In New Zealand, as a consequence of inconsistencies between the parties' factual chronologies, the court may direct that evidence is to be given orally. However, as noted in a recent decision in Gestmin SGPS SA v Credit Suisse (UK) Ltd  EWHC 3560 (Comm), an obvious difficulty with oral evidence based on recollection of historical events is the unreliability of the human memory.
As with written evidence, in the case of inconsistent oral testimony, the court will have to assess the credibility of the witnesses and determine which evidence it prefers. In these circumstances, it is not uncommon to assume that the stronger and more vivid a person's feeling or experience of recollection, or the more confident that person is in their recollection, the more likely the recollection is to be accurate.
Underlying such assumption is the misconceived belief that the memory is a mental record that is fixed at the time of an event and then fades slowly over time. Instead, as the Court noted, memories are changeable, can be rewritten when they are retrieved, and external information can intrude into a witness's memory, as can his own thoughts and beliefs, and cause dramatic changes in the recollection.
The judgment discusses the impact of litigation on a witness's memory, particularly when the witness is a party to the proceeding or has a tie of loyalty to a party, such as an employment relationship. In such circumstances, "a desire to assist, or at least not prejudice, the party who has called the witness… as well as a natural desire to give a good impression in a public forum, can be significant motivating forces."
The process of preparing for litigation itself may considerably interfere with a witness's memory. In particular, a witness's recollection of an event may be altered by the length of time between the event and the preparation of the witness's statement, and the involvement of lawyers in the preparation of such statements and their awareness of the significance of what the witnesses does or does not say. Further, multiple iterations before the witness statement is finalised, together with numerous reviews of supporting documentation, may cause the witness's memory of events to be based increasingly on this documentation and later interpretations of it, rather than on original experience of the events.
While suggesting that judges should place little reliance on witness's recollections and instead base factual findings on inferences drawn from the documentary evidence and known and probable facts, this judgment also acknowledges the utility of oral evidence. The value of oral testimony lies largely in the opportunity to subject documentary records to critical scrutiny, and to gauge the personality, motivations and working practices of a witness by way of cross-examination. Above all "it is important to avoid the fallacy of supposing that, because a witness has confidence in his or her recollection and is honest, evidence based on that recollection provides any reliable guide to the truth."