Source: Association for Psychological Science

Even if you have never had any direct experience with the police, you have probably seen police lineups on television and in the movies. A group of suspects marches into a room and an eyewitness attempts to pick out the one who committed the . In real life, lineups are rarely conducted with live suspects the way they are depicted in movies. Instead, photo lineups are used. The photo of the suspect is shown along with several others (called distractors), and the witness is asked to identify which person they saw.

There are several important issues to think about with lineups. First, the person the police suspect of having committed the crime may not be the actual perpetrator of the crime. So, there is some danger that—even if the witness selects the suspect the police have apprehended—the real criminal has not been caught. Second, the ability of the witness to pick out the suspect depends both on the match between the suspect and the of the witness and also on the comparison between the suspect and the distractor faces.

Over the years, standard police procedures have been adapted based on psychological research. One of the ways that procedures have changed relates to distinctive features of suspects. For example, suppose that the eyewitness recalls that the person they saw had a unique birthmark near his eye. The police apprehend a suspect with a birthmark. If they set up the lineup so that the suspect is the only one with a birthmark, then the witness will probably select that person.

Does that mean they got the right person?

To guard against the prospect that people will focus just on a particular unique feature, the police will often find a way to minimize the influence of that unique feature on eyewitness identification. They might use photo software to replicate that birthmark on all of the photos. They might pixelate the photos in the area of the birthmark so that it is hard to see. They might actually black out that area of the photo.

Do these techniques work?

This question was addressed in a paper by Melissa Colloff, Kimberley Wade, and Deryn Strange presented in the September, 2016 issue of Psychological Science.

They ran a study with over 8,000 participants. Each participant watched one of four (staged) crimes being committed. The perpetrator of the crime had a distinctive facial feature. Participants were told to pay careful to the crime, because they would be asked questions about it later. Then, they did some filler tasks for 8 minutes. After that, they were shown a lineup of six faces. The faces were drawn from a database of criminal suspects. Each face was converted to a black-and-white picture and the clothing was altered to look like the suspect was wearing a black t-shirt.

The particular lineup a participant saw used one of four procedures. Three of the procedures took the distinctive feature into account and either added it to all of the faces, pixelated the area, or blacked it out. The fourth procedure showed people a lineup with one face that had the distinctive feature and distractors that did not. Half of the participants in this condition saw the face of the actual perpetrator in the lineup and half did not.

Overall, the procedures that minimize the impact of the distinctive facial feature were effective compared to the procedure where only one face had the distinctive facial feature. In particular, the procedures that gave the distinctive feature to every suspect led to few cases in which the innocent suspect was selected from the lineup. The guilty suspect was not always found in these lineups either, but they were found more often than innocent people were picked from the lineup. In contrast, people shown a lineup in which only one suspect had a distinctive feature often selected that face, whether it was the actual perpetrator or an innocent suspect.

What happens in the badly constructed lineups is that witnesses focus primarily on the distinctive feature. As a result, they do not notice other important aspects of the face that might allow them to distinguish between the actual perpetrator and other people who have that unique property. This research demonstrates why it is important that police procedure has been influenced by psychological work on eyewitness memory.

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