The most common purpose of setting up a suspect lineup is to test the durability of a guilt hypothesis. However, the validity of this test is directly dependent on how the lineup is set up, conducted and evaluated. Even seemingly subtle factors such as the number of times lineup photos are displayed to witnesses can impact on the risk of error, both false positives and false negatives. In an applied setting, a decision by investigators to “try again” and go for another lap of displaying the photos, for example when the witness did not identify anyone in the first lap, can be due to a genuine will to prevent false negatives. For example, investigators may believe that the witness recognizes the suspect as the perpetrator but needs another lap to be confident enough to make an identification. However, the perception that there is a risk of a false negative can stem from a confirmation bias, that is, that investigators maintain their belief that they have identified the right suspect even in the face of information that potentially challenges that hypothesis (that the witness does not recognize the suspect as the perpetrator).
The purpose of this research was to test whether and how witness accuracy and confidence are impacted by the number of laps used in a sequential photo lineup, comparing specifically one lap (original version) to two laps (two laps version). To this end, a scenario-based experiment was conducted in which participants (n = 164) were shown a film clip of a suspected rape and 1 week later suspect lineups (both target absent and present lineup) were conducted with the witnesses.
Overall, the results suggest that witnesses tested with the original version more rarely erroneously identified someone when the perpetrator was absent, compared to when the two laps version was used. Yet, the ability to identify the perpetrator (when the perpetrator was present) was not significantly different among participants tested with the different methods. Overall, witnesses tested with the two laps version stated a higher degree of confidence in their decisions than did witnesses tested with the original version. The average deviation between stated confidence level and accuracy was somewhat, although not significantly, higher with the two laps version than with the original version.
Hence, while investigators may fear that there is a substantial risk of a false negative if they do not continue with another lap after a witness has been unable to identify the suspect after the first lap, the present results in fact suggest that a second lap does not significantly reduce the risk of a false negative. However, a second lap does significantly increase the risk of a false positive. Taken together, the results imply that the original version of a sequential lineup (one lap) is a better option than the two laps version, although none of the versions are fool-proof.
Cham: Springer, 2022. p. 49-78