At common law an interested party in a court proceeding was not a competent witness in his own behalf. The basis for disqualifying parties was a notion that the temptation to falsify was too great for the average witness to resist. Any party, therefore, who had any interest whatsoever in the case could not testify. A riper wisdom and experience, however, brought to light the fact that it was better not to close the mouths of interested parties in court proceedings, and statutes were subsequently passed in practically all the English-speaking jurisdictions that would allow an interested party to be a competent witness, and consequently a defendant in a criminal case could testify and be a witness at his own trial.
Pack, S. Dan
"Defendant as a Witness in a Criminal Proceeding,"
Mercer Law Review: Vol. 3:
2, Article 9.
Available at: https://digitalcommons.law.mercer.edu/jour_mlr/vol3/iss2/9