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Why Police Lineups Will Never Be Perfect

By Virginia Hughes | The Atlantic | October 2, 2014

One night in 1984, a man broke into 22-year-old Jennifer Thompson’s apartment, threatened her at knifepoint, and raped her. While it was happening she tried to memorize everything about him—his  face, hair, clothes, body type. Later that day, she recounted those details to a police sketch artist.

Two days later, a detective showed Thompson a photo lineup of six men. She ruled out four of them right away, and stared at the other two pictures for four or five minutes. Finally she chose one. “Yeah. This is the one,” she said, as recounted in the book Picking Cotton. “I think this is the guy.”

“You ‘think’ that’s the guy?” one of the detectives asked her.

“It’s him,” she said.

“You’re sure?” asked another detective.


She wrote her initials and date on the back of the photo, then asked them, “Did I do OK?”

“You did great, Ms. Thompson.”

The man she identified, Ronald Cotton, was convicted and sentenced to a life in prison. More than 10 years later, a DNA test revealed that Thompson had pointed to the wrong guy. Cotton was innocent.

Eyewitness testimony is hugely influential in criminal cases. And yet, brain research has shown again and again that human memory is unreliable: Every time a memory is recalled it becomes vulnerable to change. Confirming feedback—such as a detective telling a witness she “did great”—seems to distort memories, making them feel more accurate with each recollection. Since the start of the Innocence Project 318 cases have been overturned thanks to DNA testing. Eyewitness mistakes played a part in nearly three-quarters of them.

For three decades psychology researchers have been searching for ways to make eyewitness identifications more reliable. Many studies have shown, for example, the value of “double-blind” lineups, meaning that neither the cop administering the lineup nor the witness knows which of the photos, if any, is the suspect.

But injecting science into the justice system is tricky. For one thing, most criminal investigations happen at a local level. The U.S. has roughly 16,000 law enforcement agencies and few nationally mandated standards. The other big problem is the nature of science itself: Evidence for a given idea builds gradually, as scientists try to replicate others’ work. It can take years or even decades for a clear picture to emerge, and in the meantime scientists may vigorously disagree. While they argue, cases are opened and closed, and people, sometimes the wrong people, go to prison.

Some helpful guidance came today from the National Academy of Sciences. Last year the Academy asked a panel of top scientists to review technical reports and expert testimony about eyewitness identifications and make some solid recommendations. The resulting 160 page report offers many concrete suggestions for carrying out eyewitness identifications. For example, the Academy recommends using double-blind lineups and standardized witness instructions, and training law enforcement officials on the fallibility of eyewitness memory.

On one question, though, the Academy offers no clear answer: What’s the best way to present a photo lineup to a witness? This fuzziness reflects a hot debate bubbling in the scientific literature.

* * *

In 1984, psychology researchers Wells published a study of eyewitness memory by setting up a mock crime in his laboratory. College-student volunteers were told they were about  to participate in an experiment about video games. While they were waiting for the experiment to begin, they witnessed someone steal the game. Then Wells’s team asked them to pick out the thief from a photo lineup. Some of the lineups contained the actual thief but others did not.

The study found that when volunteers made mistaken identifications, it was usually in lineups that did not contain a photo of the real culprit. Wells has a hunch as to why that might be. “Witnesses have a natural propensity to identify the person in the lineup who looks most like the perpetrator relative to the others,” he says. “The problem with that is that if the real perp’s not there, there’s still somebody who looks more like the perpetrator than others.”

Based on those findings, Wells thought that some false IDs could be avoided by using what’s called a ‘sequential lineup’, in which witnesses see photos one at a time and make a decision, yes or no, after each. Over the next few years he carried out more mock-crime experiments and reported exactly that—when images were shown one by one people were less likely to falsely accuse someone than if the images were shown all together.

In fact, Wells’s studies found that sequential lineups slashed the rate of false positives considerably. His first study showed a drop in incorrect accusations  43 to 17 percent. Sequential lineups also slightly increased the number of missed identifications, in which the perpetrator is in the lineup but not fingered by the witness. But because the ratio of true positives to false positives—the so-called ‘diagnosticity ratio’—was much higher for sequential procedures, Wells argued they were superior.

At the same time, Wells’s group was also publishing on other ways to improve eyewitness identifications, such as instructing the witness in an unbiased way, using double-blind administrators, and picking appropriate “filler” photos— photos that all share whatever physical characteristics were noted by the witness, such as race and hair color. These findings gradually seeped into official policies. In 2001, the New Jersey attorney general mandated each of these lineup reforms, and a dozen other states have implemented similar changes. Many local agencies adapted their policies voluntarily. A 2011 survey found thatroughly one-third of police departments were using the sequential lineup method.

Read the full piece in The Atlantic.