Suspect-filler similarity: Replicating distinctive features in police lineups

Worldwide, police test eyewitness memory using a lineup, containing an innocent or guilty suspect and several fillers. How similar-looking the fillers should be to the suspect has not been sufficiently answered, possibly due to a lack of guiding cognitive models. We made predictions using a new signal-detection model assuming feature-matching logic and discounting of shared features. Participants encoded a perpetrator with a distinctive feature. In two pre-registered experiments, we tested how replicating a similar but non-identical feature across the fillers (low feature-similarity replication) influenced identification accuracy compared to replicating an identical feature (high feature-similarity replication), and an unfair condition in which the feature was not replicated. In Experiment 1 (N=4,915), the innocent suspect’s feature matched the description of the perpetrator’s. As predicted, low compared to high feature-similarity replication increased the hit rate without affecting the false alarm rate and increased ability to discriminate innocent from guilty suspects. In Experiment 2 (N=1,964), the innocent suspect’s feature was identical to the perpetrator’s and, as predicted, the effect on discriminability was reversed. The model provides a useful theoretical framework, and police should not needlessly match features across a lineup that could be useful cues to identity, as this may harm witness performance.