In criminal investigations, eyewitnesses are routinely required to identify a previously seen perpetrator from a police lineup. In the UK alone, tens of thousands of identity lineups are administered for this purpose every year (e.g. http://www.viper.police.uk). Under these circumstances, many individuals make perfectly accurate eyewitness identifications. However, eyewitness misidentifications are also frequently made, whereby innocent people in a lineup are mistaken for a perpetrator (e.g. Memon, Havard, Clifford, Gabbert, & Watt, 2011; Slater, 1994; Wright & McDaid, 1996). For investigators, the difficulty therefore arises in differentiating individuals that are accurate from those that are inaccurate eyewitnesses.
One approach under investigation in psychology to address this problem is a multiple-lineup method. In this method, eyewitnesses are required to identify a perpetrator repeatedly, but from different person aspects that might have been observed at a crime scene, such as the face, body, voice, clothing, or accessories (Lindsay, Wallbridge, & Drennan, 1987; Pryke, Lindsay, Dysart, & Dupuis, 2004; Sauerland & Sporer, 2008; Sauerland, Stockmar, Sporer, & Broers, 2013). These studies show that the selection of the same identity from different lineup combinations can be used to assess the likelihood that a correct target selection has been made. Sauerland and Sporer (2008) found, for example, that identification of a suspect’s body from a lineup indicated only a .60 probability that the identified person was, in fact, guilty. However, this number rose to .91 when the separate identification of body and face cues, from two different lineups, were considered together.
These findings illustrate the promise that multiple lineups hold for assessing the accuracy of individual eyewitnesses. However, the results of these studies are curtailed by the poor identification accuracy for some person aspects. For example, in studies that have examined multiple-lineup procedures, correct identifications of voices were obtained on only 27% of trials (Pryke et al., 2004) and this number was lower still for bodies and accessories, at 18% and 11%, respectively (Sauerland & Sporer, 2008). In comparison, identification accuracy was much higher for frontal views of faces, at 72% (Pryke et al., 2004) and 61% (Sauerland & Sporer, 2008), and combinations of other person aspects with such frontal face portraits were most useful for diagnosing eyewitness accuracy.
In this study, we sought to investigate this face advantage further, by exploring a new variant of this procedure in which multiple lineups only comprised faces. This manipulation makes good sense given the comparatively high recognition accuracy for faces in previous multiple-lineup studies, but it also has a strong theoretical grounding in the face perception literature. According to cognitive theories of face processing (e.g. Bruce & Young, 1986; Burton, Bruce, & Johnston, 1990; Burton, Jenkins, Hancock, & White, 2005; Haxby, Hoffman, & Gobbini, 2000; Schweinberger & Burton, 2003), the successful recognition of familiar people, such as family, friends, or colleagues, is highly robust and can be triggered by any instance of their face. The ultimate hallmark of accurate person identification is therefore the ability to recognize the same person’s face repeatedly, across many different encounters.
In line with this research, eyewitness identification errors are made rarely when the perpetrator is someone that is already known to a witness (e.g. Memon et al., 2011). Familiarity with a face is, however, a continuum (e.g. Clutterbuck & Johnston, 2002, 2004), which reflects the exposure duration to an identity (e.g. Bornstein, Deffenbacher, Penrod, & McGorty, 2012; Memon, Hope, & Bull, 2003; Roark, O’Toole, Abdi, & Barrett, 2006) as well as the variability in a person’s appearance across different exposures (Andrews, Jenkins, Cursiter, & Burton, 2015; Burton, Kramer, Ritchie, & Jenkins, 2016; Dowsett, Sandford, & Burton, 2016; Murphy, Ipser, Gaigg, & Cook, 2015). Eyewitnesses who are initially unfamiliar with a perpetrator cannot acquire strong familiarity (e.g. as they would in the case of the faces of family, friends, celebrities) with this person’s face at a crime scene. As a consequence, identification of such unfamiliar people can be rather difficult, even under best-possible conditions (e.g. Bruce, Henderson, Greenwood, Hancock, Burton, et al., 1999; Henderson, Bruce, & Burton, 2001; Megreya & Burton, 2006, 2008).
Another factor appears to determine eyewitness performance, as identification accuracy varies even when exposure to a perpetrator is held constant across participants. One possibility is that this reflects individual differences in the ability to recognize unfamiliar faces, whereby some individuals are naturally equipped better than others for this task (e.g. Wilmer, Germine, Chabris, Chatterjee, Williams, et al., 2010; Zhu, Song, Hu, Li, Tian, et al., 2010). Support for this reasoning comes from studies that have revealed correlations of eyewitness accuracy with tests of face recognition (e.g. Geiselman, Tubridy, Blumkin, Schroppel, Turner, et al., 2001; Hosch, 1994; Morgan, Hazlett, Baranoski, Doran, Southwick, et al., 2007). Bindemann, Brown, Koyas, and Russ (2012) showed, for example, that eyewitness identification accuracy for the perpetrator of a staged crime correlated with performance on a laboratory test of face recognition, in which observers had to select newly learned face targets from identity lineups. So far, however, these studies have only examined this link for a single eyewitness identification.
