Exposure Time and Eyewitness Memory

When assessing the potential of an eyewitness, among the first things an investigator has to decide is whether or not the witness had an opportunity to observe what took place for a sufficient time. The decision is likely to be influenced by a witness’s assessment of the length and quality of exposure to a perpetrator’s face. A longer exposure time can increase the ease with which details come to mind at the time of remembering and increase the likelihood that witnesses will correctly recognize a face from an identification lineup and provide a more detailed description. However, an extended exposure could make the witness more confident in their identification ability even when they are wrong. It has been recommended that investigators should not rely too heavily on witness confidence as an indicator of accuracy.

There are two points to bear in mind when examining the relationship between eyewitness memory and exposure duration. First, eyewitnesses are not very good at making estimates of the duration of a given event, and witnesses may overestimate the length of exposure to a face. Second, a longer exposure to a face can make a witness more confident in their ability to make an identification, although there are numerous other factors that could inflate (or deflate) a witness’s confidence.

When witnesses are asked whether or not they could identify someone seen earlier, they will rely on various sources of information when making a judgment about the strength of their memory. One source of information that could influence their decision is “availability” or the ease with which information can be brought to mind. A longer exposure is associated with an increase in availability, and this can have interesting consequences for the accuracy of an eyewitness’s identification.

Don Read was the first to examine the use of the availability heuristic in an eyewitness identification setting. He found that participants who interacted with store clerks for a longer duration (4-15 minutes as compared with 30-60 seconds) made a higher number of correct choices from lineups in which the culprit was present. However, it was found that the false identification rate in the target-absent lineups were inflated if the store clerks received additional information (cues) about the target. The latter finding fits with the hypothesis that availability of additional cues can sometimes lead a witness to believe that they have a stronger memory for the target, and in a target-absent lineup this can have potentially serious consequences.

The question of why an increase in exposure would lead witnesses to overestimate their ability to make an accurate identification from a lineup was explored by researchers at Aberdeen University using “mock” eyewitnesses (aged 17 to 81 years). The witnesses individually viewed a video reconstruction of a robbery at a savings bank. No weapons were seen in the video, although the culprit indicated to the clerk that he had a gun. The critical aspect of this video for the purposes of the study was the length of exposure to the culprit’s face in the video. Two versions of the video were created. In one version, the culprit’s face (full-frontal and profile view) was visible for 45 seconds, and in another version, the culprit’s face (full-frontal and profile view) was visible for 12 seconds. No other details differed, and the videos were of the same duration (1 minute 40 seconds). About 35 to 40 minutes after witnessing the robbery, witnesses in the long-exposure group made more correct identifications of the robber when he was present in the lineup. They also provided more correct descriptions of the robber under the long-exposure condition. A longer exposure did not appear to inflate false identifications when the culprit was absent from the lineup in the Aberdeen study.

One additional finding from the comparison of witnesses exposed to a target for a shorter or longer duration in the Aberdeen study could be of use to investigators. Witnesses in the long-exposure condition were more confident in their identification decisions than were witnesses in the short-exposure condition. However, they were confident even when they were inaccurate. In other words, confidence was not a reliable indicator of accuracy under long exposure. This effect was most marked in the culprit-absent conditions. This finding becomes more meaningful when the implications for assessing witness credibility are examined. When deciding whether or not a given witness is likely to be reliable, a police officer or a juror may rely on that witness’s verbal expression of their confidence. To summarize, the research suggests that the likelihood of a witness making an accurate identification is increased if he or she has seen the perpetrator’s face for a longer period of time. However, an extended exposure could make witnesses more confident in their identification ability even when they are wrong. Therefore, while a longer exposure increases the chances of an accurate identification, investigators should not rely too heavily on witness confidence as an indicator of accuracy.

So far it has been proposed that the extent of time of exposure to a face could be useful information when assessing the potential of an eyewitness to aid an investigation and the administration of justice. One of the limitations of prior research on exposure duration is that it has been assumed that a witness who is exposed to a perpetrator for a longer time will be paying more attention to the face and processing it more “deeply,” thereby providing a stronger and more accessible memory trace. However, this assumes that there is nothing else at the scene of the crime to attract one’s attention. This is typically not the case. For example, research has shown that when a perpetrator is holding a weapon, a witness’s attention may be drawn to that, and consequently, the witness may spend less time looking at the face (referred to as the weapon-focus effect). It is important in future research to identify various situational factors that alter the relationship between degree of exposure and memory for an event.

References:

  1. Memon, A., Hope, L., & Bull, R. H. C. (2003). Exposure duration: Effects on eyewitness accuracy and confidence. British Journal of Psychology, 94, 339-354.
  2. Pedersen, A., & Wright, A. (2002). Do differences in event descriptions cause differences in duration estimates? Applied Cognitive Psychology, 16, 769-783.
  3. Read, J. D. (1995). The availability heuristic in person identification: The sometimes misleading consequences of enhanced contextual information. Applied Cognitive Psychology, 9, 91-121.

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