You see a man grab a woman's purse. You stare at him, memorizing the details as he runs away: blue hat, average height, dark jacket, black-and-white sneakers. Brown hair? Brown hair.
You feel confident that you could identify him again, you tell the responding police officer: "I saw him with my own eyes."
But just what did you see and how much detail will you remember?
Not as much as you might think.
Witnessing a crime doesn't lead to a "flashbulb" memory, said Gary L. Wells, who has studied witness identification since 1975. Wells, a psychology professor at Iowa State University, will speak at 2 p.m. Thursday, March 21, at Black Hills State University (see box).
"We now know that things like stress and fear actually interfere with the formation of detailed memories," he said. "You'll never forget that it happened, but remembering the details is a whole other story."
Wells also is the director of social science for the American Judicature Society's Institute of Forensic Science and Public Policy, with a research interest in the integration of cognitive and social psychology as it relates to the law. He has completed extensive research on lineup procedures and the accuracy of witness identification. He is consulted by court officers in federal and state criminal cases regarding witness memory, investigation procedures and evidence evaluation.
But his studies, conclusions and proposals were a lonely business in the 1970s and well into '80s — "I'm not sure that anyone really fully accepted the premise," Wells said — until DNA testing revolutionized forensic investigation in the late '80s.
Mistaken identification has been a factor in more than 75 percent of the 179 DNA-exonerated cases across the nation since 1989, he said.
At a crime scene, lighting conditions, distance and distractions can cloud a witness' memory.
"You've got simple things that have huge effects: a person wearing sunglasses or a hat, a disguise that doesn't look like a disguise," Wells said. "If somebody's got a weapon, your attention goes to that," he added, noting that witnesses typically focus on what the perpetrator is wearing rather than more permanent distinguishing characteristics, such as a scar or mole.
While the memory is being retained, other factors can intrude on its preservation, he said.
"People will go back and, without realizing it, fill in details. The filling-in process is not really memory," Wells said, but more like manufactured recall. "Somebody says something about a mustache and then when you go back to try to remember, you're putting a mustache on it."
Police lineups — "the ultimate eyewitness evidence" — can be problematic as well, he said.
"You're trying to recognize a person you (may have seen) only briefly under sub-optimal conditions," he said. "People believe they should be able to do that. They go into that (lineup procedure) pretty motivated that they can help solve this crime."
The tendency of most witnesses is to make a relative judgment, Wells said, choosing the person who looks most like the memory of the person they remember — even if the suspect is not in the lineup.
"It's harder to recognize the absence of a perpetrator than anything else. If the real person is not there, somebody's at risk," he said.
Capt. James Johns, head of investigations for the Rapid City Police Department, said people do witness things differently.
“From our point of view, eyewitness testimony, or a pick out of a lineup, is only one piece of the puzzle,” Johns said. Witness testimony must be corroborated and backed-up, he said, by solid police work.
Johns said he could not think of a case where a lineup was the only evidence used to identify a suspect.
“We like to use that as a lead and then use the hard evidence to corroborate what the witness is telling us,” Johns said. “There has to be a balance.”
Complicating matters can be lineups of suspects who don't fit the description or the failure to warn the victim or the witness that the person who robbed her might not be in the room. In fact, most of the DNA-exoneration cases have occurred because the actual perpetrator was not present to be identified, Wells said.
That's why he said he has focused much of his research on giving law enforcement the tools they need to help eliminate the number of mistaken-identity cases.
"They can choose to give that warning prior to the lineup, or not. They can choose to use 'good lineup members' who all fit the description, or not," he said. "I don't think the problems we've been looking at in these cases are in any case intentional. They didn't really know how fragile memory is and how carefully you have to construct these identification tests."
Wells said he also helps train law enforcement officials to refrain from giving any feedback to witnesses to avoid nudging the witness toward or away any particular suspect.
"Give someone confirming feedback on a mistake and suddenly ... they claim they had a better view than they really did; they were paying better attention; they're more certain," he said. "Witnesses are looking for cues."
“You want to be careful," Johns said, "because you don’t want to interject something that occurred and then it becomes the person’s memory."
The Rapid City Police Department is careful about how it conducts lineups, he said.
“We don’t prejudice our lineups by encouraging people to pick someone out,” Johns said. “It’s either there or it’s not.”
When DNA-exoneration cases first began to unfold, Wells said, authorities were just as surprised as witnesses that the supposed positive identifications were wrong.
"They were positive, but (they were also) mistaken," he said. "They're shocked when DNA evidence proves they were wrong. There are still a number of victim-witnesses who still can't really accept that. They think back to that crime scene and they see the person they identified. I'm not talking about anything sinister, just natural mistakes."
In his talk at BHSU, Wells said he would focus on the kinds of reforms that have taken place. He has worked with law enforcement officials in Ohio, North Carolina, New Jersey, Connecticut and in Washington, D.C., to implement some of his methods into their policing systems.
"There's actually been a lot of improvement, a lot of change. But there's still a long way to go," Wells said. "It's a success story, but it's far from complete."