Thinking outside the red box: Does the simultaneous Showup distinguish between filler siphoning and diagnostic feature detection accounts of lineup/Showup differences?

Lineups are considered a superior method of identification to showups, but why is contested. There are two main theories: diagnostic feature detection theory, which holds that surrounding the suspect with fillers causes the eyewitness to focus on the features that are most diagnostic, and differential filler siphoning theory that claims that the fillers draw incorrect choices away from the suspect. Colloff and Wixted (2020) created a novel identification task, called a simultaneous showup, designed to prevent filler siphoning, while still allowing comparison to occur between members of the array. However, even in the simultaneous showup, it is possible that covert filler siphoning occurs. In Experiment 1, we replicated the simultaneous showup condition and also asked participants if the other photos affected their decision making; we found evidence that participants self-reported both diagnostic feature detection and covert filler siphoning. In Experiment 2, we replicated Colloff and Wixted (2020, Experiment 3) main findings. Additionally, we found that participants self-reported both diagnostic feature detection and covert filler siphoning. This led us to conclude that the simultaneous showup procedure could not fully exclude covert filler siphoning from occurring.