Eyewitness Identification Evidence and Innocence Risk

Article excerpt

It is well known that the frailties of human memory and vulnerability to suggestion lead to eyewitness identification errors. However, variations in different aspects of the eyewitnessing conditions produce different kinds of errors that are related to wrongful convictions in very different ways. We present a review of the eyewitness identification literature, organized around underlying cognitive mechanisms, memory, similarity, and decision processes, assessing the effects on both correct and mistaken identification. In addition, we calculate a conditional probability we call innocence risk, which is the probability that the suspect is innocent, given that the suspect was identified. Assessment of innocence risk is critical to the theoretical development of eyewitness identification research, as well as to legal decision making and policy evaluation. Our review shows a complex relationship between misidentification and innocence risk, sheds light on some areas of controversy, and suggests that some issues thought to be resolved are in need of additional research.

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The ever-increasing number of cases in which DNA evidence has led to the exoneration of innocent people has focused a bright light on the problem of false identification. In 161 of the 218 DNA exonerations listed on the Innocence Project Web site (www.innocenceproject.org, last visited August 5, 2008), false identification was part of the evidence brought forward at trial, leading to the wrongful conviction. These DNA exonerations, as well as a number of other archival analyses, have led many to the conclusion that false identification is the primary cause of wrongful convictions in the United States (Gross, Jacoby, Matheson, Montgomery, & Patil, 2005; Huff, Rattner, & Sagarin, 1996; Scheck, Neufeld, & Dwyer, 2000; Wells et al., 1998).

The next question seems obvious: If false identifications are the primary cause of wrongful convictions, what are the causes of false identifications? The logic here is straightforward. If A causes B, it should be useful to know what causes A. However, the relationship between false identification and wrongful conviction is not so straightforward.

Psychological research, going back at least 100 years to Munsterberg's (1908) seminal book On the Witness Stand, up to, more recently, Cutler and Penrod (1995), Loftus (1979), and Wells (1993; Wells, Memon, & Penrod, 2006), has demonstrated the frailties of memory and the influence of suggestion, so there is little doubt but that witnesses make mistakes. However, a wrongful conviction requires a particular kind of eyewitness mistake-specifically, a false identification of a person who is suspected by the police but innocent.

The 161 false identifications noted above make the point that the identification of the suspect is not equivalent to the identification of the perpetrator. This points to the central question of this article: What does it mean when the witness identifies the suspect? Is it a correct identification of the perpetrator, or a false, mistaken identification of an innocent suspect?

A mistaken or false identification can be viewed as a conditional probability-specifically, the probability that the suspect will be identified, given that he is innocent. However, the question we raise points to another important conditional probability: the probability that the suspect is innocent, given that he was identified. For brevity, we will refer to this probability, which lies at the core of the question before the jury, as innocence risk.

Simply put, innocence risk depends on two response probabilities: the probability that the suspect will be identified given that he is guilty (a correct identification) versus the probability that the suspect will be identified given that he is innocent (a false identification). Innocence risk is simply the proportion of all suspect identifications that are false identifications of an innocent suspect. …