Postevent Information and Eyewitness Memory

Human memory, however accurate generally, is not a perfect processing system. Over time, our memory becomes less accurate, primarily for two reasons. First, our memory is not permanent, and information fades from memory over time. Most people are familiar from experience with this unfortunate feature of memory but are less familiar with the second factor that influences the accuracy of memory—memory can be distorted by the influence of postevent information. Although memory can be influenced by subsequent experiences, there are constraints on the conditions under which this is likely to occur. Nonetheless, when memory accuracy is a premium, such as in forensic situations involving eyewitness memory, it is important to recognize that eyewitness memory can be suggestively influenced. In these situations, the impact of postevent information should be minimized by avoiding misleading questions, and when it is relevant to do so, jurors should be informed about the potential fallacies in eyewitness memory that can result from a suggestive interview.

[Page 608]How can postevent information influence memory? Take the example of eyewitnesses who observe a convenience store robbery. From their observations, they construct a memory for what transpired during the robbery. Most of the time, it is this memory that police officers want to examine. However, virtually all eyewitnesses to crimes who eventually testify in court are interviewed by police officers at least once and typically multiple times. In police interviews, the eyewitness is questioned about what happened, and if the investigating officer has specific suspicions about what occurred, the interview may include some leading questions (e.g., “Was it a white four-door sedan?” “Was he or she wearing athletic clothing?” “Was he or she carrying anything?” “May he or she have had a gun in his or her hand that was in his or her jacket pocket?”). Questioning such as this presents one source of postevent information. Another source of postevent information is self-generation; that is, the eyewitness may introduce new information by just thinking about or talking about the robbery. Either way, the postevent information affects one's memory of the original observed event, and over time, individuals become less able to differentiate between the information that is in their memory because it was actually observed and the information that was introduced after the event by postevent information.

Most of the time, the influence of postevent information is minimal and inconsequential. However, in the case of an eyewitness to a crime, when it is important to know exactly what transpired, postevent information may be an important source of memory error. For this reason, the distortion in memory that results from postevent information is often referred to as the misinformation effect.