Lineup Fillers

Lineup fillers prevent unreliable witnesses from guessing the identity of the police suspect and should allow for a fair recognition test for those witnesses who do remember the culprit. The primary strategies for selecting fillers for criminal identification lineups are presented in this entry. The suspect-matched and perpetrator-description-matched strategies are two methods of constructing lineups that have been compared by researchers. Additionally, care should be taken to ensure that the structure of the lineup is uniform across members. To assess the fairness of a lineup, several indices that measure lineup bias and lineup size have been developed.

The Function of Lineup Fillers

Lineup fillers, also known as foils (an innocent person in a police lineup), serve the major purpose of testing an eyewitness’s recognition memory for a criminal perpetrator so as to establish evidence that the suspect is guilty of the crime. Fillers also serve to screen out unreliable witnesses: Witnesses who identify foils may have a weak memory for the perpetrator or may be guessing. With respect to the problem of guessing, the probability that a witness will select the suspect from a lineup based on chance alone equals 1/k, where k equals the number of foils in the lineup. Having more options during the identification test decreases the probability that witnesses will identify the suspect by guessing alone. Additionally, presenting foils that resemble the suspect works toward preventing the witness from being able to deduce who the suspect is simply by eliminating improbable choices from the lineup.

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Filler Selection Strategies

There are two primary filler selection strategies that have been investigated by researchers. First, foils may be selected for the lineup on the basis of their similarity to the physical appearance of the suspect, a procedure that is known as the suspect-matched strategy. Second, foils may be selected based on their resemblance to a physical description of the perpetrator given by the eyewitness, a procedure that is termed the perpetrator-description-matched strategy.

Two main concerns arise when foils are selected for the lineup on the basis of the suspect-matched strategy. First, if the suspect is not the culprit and is in fact innocent, then selecting the foils based on their match to the innocent suspect may result in a lineup in which the similarity of the foils to the perpetrator is low. This is a concern in cases in which the suspect is apprehended because he or she is physically similar to the description of the culprit given by an eyewitness. In such cases, the suspect may be the only one in the lineup that resembles the perpetrator. As a result, the innocent suspect might be frequently identified from lineups in which the foils are chosen on the basis of their match to the innocent suspect’s appearance, a consequence that is known as the backfire effect. Another concern that arises when the foils are chosen for the lineup using the suspect-matched strategy is that if the suspect is in fact the culprit, then the foils could potentially be too similar to the suspect, and thereby decrease the odds that a witness who remembers the perpetrator can distinguish the guilty suspect from the foils.

In view of these concerns, the perpetrator-description-matched strategy has been proposed. In the event that an innocent suspect is in the lineup, the perpetrator-description-matched strategy is thought to ensure that the innocent suspect and the foils have the same probability of being chosen. The rationale is that if investigators select the foils and the suspect for the lineup using the same criteria (i.e., their match to the witness’s description), then the foils should look no more like the perpetrator than does the innocent suspect. Additionally, for a witness who remembers the perpetrator, the perpetrator-description-matched strategy allows for propitious heterogeneity, a term that refers to having sufficient variability across lineup members to allow the witness to recognize a guilty suspect.

Some researchers studying lineup identification in the laboratory employ a hybrid of the suspect-matched and perpetrator-description-matched strategies. A pool of potential foils that fit the modal description of the target (i.e., the “perpetrator”) is obtained. Participant raters then judge the similarity of each face in the pool to the target. The faces that are rated as being the most similar to the target are selected as fillers. An additional method on the horizon for the selection of fillers for lineups is the use of principal components analysis (PCA). PCA represents the similarity of faces on multiple dimensions using Euclidean distances. Results derived from PCA have been shown to relate to lineup identification performance and to measures of lineup fairness.

Special Considerations in Selecting Fillers

In employing the perpetrator-description-matched strategy to select fillers, a number of issues may arise. One difficulty is that the witness may provide an inadequate number of details regarding the perpetrator’s appearance for selecting fillers for the lineup. Selecting foils when there is little information regarding the culprit’s appearance may result in a lineup in which the members are highly dissimilar in relation to one another and to the suspect. As a consequence, the perpetrator-description-matched strategy might in some cases increase the rate at which innocent suspects are identified. Additionally, the degree to which the lineup members are similar can affect the eyewitness’s decision standard. If the foils in the lineup are relatively low in their similarity to the culprit, then witnesses may be less cautious in identifying a face compared to when there is a higher degree of similarity across lineup members. Finally, research comparing the suspect-matched and perpetrator-description-matched strategies has produced findings that are mixed, thereby leading some researchers to maintain that it is premature to recommend one strategy over the other at this time.

