Cognitive psychologists have conducted literally thousands of experiments examining factors that might affect the accuracy of eyewitness identifications. These experiments give us reason to be skeptical of a crime victim who points to the defendant in court and professes certainty. (For information on the process of indentification in criminal cases, see Nolo's page for Eyewitness Identification issues.)
Even identifications that sound quite convincing can be mistaken. The human memory doesn't act like a machine, accurately recording, storing, and retrieving images on demand. Eyewitnesses, like all of us, construct and interpret what they as they see it. And the construction and interpretation process occurs well past the event itself. 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." (Geiselman, Eyewitness Expert Testimony, pp. 74-75, Eagle Publishers, 1995.)
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, you don't have to be a cognitive scientist to know that lighting, distance, and the witness's physical condition (for example, fatigued) can also compromise an identification. But here 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 the culprit.
Presence of a weapon. Eyewitnesses confronted by a weapon are apt to focus on the weapon rather than 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 asked to identify someone 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 pressure to make an identification, even if they are told that they don't have to make a choice.
Influence after the fact. 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 because they saw 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.
Many cognitive psychologists not only do research experiments, but also testify as expert witnesses 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, eyewitness identification experts don't usually opine as to whether the identifications in that case are accurate. 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.
Example: Sal Mander, a Caucasian male, is on trial for robbing Delores, an African American female. After Delores identifies Sal as her attacker, Mander's eyewitness identification expert testifies about factors that existed at the time of the robbery that might cast doubt on Delores's ability to observe and recall accurately. However, the expert probably cannot testify, "In my opinion, there's less than a 50% chance that Delores's identification is accurate." Eyewitness identification experts can talk about factors that have been associated with mistaken identifications in experiments, but they themselves admit that they cannot authoritatively determine the accuracy of a particular identification.
The testimony of eyewitness identification experts can aid jurors in understanding the psychology of mistaken identifications, but jurors ultimately have to try to decipher the accuracy of an identification on their own. Given what we know about the vagaries of eyewitness ID, this can be a difficult task.