The identification of criminal perpetrators by eyewitnesses is a staple form of evidence in courts of law. In the 1970s psychological scientists began a systematic study of eyewitness identification using controlled experiments, and much has been learned through thousands of lab studies about factors that put the reliability of eyewitness evidence at risk. Laboratory science has soundly established that the rate of mistaken identification varies as a function of conditions during the witnessed event, the retention interval, and the method of conducting an identification procedure. A useful distinction has been made between two types of factors that influence eyewitness accuracy: system and estimator variables. System variables are those that are (or could be) controlled to improve the accuracy of eyewitness reports, particularly through science-informed police procedures such as non-leading interview protocols, fair lineup structure, and double-blind lineups. Estimator variables influence eyewitness memory and accuracy but are not under the control of the criminal justice system, such as conditions during the crime event (e.g., lighting, distance, obstacles to view), aspects of the criminal (e.g., number of perpetrators, facial disguise), and characteristics of the witness (e.g., visual acuity, stress). One estimator variable—referred to as the weapon focus effect—describes the phenomenon in which the presence of a weapon at the scene of a crime impairs the witness’s memory for the perpetrator and other details of the crime. The basic idea is that the weapon captures the visual attention of the witness and thereby distracts witness attention from other important features of the visual scene such as the face of the perpetrator. The eyewitness is simply less able to adequately encode and subsequently recall details peripheral to the weapon. The weapon focus effect has garnered attention in labs and courtrooms over the more than forty years of eyewitness research. The cumulative body of weapon focus research provides support for the claim that the presence of a weapon draws the attention of an eyewitness. In doing so, this weapon focus can diminish the ability of the eyewitness to later recognize and describe the culprit.
Kassin, et al. 1989 assessed the status of a variety of eyewitness memory topics through a survey of eyewitness experts. A number of topics were endorsed as strongly established principles (e.g., regarding biased lineup instructions), but only 57 percent of experts agreed that the weapon focus effect was reliable enough for psychologists to present in courtroom testimony. This report served as a catalyst for a meta-analytic review of the nascent weapon focus literature. Steblay 1992 located nineteen tests of the weapon focus effect from twelve articles and reported a significant overall difference between weapon-present and weapon-absent conditions, with a small effect for witnesses’ lineup identification accuracy, and a moderate impairment for witnesses’ memory of perpetrator clothing and facial characteristics (“feature accuracy”). Witness accuracy was worse in a weapon-present condition compared to a weapon-absent condition. A decade later, Kassin, et al. 2001 repeated the survey of experts, and the majority of eyewitness scientists at that point were convinced of the reliability of the weapon focus effect (WFE), a significant jump from 57 percent to 87 percent. Fawcett, et al. 2013 followed with a meta-analysis of studies to 2009. Their review supported the weapon focus effect, again noting a small impact of the weapon on witness identification accuracy and a larger impact on feature accuracy. In 2014, the National Academy of Sciences (NAS) confirmed that “the so-called weapon focus is a real-world case in point for eyewitness identification, in which attention is compellingly drawn to emotionally laden stimuli, such as a gun or a knife, at the expense of acquiring greater visual information about the face of the perpetrator” (National Academy of Sciences 2014, p. 55). Fawcett, et al. 2016 followed up the 2013 review with a summary article that encompassed previous work on the topic, including three prior meta-analyses (Steblay 1992; Fawcett, et al. 2013; Kocab and Sporer 2016). They concluded that laboratory evidence demonstrates that an unexpected weapon reduces the accuracy of subsequent suspect identification attempts and witness descriptions of the crime: “The weapon focus effect is sufficiently robust and uncontroversial to guide practice so long as consideration is given to the circumstances surrounding the criminal event with a particular emphasis on witness expectation” (p. 257).
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