The U.S. Supreme Court wrote that “the state arbitrarily denied [the defendant] the right to put on the stand a witness who was physically present and mentally capable of testifying to events that he had personally observed and whose testimony was relevant and material to the defense."
In 1865 Maine was the only state which allowed a prisoner to testify in his own behalf in a trial for murder. Even Federal Courts and tribunals followed the common law; e.g. the Lincoln assassination defendants were not allowed to testify in their own criminal prosecution, nor could they call any other defendant to testify on their behalf.
While that may seem like ancient history to you, consider that in 1967, two Texas statutes provided at that persons charged or convicted as co-participants in the same crime could not testify for one another, although there was no bar to their testifying for the State. In Washington v. Texas, 388 U.S. 14, 87 S. Ct. 1920, 18 L. Ed. 2d 1019 (1967), those statutes were stricken as a violation of the Sixth Amendment. The U.S. Supreme Court wrote that “the state arbitrarily denied [the defendant] the right to put on the stand a witness who was physically present and mentally capable of testifying to events that he had personally observed and whose testimony was relevant and material to the defense."