Eyewitness testimony refers to the descriptions that bystanders or victims give to investigators, or in the courtroom, to describe what they can recall about the incidents under investigation. Since antiquity, eyewitnesses’ testimony has overwhelmingly constituted the main basis for courts’ and juries’ decisions to convict or to acquit.

Even today, many court verdicts are based almost exclusively on evidence provided by eyewitnesses. This is true even when other available case evidence clearly contradicts the eyewitnesses’ reports and, importantly by extension, even when the witnesses are factually incorrect. The severity of this situation is epitomized by data from the Innocence Project, a public policy organization based in the USA.

Using DNA testing, more than 300 individuals have been found to be wrongfully convicted, most commonly of murder or rape, and have consequently been exonerated. It is important to consider that these cases undoubtedly represent only a fraction of the true number of wrongful convictions, not least because in many cases forensic trace evidence was never, or is no longer, available to be DNA tested. More importantly, of these 300-plus cases, more than 75% were convicted at least in part on the basis of mistaken eyewitness identification (www.innocenceproject.org; Wells and Quinlivan, 2009). This rather worrisome situation can be linked to the fact that eyewitnesses’ reports are typically perceived as more convincing than many other types of legal evidence, especially when those reports are expressed with high levels of confidence.

Any given testimony is the product of a number of cognitive (intrapersonal) and social (interpersonal) processes. Perceptual and attentional processes are involved during the moments when individuals actually witness or are victims of a crime. Memory processes are involved in encoding, storing, and later retrieving the details of what has been witnessed. Metacognitive processes are involved in witnesses’ decisions whether to report specific details that they remember. And in each of these instances, the social context within which these processes occur also plays a vital role.

Eyewitness Factors

Given the importance and the persuasiveness of eyewitness testimony, it is of utmost importance for witnesses’ reports to be accurate and reliable. Unfortunately, as confirmed now by many hundreds of studies, the reliability of eyewitness reports depends on a countless number of variables, some of which can lead the testimonial evidence to be weak, and some of which can lead it to be factually inaccurate. These variables have traditionally been divided into two types: system variables and estimator variables.

System variables are those that can be controlled through the processes and procedures of eliciting witnesses’ memory reports; for example, the amount of time that investigators invest in building rapport with the witnesses in advance of their formal interviews, or the instructions given to witnesses before they participate in a lineup identification procedure. Estimator variables, in contrast, are those that cannot be controlled, but that nevertheless play important roles in determining the reliability of the end product. For example, we are unable to control the age of the witnesses, or the amount of alcohol they had imbibed at the time the incident occurred. Other important estimator variables include the perceptual conditions under which the bystander witnessed the incident (e.g., the lighting conditions) and other attentional factors (e.g., the witness’s level of concentration and distraction).

Among the most widely studied estimator variables is the presence of a weapon during the incident, which can lead to a perceptual effect known as ‘weapon focus.’ Weapon focus refers to the tendency of witnesses to pay particularly high levels of attention to weapons when one is present during a crime. This attention to the weapon, in turn, comes at the expense of attention paid to other elements of the scene. As a result, the presence of weapons has been shown to lead to a poorer ability of witnesses to describe a perpetrator’s appearance and to identify the perpetrator at a later time (Fawcett et al., 2013).

Among the multitude of studies on this phenomenon, many authors have argued that in fact the ‘weapon focus’ effect may have little to do with weapons per se. Instead, this effect may be related to how a weapon is perceived in its situational context. Pickel (1999), for instance, showed her participants different videos depicting a perpetrator with a gun. She found that participants who saw the armed perpetrator at a baseball field were subsequently far less able to describe him than those who saw him at a shooting range.

Similarly, Mitchell et al. (1998) found a comparable ‘weapon focus’ effect among participants who saw a perpetrator holding a gun, as among different participants who saw a perpetrator holding a stick of celery. Together, these data suggest that it is the unusualness of a weapon, rather than the threat that it seems to pose, which attracts attention. What the ‘weapon focus’ effect illustrates well, though, is that the reliability of witnesses can be influenced quite easily by straightforward visual elements of the incident. With regard to estimator variables more broadly, even though these are factors that the justice system cannot control, empirical research can nonetheless help us to understand these factors as means to predicting and understanding the circumstances under which witnesses will be most reliable.

This excerpt is taken from Eyewitness Testimony by Robert A. Nash from the second edition of the International Encyclopedia of the Social and Behavioral Sciences. This article outlines some of the most important factors in eyewitness testimony– including confidence – that have been studied in the context of understanding what leads witnesses to be more versus less reliable. Read More Here

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