Imperfect recall of a crime or other dramatic event
Eyewitness memory is a person's episodic memory for a crime or other witnessed dramatic event. Eyewitness testimony is often relied upon in the judicial system. It can also refer to an individual's memory for a face, where they are required to remember the face of their perpetrator, for example. However, the accuracy of eyewitness memories is sometimes questioned because there are many factors that can act during encoding and retrieval of the witnessed event which may adversely affect the creation and maintenance of the memory for the event. Experts have found evidence to suggest that eyewitness memory is fallible.
It has long been speculated that mistaken eyewitness identification plays a major role in the wrongful conviction of innocent individuals. A growing body of research now supports this speculation, indicating that mistaken eyewitness identification is responsible for more convictions of the innocent than all other factors combined. This may be due to the fact that details of unpleasant emotional events are recalled poorly compared to neutral events. States of high emotional arousal, which occur during a stressful or traumatic event, lead to less efficient memory processing. 
The Innocence Project determined that 75% of the 239 DNA exoneration cases had occurred due to inaccurate eyewitness testimony. It is important to inform the public about the flawed nature of eyewitness memory and the difficulties relating to its use in the criminal justice system so that eyewitness accounts are not viewed as the absolute truth.
 ;templatestyles src="Template:TOC limit/styles.css" / ;
Encoding.
During the event.
Challenges of identifying faces.
People struggle to identify faces in person or from photos, a difficulty arising from the encoding of faces. When participants were given a basic memory test from an array of photos or a lineup, they struggled to accurately identify the images and had low recognition. This finding provides a starting point for estimating the accuracy of eyewitnesses' identification of others involved in a traumatic event. It can only get more challenging for a person to accurately encode a face when they themselves are experiencing a traumatic event. Because courts rely on eyewitness facial recognition, it is important to acknowledge that identification is not always accurate. Face-specific cognitive and neural processes show contributions to holistic processing and recognition in the episodic memories of eyewitnesses. Unreliability of eyewitness identifications may be a result of mismatching between how faces are holistically processed and how composite systems retrieve features in faces during an event.
Other-race effect.
The other-race effect (i.e. the own-race bias, cross-race effect, other-ethnicity effect, same-race advantage) is one factor thought to impact the accuracy of facial recognition. Studies investigating this effect have shown that a person is better able to recognize faces that match their own race but are less reliable at identifying other more unfamiliar races, thus inhibiting encoding. Various explanations for this effect have been proposed. The perceptual expertise account suggests that with an increase of exposure to one's own race, perceptual mechanisms develop which allow people to be more proficient at remembering faces of their own race. The socio-cognitive account predicts that motivational and/or attentional components over focus on the race of a person. Another hypothesis is that each race pays attention to certain facial details to differentiate between faces. However, other races might not encode these same features. A final suggestion is that faces of the same race are encoded more deeply, leading a witness to have a more detailed memory for those faces; but there has not been much research to support this hypothesis. Research on the other race effect has mainly focused on the African American and Caucasian races. Most research has shown that white eyewitnesses exhibit the other-race effect, however this effect does extend to other races too. In general, memory is an individual process and that conceptualization of race causes racial ambiguity in facial recognition. Mono-racial eyewitnesses may depend on categorization more than multiracial eyewitnesses, who develop a more fluid concept of race. Perception may affect the immediate encoding of these unreliable notions due to prejudices, which can influence the speed of processing and classification of racially ambiguous targets. The ambiguity in eyewitness memory facial recognition can be attributed to the divergent strategies that are used when under the influence of racial bias. This phenomenon is not limited to race. Stereotypes of any kind (whether they be related to age, gender, etc.) can affect the encoding of information at the time of the event. For example, if one is held at gunpoint by two individuals, one of whom is a man and the other is a woman wearing a hat, the victim may quickly fall back on the belief that men are more likely to be aggressors. Consequently, the victim may encode the situation as involving two male assailants, yielding problematic effects in the process of identifying the assailants later on.
Stress and trauma.
Stress or trauma during an event can affect the encoding of the memory. Traumatic events may cause memory to be repressed out of conscious awareness. An inability to access the repressed memory is argued to occur in cases involving child sexual abuse. Another way encoding a memory can be affected is when the person involved in a traumatic event experiences dissociation; mentally removing themselves from the situation, which may serve as a coping mechanism. Lastly, trauma may induce a flashbulb effect; the witness believes they vividly remember significant details of a salient event, although accuracy must be determined of such memories . In legal settings the mental state of an individual at both witnessing a crime and in testimony can affect the success of their memory retrieval. Stress in small amounts is thought to aid memory, whereby stress hormones released by the amygdala promote the consolidation of emotional memories. Nevertheless, stress in high amounts may hinder memory performance. Witnesses of severe crimes or trauma can develop further complications, such as posttraumatic stress disorder (PTSD) or even psychogenic amnesia.
Post traumatic stress disorder.
