Court trials involve assessing the available facts and evidence to reach a decision regarding a case. It involves listening to a witness who can provide evidence. A witness is a person who has seen the event (related to a case that is to be decided by the court), and tells the court about when, where, and how the event took place.

Since the witness has to recall and tell the story from his memory, there can be invalidities in the information provided to the court. This is because human memory is not like a perfect video recording machine as well as the memory can be affected by several factors such as the emotional state of the individual and that how much time has passed after the event took place.

In such a case, the information provided by the witness is assessed by a psychologist, called the expert witness, who determines whether the provided information is true or false. The expert witness assesses the information based on various factors that affect memory, as explained below.

The psychologist, who assess the testimony of the witness, is called the expert witness. The report prepared by the expert witness, about the testimony of the witness, is called expert testimony.

The expert witness is a well-versed psychologist and has knowledge about cognitive processes involved in memory formation. He uses his psychological expertise and knowledge of factors affecting memory to arrive at a conclusion – expert testimony. The factors, that affect memory, are as follows.

There are three stages of memory and error can occur at any stage. 

There are three stages of memory formation: 1) encoding, 2) storage, and 3) retrieval. To briefly state, encoding is a process of converting external stimuli (received through eyes and ears) into a form that can be stored in the mind. Storage is the second stage in memory formation where the storable form of external stimuli gets stored in the mind. Retrieval simply means recalling back what is stored as memory.

Errors can occur at any of these stages. Hence, the factors with regard to each stage are discussed, as follows.

1. Attention 

Attention is the most important factor in memory formation. If a person does not pay attention to an object or event, no encoding may occur, and thus, no memory may be formed. In the case of a witness, it is important to know how attentively the witness had seen the event. It is the role of a forensic psychologist to ask the witness whether or not he paid attention to what he had seen. For instance, a man walking on a roadside sees a crime scene, he may see it by chance and pass by it without paying much attention to it. In the same scenario, another man may start noticing the crime scene giving proper attention to it. The more attentively a witness has seen the event, the more reliable will be his testimony.

2.  Exposure to the event

Another factor, that affects the encoding, is exposure to the event. It includes the duration of time for which the witness has seen or attended the event, as well, the visual clarity with which the event had been seen.

To understand this, let us take an example. Suppose a person named John kills a murderer who in fact had come to kill John. John did not intend to kill the murderer but as a self-defense attempt, he mistakenly shot the murderer. If a person sees this event from beginning to the end, he will be able to tell the whole story that in fact the murderer had come to kill John and was shot dead by John by mistake as in his self-defense attempt. However, if another person comes and sees this event exactly at the end of the event where John shoots the murderer. Since this person sees a portion of the event, he (as a witness) will tell the story different from reality which will make the murderer look like a completely innocent person. Moreover, since the first person sees the whole event, the observed facts will be relatively more entrenched in his memory as compared to the second person who sees only a portion of the event.

Another factor is visual clarity. The more clearly a person sees an event, the better it will be stored in his memory. Distance from the event and the amount of light in the place of the event determine visual clarity. For instance, one person sees a crime event from the 10th floor of a building whereas another person sees it from 1st floor of the same building. The person, who sees it from near (1st floor), will have a clearer picture of the event in his mind as well as this clear picture of the event will be retained in his memory for a longer time as compared to the person observing the event from 10th floor of the building. Similarly, if an event takes place in a street at nighttime but there are bright street lights, the witness will be able to retain clear face images of the involved persons and other information in his mind. In the darkness, relatively vague images of the event will be formed in the memory.

3. Salience 

The salience of the event to which the event was important to the witness. The more an event is important, the better it will be retained in the memory. The importance of the event for the witness can also be gauged how much the event was related to the witness. For instance, if the event involves a friend or a relative of the witness, the event is more important for the witness.

Salience is also about how much the event was noticeable. Generally, the unusual and extraordinary events are remembered more.

4. Nature of the event

The nature of the event can include many things. However, the important one is whether the event was more violent or less violent. Based on some prior research studies, violent crimes are generally less remembered (as in the form of the whole event) whereas nonviolent are better remembered (as in the form of whole even). The reason for this is that in violent crimes, the attention of the witness is majorly focused on the criminal rather than context. Crime events grab all the attention of the witness but in a form that is primarily converged at the criminal, and thus, the witness does remember some central information but remember little about things in the context.

5. Witness stress

Witness stress means the level of stress that the witness had undergone while he was seeing a crime event or after seeing the event. Stress affects the cognition of a person negatively. This stance is supported by some studies which reveal that stress affects memory formation negatively, however, some studies also claim the opposite.

In fact, extreme crimes (involving weapons) induce fear and stress but may also attract the attention of the witness. Due to this mixed state of mind (stress as well as attention), the witness may remember some of the central details while forgetting other important information. 

1. Rehearsing 

Rehearsing helps in storing information in the mind and intensifying the information as part of the memory. The more frequently a witness has discussed the event with others, the better the information would have become part of his memory. Another point is that with each time the witness has discussed the event with someone, he would have extra details of the event and would be able to present a more detailed picture of the event. Therefore, the psychologist needs to ask the witness if and how many times he has discussed the event with others such as family members and friends after seeing the event.

2. Time span after the event

The stored information (memory) in the mind fades away with time. If the event has occurred in the near past (as a day or week ago), the information is likely to be stored in its entirety in the memory of the witness. As more time passes, the stored information becomes relatively vague in the memory.

Retrieval is the process of recalling back the memory. This process operates under conscious control and is not much affected by external factors. However, one way to enhance retrieval is to take the witness to the place where the event had occurred. There are cues present in that places which will enhance the retrieval process.

Previously hypnosis was used to enhance retrieval but later on, it was seen that hypnosis caused further problems in testimony. Hence, the practice of hypnosis is now generally not used in court.