Could precise and replicable manipulations of suspect-filler similarity optimize eyewitness identification performance?

The optimal level of suspect-filler similarity in police lineups remains undefined. Difficulties inherent in pinpointing precise and replicable variations in face similarity create challenges for examining the effects of suspect-filler similarity on identification outcomes and providing decisive lineup construction recommendations. We tested the relationship between suspect-filler similarity and identification outcomes using stimuli developed with a combination of face matching and morphing software that could potentially be used by police investigators to standardize lineup composition. A group of fillers selected to be as low in similarity to the suspect as possible, while still matching a basic
perpetrator description, were morphed to the suspects. Participants viewed lineups in which the fillers were either unmorphed, a 33% morph, or a 50% morph to the suspect. Results showed a tendency for identification performance
—including accuracy, discriminability between guilty and innocent suspects and the confidence-accuracy relationship—to suffer as similarity levels increased beyond matching a basic perpetrator description. Thus, despite the attraction of exploiting modern technology to standardize suspect-filler similarity relations, our findings across multiple sets of encoding and test materials broadly cohere with Wells and colleague’s argument (Luus & Wells, 1991; Wells et al., 1993) that identification performance may be optimized when fillers are selected to be as low as possible in similarity to the suspect while still matching description. However, before unequivocally endorsing such a lineup construction approach, it is necessary to conduct a systematic examination of precisely what constitutes an adequate perpetrator description and how this might be consistently obtained.