Explicit memory (used in legal testimony) is affected by post traumatic stress disorder (PTSD); individuals diagnosed with PTSD can struggle to recall explicit events from their memory, usually those which are especially traumatic events. This may be due to the individual preferring not to think about the unpleasant memory, which they may rather forget. Implicit memory, on the other hand, does not seem to be affected in the same way that explicit memory does, rather some individuals with PTSD may score higher on implicit memory tests than non-PTSD individuals.
Psychogenic amnesia.
Psychogenic amnesia (or dissociative amnesia) can affect explicit memory for a particular event. Those with psychogenic amnesia present impaired memory functioning in their personal life histories without a discernable neurological cause. Most often cases of psychogenic amnesia occur after witnessing an extremely violent crime or trauma, such as war. This also goes for experiencing a crime, such as childhood sexual abuse. The dissociation after the event can lead to a higher chance of revictimisation. 
Mood-congruency effect.
Everyday memory can be affected by factors such as stress or mood. The 'mood congruency' effect refers to memory being aided by a matching of mood at the encoding/learning stage to the retrieval stage. If a memory is encoded under stressful conditions it may be more likely that the memory is better recalled if stress levels at retrieval are congruent to stress levels at encoding. Mood congruency may affect a witnesses ability to recall a highly stressful crime, if conditions of encoding and retrieval are different. Moderate amounts of stress may be beneficial to memory by the release of corticosteroids. Conversely, too much stress (and therefore an extreme influx of corticosteroids) can affect function of the hippocampus and therefore hinder memory. Very high levels of corticosteroid release may be very detrimental for memory.
Weapon focus.
The weapon focus effect suggests that the presence of a weapon narrows a person's attention, thus affects eyewitness memory. A person focuses on the central detail (for example, the weapon) and loses focus on the peripheral details thus resulting in worse perpetrator recall. While the weapon is remembered clearly, the memories of the other details of the scene suffer. The weapon focus effect occurs because additional items require more visual attention, therefore they are frequently not processed. This increased focus of attention on central aspects takes away attentional resources from peripheral details. For example, if a gun was brought into a school, it would attract significant amount of attention, because students are not used to seeing that item. When participants were watching a slideshow, and were seeing an unusual stimulus item, their reaction times were slower (regardless whether the stimulus was dangerous) in comparison to reaction times for more frequent stimulus. When the item was dangerous (i.e. a weapon), participants had a lower accuracy and confidence than the control group's. Another hypothesis is that seeing a weapon might cause an aroused state. In an aroused state, people focus on central details instead of peripheral ones.
Interference.
The testimony of a witness can lose validity due to too many external stimuli, that may affect what was witnessed during the crime, and therefore obstruct memory. For example, if an individual witnesses a car accident on a very public street, there may be too many cues distracting the witness from the main focus. Numerous interfering stimulus inputs may suppress the importance of the stimulus of focus, the accident. This can degrade the memory traces of the event, and diminish the representation of those memories. This is known as the cue-overload principle.
After the event.
Memory becomes susceptible to contamination when witnesses discuss the event with others and as time passes. This is because memory traces blend with other stories and events that the witness is exposed to after the stressful or traumatic event Because memory is subject to contamination, the most reliable test of a memory is the initial test. Police procedures can reduce the effects of contamination on memory with proper testing protocols.
Misinformation effect.
Witnesses can be subject to memory distortions that can alter their account of events. It is of particular interest that the memory of an eyewitness can become compromised by other information, such that an individual's memory becomes biased. This can increase eyewitnesses sensitivity to the misinformation effect. Individuals report what they believe to have witnessed at the time of the crime, even though this may be the result of a false memory. These effects can be a result of post-event information. It is very important to provide witnesses with helpful response options on memory tests and to be warned of misleading influences that might affect how the memory of the event is recalled at a later time. Many employees, police force workers, and others are trained in post-warning in order to reduce influences on the misinformation effect, which can be predicted before crime. In their studies, many researchers use eyewitnesses to study retrieval-blocking effects, which interfere with a witness' ability to recall information. Misleading information prior to the event can also influence misinformation effects. Other studies also address how the misinformation effect seems to amplify over increasing recall. Discussing events and being questioned multiple times may cause various versions of the testimonies. However, the earliest records prove to be the most accurate due to a minimized misinformation effect. In one study, repeated recall of a traumatic event did not increase resistance to misinformation, nor did it increase susceptibility to information. This result may be due participants being presented both forms of additive and contradictory misinformation.
Unconscious transference.
Many mistaken identifications are the result of unconscious transference, or the inability to distinguish between the perpetrator and another person who was encountered in a different context. In many of these cases, the culprit is confused with a different person who was present at the crime scene. Implicit processing takes place during the event, in which the witness encodes the general features of innocent bystanders, creating a sense of familiarity. At retrieval, this familiarity could cause people who were merely present in the crime scene to be confused with the culprit. After viewing a video of a crime involving a thief and two innocent bystanders, participants were asked to identify the perpetrator from a lineup including the three persons present in the video and three other people never before encountered. Most participants falsely identified an innocent person from the lineup. Furthermore, participants were more likely to misidentify one of the two innocent confederates in the