This is an important issue as there is evidence to suggest that, in contrast to familiar face recognition, the repeated recognition of unfamiliar faces might also be particularly difficult. Studies that speak to this issue have focused primarily on unfamiliar face matching tasks, in which observers have to decide whether side-by-side photographs of two unfamiliar faces depict the same person or different people. For example, in these studies observers often decide that a face pair depicts two different people in one block of trials, but classify this pair as depicting the same person in a subsequent block (Alenezi & Bindemann, 2013; Alenezi, Bindemann, Fysh, & Johnston, 2015) or on a different day (Bindemann, Avetisyan, & Rakow, 2012). When these effects are assessed at a group level, by averaging performance across participants, identification accuracy declines with each repetition of the face pairs. However, observers also exhibit broad inter-individual (e.g. Burton, White, & McNeill, 2010) and intra-individual differences (Bindemann et al., 2012).
The differences between familiar and unfamiliar face processing, and the individual differences that are observed in unfamiliar face matching tasks, have important implications for our understanding of eyewitness identification, for two reasons. First, a lineup task is essentially a test of the familiarity that an eyewitness has gained with a perpetrator’s appearance at a crime scene. Following only a brief exposure to a perpetrator (as is often the case at crime scenes), one should therefore expect that eyewitnesses generally have relatively poor memory for a perpetrator’s facial appearance and identification of this person should be taxing. Second, this issue should be compounded by inter- and intra-individual differences in the ability to process faces. Thus, observers at the lower end of face processing ability should be more prone to make errors in a single eyewitness identification and those who tend to be more inconsistent in their identification decisions should also be less capable of identifying a perpetrator repeatedly. In turn, an eyewitness’ ability to correctly recognize an unfamiliar perpetrator across multiple instances should greatly increase one’s confidence in the accuracy of their identification.
Exploring this issue will reveal new information about the robustness of facial representations that eyewitnesses hold of a perpetrator. We therefore assessed the extent to which individual eyewitnesses can identify the same person’s face repeatedly from different identity lineups and, equally, whether they can consistently indicate the absence of a target when he or she is not included in a lineup. To fully explore this question, it is important to compare consistent target identifications with the repeated selection of other, non-target lineup identities. Outside of the laboratory, an identity lineup always includes a suspect, but this person may be the sought-after perpetrator of a crime or an innocent person. The purpose of a lineup is essentially to determine whether a witness selects the suspect, thereby seemingly confirming them as the target, or does not select this individual. The remaining faces act as “fillers” that are known innocents that would not be charged if they were selected by an eyewitness. To determine the extent to which observers might repeatedly identify the same non-target face in a multiple-lineup procedure, one could therefore replace the target with another identity that acts as a designated innocent suspect. By comparing repeated target identifications with selections of the innocent suspect, it would then be possible to determine whether the consistency of the responses of individual observers across multiple lineups can dissociate correct from incorrect eyewitness identifications.
While this approach has obvious applied value, the designation of innocent suspects poses problems in experimentation (Pryke et al., 2004; Sauerland & Sporer, 2008). In police investigations, suspects are arrested on the basis of their similarity to a witness’ description. However, it can be difficult to establish the perceived similarity of targets and suspects in advance. Different strategies for designating innocent suspects and lineup fillers appear to influence eyewitnesses’ identification decisions (Lindsay, Martin, & Webber, 1994; Luus & Wells, 1991; Wells, Rydell, & Seelau, 1993), but the study of such strategies has also yielded inconsistent results (e.g. Darling, Valentine, & Memon, 2008; Tunnicliffe & Clark, 2000). In addition, studies of unfamiliar face matching demonstrate that people vary considerably in how they detect the resemblance of faces in person identification tasks (e.g. Bindemann et al., 2012; Burton et al., 2010).
In light of these problems, we adopted a different approach. Instead of pre-selecting a designated suspect, this identity was defined a posteriori. We provide two contrasting approaches for this analysis. For the first approach, we assess identifications of the non-target identity that was selected first by an eyewitness in the multiple-lineup procedure. This approach minimizes data loss by including all incorrect eyewitnesses in the analysis and provides a “worst case” scenario by comparing consistent target selections with the greatest possible number of the corresponding non-target identifications. With a second approach, we focus on the non-target that was selected most often as the target by all observers during the course of the entire experiment. This “worst non-target” approach provides the highest number of comparison identifications for the target when these are defined by only a single non-target identity (for similar approaches, see, e.g. Pryke et al., 2004; Sauerland & Sporer, 2008). The consistency of these non-target selections across lineups served to contextualize the extent of consistent target selections.