On the diagnosticity of multiple-witness identifications

It is not uncommon for there to be multiple eyewitnesses to a crime, each of whom is later shown a lineup. How is the probative value, or diagnosticity, of such multiple-witness identifications to be evaluated? Previous treatments have focused on the diagnosticity of a single eyewitness's response to a lineup (Wells and Lindsay, Psychol. Bull. 3 (1980) 776); however, the results of eyewitness identification experiments indicate that the responses of multiple independent witnesses may often be inconsistent. The present paper calculates response diagnosticity for multiple witnesses and shows how diagnostic probabilities change across various combinations of consistent and inconsistent witness responses. Multiple-witness diagnosticity is examined across variation in the conditions of observation, lineup composition, and lineup presentation. In general, the diagnostic probabilities of guilt were shown to increase with the addition of suspect identifications and decrease with the addition of nonidentifications. Foil identification results were more complicated-diagnostic of innocence in many cases, but nondiagnostic or diagnostic of innocence in biased lineups. These analyses illustrate the importance of securing clear records of all witness responses, rather than myopically focusing on the witness who identified the suspect while ignoring those witnesses who did not.