Why are lineups better than showups? A test of the filler siphoning and enhanced discriminability accounts

Presenting the police suspect alongside similar-looking people (a lineup) results in more accurate eyewitness identification decisions than presenting the suspect alone (a showup). Why are lineups better than showups? Diagnostic-feature-detection theory suggests that lineups enhance witnesses' ability to discriminate between innocent and guilty suspects, because facial features can be compared across lineup members. Filler-siphoning suggests that the presence of other lineup members siphons some of the incorrect identifications that would otherwise land on the innocent suspect. To test these 2 accounts, over 3,600 subjects across 3 experiments watched a mock-crime video and were presented with either a showup, a simultaneous lineup, or a simultaneous showup (a novel procedure). Subjects in the simultaneous showup condition saw the suspect and 5 similar-looking faces, but, unlike a lineup, could not identify the other faces. Presenting similar-looking faces alongside the suspect (simultaneous showup and lineup) enhanced subject's ability to discriminate between innocent and guilty suspects compared with presenting the suspect alone (showup) as measured by Area Under the ROC Curve (pAUC) and fitting a signal-detection model. These results show, for the first time, that the discriminability advantage in simultaneous lineups is because of the comparison of multiple faces as predicted by diagnostic-featuredetection theory, but not the filler-siphoning account.