The primary function of most witnesses is to give their recollection of events. Different people honestly perceive the same event differently. They are subject to conscious and subconscious influences. Powers of observation and recollection are affected by past events, prejudices, habits, our imagination, suggestion, state of mind, health, and the effects of alcohol or drugs at the crucial moment of the incident. If the events happen quickly and unexpectedly, or if the witness was frightened or surprised at the crucial moment, then the ability to record the event accurately may be even more negatively affected.

Other factors affecting the trustworthiness of testimony could be: the age or the intelligence of the witness; whether the witness has impaired eyesight or hearing; whether the witness was thinking of something else, such as business or family worries at the time of the event; whether the witness was interested in what was happening; whether there were any distracting sounds such as a loud radio; wakefulness; and how good the opportunity was to observe, taking into consideration such matters as distance, lighting, atmospheric conditions and obstructions.

Sounds can play tricks on the mind. Regular sounds can often assume to be heard rather than actually heard.

The opportunity to observe is not in itself enough. Interest and attention are preconditions of accurate observations. Every day, newer things are said and done in our presence which we hardly notice. Evidence given by a witness about matters in which there was no interest at the time is likely to be vague: positive statements by such a witness are likely too unreliable, as they may have been built up after the event by interference, influence, or imagination. For example, often in an ordinary road accident, bystanders are unlikely to have seen much until their attention has been attracted by a loud crash.

Where a person reports a casual conversation which was heard some months before, and which may have become important only because of a dispute, it is improbable that the person would notice and remember the exact words which were used.

There are other limitations to accuracy: if events happen quickly or there is a great deal of talking going on within a short space of time, an impartial witness will see or hear only a fraction of what is happening. Surprise, excitement and rush will make the picture confused and its detail obscured. If a witness has a personal interest or a bias, his attention will tend to be concentrated on facts or remarks that are favourable to him, to the exclusion of others. Such a witness is referring, of course, not to a deliberate choice, but to one which takes place subconsciously. It is important to realize that bias can intervene without any dishonest intention, especially as the observation ages and before memory or imagination have started to work.

A further factor which may interfere with accuracy is the presence of intense pain or shock, or strong emotion: all these things may prevent the senses from operating in a natural way, and they produce sounds or pictures which are distorted or totally imagined. A person who easily hears imaginary voices may be quite convinced that they are real. The reason is that an abnormal physical state exists when there is not enough interest in exterior happenings and the imagination takes the place of exterior senses. Strong emotion may have the same effect as shock or pain, though the effect is not so obvious. Any condition which concentrates the attention on one's own interior feelings, mental or physical, is open to suspicion, because it draws attention away from the outside world and gives scope to the imagination.

A line of questioning will open up for the cross‑examiner if the witness did not give a statement shortly after the event. This statement is used by the witness in order to refresh his or her memory before testifying. If the witness is asked to give a statement or recollect an event for the first time, perhaps weeks or months after the event, the questioner might ask the witness in the name of fairness how he could remember the event and that he may not be reporting accurately or completely what he is trying to remember so long after the event. Counsel may wish to inquire in a subtle way whether there is any reason for the witness to recall the event in question so that the details would be impressed in his memory. Was the witness interested or attentive at the time of the event so that the incident would be more accurately imprinted in their mind? An unusual event would be more readily remembered than a common place, trivial or an unimpressive event. If the witness is prepared to admit that the matter was relatively insignificant at the time and that there is no particular reason to remember, it may then cast doubt on the accuracy of the observations.

Often, imagination can take over where there is vagueness in recollection and provide details that did not exist. Witnesses may supply memory details through their imagination to compensate for undetected vagueness. The attempt to see faces in the moon is analogous to this experience. With a time outline, there is always a strong tendency to fill in the outline with what is in one’s mind rather than a true recollection. There are several factors which tend to promote this natural tendency for the imagination to supplement the memory.

The first of these factors is a sudden happening, just after the events in question, which focus the attention of the observer, whose attention was previously roaming at large in a superficial manner. An example of such a happening is the crash in a motor vehicle collision. On these occasions, the mind of the observer flashes back to the preceding time period and commences to reconstruct events from memory together with imagination. Afterwards, the vague impression received at the time of the event would be blended in with the reconstruction, and both will be remembered together as a single vivid picture of what was seen at the time.

The second factor is the effect on memory when witnesses talk among themselves and think about the dramatic incident. Each time the picture may be filled in a little, or something may be omitted, and when it is again restored, the witness’s memory will be modified.

A third factor is the effect of local rumours and discussions which the witness unconsciously absorbs and which form a prejudicial background to the evidence.

The last factor is the effect of suggestive questions and it is for this reason that an impartial investigator is careful to avoid suggestive questions in the early stages of an inquiry.  The technique of insinuation, which is inherently suggestive, is brought to bear only when the evidence has been crystallized in a one‑sided form.

For an interesting paper on asking questions of child witnesses please visit the Social Science Research Network Electronic Library or click here for the html version.