In an adversarial system, the Judge will attach a lot of importance to the evidence given by a witness, both in writing and in person. The basic rule is ‘he who asserts, must prove’ and the standard of proof in family cases is the ordinary civil standard i.e. ‘on the balance of probabilities’.
It is often said that the Judge who sees and hears the witness give evidence has a significant advantage in being able to assess their credibility ‘on the spot’. Mostyn J commented in a speech at Bristol University in December 2014 about the craft of judging and the fallibility of memory (see paragraph 5):
If I were to ask you what was the key factor in finding facts in a trial you might reply “credibility”. Who does the judge believe? The primacy of the factor of credibility has an iconic, almost canonical, status. Thus Posner writes at page 123: “No legal catchphrase is more often repeated than that determinations by a trial judge whether to believe or disbelieve a witness can be overturned on appeal in only extraordinary circumstances. The reason is said to be the inestimable value, in assessing credibility, of seeing and hearing the witness rather than reading a transcript of his testimony, since the transcript eliminates clues to veracity that are supplied by tone of voice, hesitation, body language, and other non-verbal expressions.”
However, given what is widely understood about the fallibility of memory, Mostyn J and many other judges and commentators have cast considerable doubt on this view that assessing the credibility of witness evidence in court is really going to help the Judge a great deal in determining where the truth can be found. Memories are notoriously susceptible to subsequent alteration by unconscious bias or ‘wishful thinking’. Our memories of what happened can change over time and harden into something very different from what actually happened.
The case of Excelerate Technology v Cumberbatch  provides some useful discussion about how Judges assess credibility. ‘Credibility’ is not the same as ‘demeanour’ which is concerned with whether or not a witness appears to be telling the truth.
Credibility deals with the following issues:
- is the witness a truthful or untruthful person?
- If truthful, is he telling something less than the truth on this issue
- if untruthful is he telling the truth on this issue? Not all liars lie all the time and motivations for lying can vary; see the Lucas direction.
- If truthful and telling the truth as he sees it, can his memory be relied upon?
- Is what is asserted so improbable that it is on balance more likely than not he was mistaken in his recollection?
The court considered the dissenting speech of Lord Pearce in the House of Lords in Onassis v Vergottis  2 Lloyds Rep 403 at p 431.
Witnesses, especially those who are emotional, who think that they are morally in the right, tend very easily and unconsciously to conjure up a legal right that did not exist. It is a truism, often used in accident cases, that with every day that passes the memory becomes fainter and the imagination becomes more active. For that reason a witness, however honest, rarely persuades a Judge that his present recollection is preferable to that which was taken down in writing immediately after the accident occurred.
The importance of written contemporaneous evidence
Mostyn J referred approvingly in his speech of December 2014 to comments in the case of Gestmin SGPS SA v Credit Suisse (UK) Ltd & Anor  EWHC 3560 (Comm) (15 November 2013).
In the light of these considerations, the best approach for a judge to adopt in the trial of a commercial case is, in my view, to place little if any reliance at all on witnesses’ recollections of what was said in meetings and conversations, and to base factual findings on inferences drawn from the documentary evidence and known or probable facts. This does not mean that oral testimony serves no useful purpose – though its utility is often disproportionate to its length. But its value lies largely, as I see it, in the opportunity which cross-examination affords to subject the documentary record to critical scrutiny and to gauge the personality, motivations and working practices of a witness, rather than in testimony of what the witness recalls of particular conversations and events. 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.
Test witness evidence against objective and independent facts
The best way of testing witness credibility is likely to be to test their veracity by reference to objective facts which are independent of their testimony. This was considered by Lord Goff in Armagas Ltd v. Mundogas S.A. (The Ocean Frost),  1 Lloyd’s Rep. 1, p. 57:
Speaking from my own experience, I have found it essential in cases of fraud, when considering the credibility of witnesses, always to test their veracity by reference to the objective facts proved independently of their testimony, in particular by reference to the documents in the case, and also to pay particular regard to their motives and to the overall probabilities. It is frequently very difficult to tell whether a witness is telling the truth or not; and where there is a conflict of evidence such as there was in the present case, reference to the objective facts and documents, to the witnesses’ motives, and to the overall probabilities, can be of very great assistance to a Judge in ascertaining the truth.
We are almost certainly going to see an increase in both audio and visual recordings of interactions between parents and professionals, given the increasing availability of cheap and reliable recording equipment. It may be that we will soon see reliance being placed first and foremost on such recordings; rather than the inherent fallibility of recalling memories.