Why Do Witnesses Swear? The Oath, the Penalty and What Either Achieves

Requiring a witness to swear serves several purposes that are usually run together. It marks the moment at which ordinary speech becomes legally consequential, which is a genuine function since the same person may talk informally to investigators without penalty and then give the same account under a regime where a lie is a crime. It historically invoked divine punishment, which was the original point in a society where that was widely believed and which explains why the form survives with religious wording available. It creates a solemnity intended to affect the witness psychologically. And it establishes the factual basis for a later prosecution, since the offence of lying under oath requires proof that the oath was administered. Most jurisdictions now permit a non-religious affirmation with identical legal effect, which separates the ritual from the consequence.

Prosecutions for lying under oath are rarer than the amount of lying would suggest, for structural reasons:

• •The statement must be shown to be false, which requires proving the underlying fact
• •It must be shown that the witness knew it was false rather than being mistaken
• •The falsehood must generally concern something material to the proceedings
• •Memory is genuinely unreliable, which supplies an honest explanation for most discrepancies
• •Some systems historically required more than one witness to the falsehood
• •Prosecutors weigh whether pursuing it is worth the resources when the main case is over

The evidence on whether swearing changes behaviour is more interesting than the confidence on either side suggests. Experimental work on honesty finds that reminders of moral standards immediately before an opportunity to cheat reduce cheating, with signing a declaration before completing a form rather than after being one widely publicised finding, though that particular result has had serious replication problems and one of the underlying datasets was found to be fabricated, which is a caution worth stating plainly. The general principle that salience of honesty at the moment of reporting matters retains support from other work. What is clearer is that the penalty rather than the ritual does most of the work legally, and that the main protections against false evidence are cross-examination, corroboration requirements and the ability to test an account against documents rather than the promise itself.

Rules about who may give evidence at all have a long and revealing history. Parties to a case were barred from testifying in their own cause in English law until the middle of the nineteenth century, on the grounds that their interest made them untrustworthy, which meant the people who knew most about the dispute could not speak. Defendants in criminal cases could not give evidence on oath until 1898. People who did not believe in divine punishment were excluded, since the oath was thought to have no hold on them, which barred atheists until affirmation was permitted. Various groups were excluded or their evidence discounted by race, by religion or by conviction of a previous offence. Children were assessed for whether they understood the oath. Nearly all of these bars have been replaced by letting the evidence be given and leaving its weight to the court, which is the modern approach across most of the area.

The larger difficulty is that most inaccurate testimony is given by people telling the truth as they understand it. Memory is reconstructive rather than a recording, and research has shown repeatedly that confident, detailed, sincerely held recollections can be substantially wrong, that questioning can alter what a witness remembers, that identification across unfamiliar faces is unreliable and that confidence correlates poorly with accuracy once a witness has been through an investigative process. None of that is addressed by an oath, since the witness is not lying. Reforms addressing it work on procedure instead, including how identification parades are conducted, how interviews are structured to avoid suggestion, recording interviews in full, and instructing juries about the limits of such evidence. Those measures have done considerably more for the reliability of testimony than the law on perjury.

The oath marks the point where speech becomes legally consequential and establishes the basis for prosecuting a lie, with affirmation now available and identical in effect. Prosecutions are rare because falsity, knowledge and materiality must all be proved and honest error explains most discrepancies. Most inaccurate testimony comes from sincere witnesses, which procedure rather than the oath addresses.