Nevertheless, additional procedural safeguards have been suggested for selecting fillers when using the perpetrator-description-matched strategy. For example, if the witness does not mention a characteristic, such as facial hair, the police might assume the perpetrator has the default value of the characteristic, and assume that the perpetrator was clean shaven. Though placing the clean-shaven suspect among foils with facial hair would not violate the witness’s description, doing so would draw attention toward the suspect and perhaps bias the witness to choose the suspect only because the suspect’s appearance is different from the others. Therefore, to avoid this possibility, if the suspect is clean shaven, then he should be placed among clean-shaven foils. Additionally, when the suspect does not fit the description given by the eyewitness on some level, it has been recommended that investigators select foils based on their match to the appearance of the suspect on the features in which the description of the culprit does not match the suspect. Along these same lines, if there are multiple witnesses involved in a case, a recommendation that has been made is to create a separate lineup for each eyewitness. In laboratory studies in which researchers feel that it is not feasible to create lineups for each participant, the foils can be selected based on their match to the typical or modal description given by research participants viewing the suspect.

Finally, care should be taken to ensure that the arrangement of the lineup is uniform. The photographs themselves should be similar and presented to witnesses in a standardized fashion. For example, if one photograph is slightly tilted away, or the focal length is greater, or the facial expression differs from the others, responses may be biased toward that photograph simply because it stands apart from the others in its presentation. Additionally, uniformity across members with respect to clothing should also be achieved. Moreover, if the eyewitness describes the perpetrator as wearing a particular type of clothing, the suspect should not be the only one wearing that type of clothing in the lineup.

Assessing Lineup Fairness

Researchers use the mock-witness procedure to determine lineup fairness, or the adequacy of lineup fillers. In the mock-witness procedure, participants who have not seen the perpetrator are given a description of the perpetrator along with a lineup. They are asked to pick the member of the lineup that most closely resembles the perpetrator’s description. If the lineup is fair, then mock witnesses should not select the suspect at a rate significantly above chance.

Mock-witness choices can also be used to determine whether the fillers that have been selected for the lineup bias choices toward or away from the suspect. Bias measures include functional size and defendant bias. Mock-witness choices are also used to determine lineup size, which refers to the extent to which identification responses are distributed evenly across lineup members. Measures designed to examine lineup size include effective size, number of acceptable foils, and Tredoux’s E.

Lineup bias and size can vary depending on how the members of the lineup are arranged. For instance, if the suspect is placed between two foils that are low in similarity relative to the suspect, the suspect may “pop out” and hence be chosen significantly more often than when the suspect is placed between two other lineup members that have a greater resemblance to the suspect. This problem may arise even when the average pair wise similarity rating across the lineup members is high. Therefore, it is important that the foils are selected for the lineup using the same criteria. Another method for circumventing possible popout effects is for researchers to systematically rotate, or counterbalance, the position of the suspect in the lineup across participants.

Counterbalancing the position of the suspect in the lineup also controls for the possibility that the decision standard varies along with the number of faces that are viewed. This may be problematic especially in sequential lineups, in which the witness views each face one at a time. In particular, there is some evidence that the decision standard that witnesses use in making a positive identification may be lowered as they progress through the series of faces in a sequential lineup. As such, innocent suspects might have a higher probability of being chosen if they are positioned later in the sequence, as opposed to earlier.


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  2. McQuiston, D. E., & Malpass, R. S. (2002). Validity of the mockwitness paradigm: Testing the assumptions. Law and Human Behavior, 26(4), 439-153.
  3. Tredoux, C. (2002). A direct measure of facial similarity and its relation to human similarity perceptions. Journal of Experimental Psychology: Applied, 8(3), 180-193.
  4. Tunnicliff, J. L., & Clark, S. E. (2000). Selecting foils for identification lineups: Matching suspects or descriptions? Law and Human Behavior, 24(2), 231-258.
  5. Wells, G. L., Rydell, S. M., & Seelau, E. (1993). The selection of distractors for eyewitness lineups. Journal of Applied Psychology, 78(5), 835-844.

Return to the overview of Eyewitness Memory in Forensic Psychology.