Cognitive psychologists have conducted literally thousands of experiments examining factors that might affect the accuracy of eyewitness identifications.
Is there reason to be skeptical of a crime victim who points to the defendant in court and says "I'm absolutely sure that's the robber, I'll never forget that face"?
Yes. Even the most convincing-sounding identifications can be mistaken because human memory does not act like a machine, accurately recording, storing, and retrieving images on demand. Like all of us, eyewitnesses construct and interpret what they see while events are ongoing. The process continues while images are stored in eyewitnesses' memories, and when they're called upon to "retrieve" the image when a police officer asks, "Do you recognize this person?" As one expert puts it, "Some memories are elaborations created by witnesses over time based on their own rationalizations for what must have happened and suggestions from others."
What factors tend to cause eyewitnesses to identify the wrong person?
Some of the factors associated with mistaken identifications are matters of common sense and everyday experience. For example, all of us recognize the difficulty of making an accurate identification based on a "quick glance" as opposed to a "long look." Similarly, one does not have to be a cognitive scientist to know that lighting, distance, and an eyewitness's physical condition (for example, just awakened) can also cause an eyewitness to identify the wrong person. Below are some of the less obvious factors that have led eyewitnesses to make mistakes:
Stress. While many people tend to believe that "stress sharpens the senses," research consistently shows that people who are under stress when they observe an event are more likely to misidentify a culprit.
Presence of a weapon. Eyewitnesses confronted by a weapon are apt to focus on the weapon and not on the person holding it.
Confidence level. Eyewitnesses who express great confidence in their identifications are no more accurate than those who admit to uncertainty. Confident eyewitnesses sometimes have higher error rates.
Cross-racial identification. Eyewitnesses are less accurate when the person they are asked to identify is of a different race. This factor affects members of all racial groups.
Pressure to choose. Eyewitnesses are more likely to make mistakes when they feel under pressure to make identification, despite an admonishment that they don't have to make a choice.
Postevent influence. Eyewitnesses are more likely to make mistakes when they rehash events with other observers. In these situations, witnesses may alter their memories so that they can be in agreement with others.
Transference. Eyewitnesses may make a mistaken identification based on having seen the person they identify on a different occasion.
Multiple perpetrators. Identification accuracy decreases as the number of people involved in an event increases.
Absence of an "employment boost." Eyewitnesses who regularly interact with the public (store cashiers, bank tellers) are no better at making identifications than other people.
How do judges and jurors find out about factors that may lead to mistaken identifications?
Many cognitive psychologists not only do research experiments, but also qualify as expert witnesses and testify at trial. Based on the factors surrounding the commission of a crime, they can testify to how those factors might have affected eyewitness's ability to make an accurate identification.
Defendants who can't afford to hire a cognitive psychologist as an expert may ask a judge to appoint an expert at government expense. However, few court systems have enough money to allow judges to appoint eyewitness identification experts in every case in which their testimony is relevant. A less expensive option is for a judge to give a jury instruction that summarizes factors that might affect eyewitness' accuracy.
When they testify at trial, do eyewitness identification experts give an opinion about whether the identifications in that case are accurate?
No. Qualified experts can "educate the jury" by talking generally about factors that studies have shown tend to lead to inaccurate identifications. But experts have no way of assessing whether a particular eyewitness is accurate.