Police interview a man who just witnessed a shooting.  Months later, once the trial begins, the story he tells the jury has changed.  Drastically.  Is the eye witness lying under oath? Or, have his memories been changed?

Eyewitness identification expert, Dr. Jennifer Dysart, says it’s unlikely that the eyewitness is lying, but that his memory has been altered or ‘contaminated’ during the course of pre-trial preparation… and time.

Memory contamination happens as an individual adds new information to her memory of an event.  It typically happens without the person realizing her own recollection is mutating.

“In general, we’re not very good at remembering the context or the source of where we learned information,” Dysart told us as she waited to testify in a Shelby County trial. “So when an eyewitness is trying to recall whether they remember something from a crime or if they remember something from talking to other witnesses or seeing something in the newspaper or seeing a photograph on television or seeing something in court, it’s very, very difficult for the witness to be accurate when they are trying to remember, ‘How do I know that? How do I have this information essentially in my head?’”

Dysart says an eyewitness’ memory can be contaminated if law enforcement, prosecutors or defense attorneys bring him back to the crime scene, show him photos of the defendant in a different context, share evidence or other information he would not know otherwise, or even provide additional details about the defendant that might change his view of the accused.

Given that our memories can be so easily influenced by outside information, Dysart says it is inevitable that some memory contamination will occur.  That’s why she believes the best testimony is the first.

“Generally speaking the most accurate report, the closest to the person’s real memory of the event, is the report that happens first or right away. That is the way in which memory works – our memories are subject to contamination over time.”

In our daily lives, this inability to recall accurately may mean mistaken names or story details, but the stakes are much higher in the criminal justice system where false eyewitness identification can put innocent people behind bars… or even to death.

Dysart says of the 305 wrongful convictions overturned by DNA evidence in the U.S., false identification of an innocent person is the most common factor among wrongful convictions.  In fact, 75% of those cases involved one or more eyewitnesses mistakingly identifying an innocent defendant. Eighteen of those wrongfully convicted people had been sentenced to death.

See the Innocence Project for more information about wrongful convictions. 

Dysart argues that because the initial statement made by a witness is most accurate, it’s crucial that law enforcement do a thorough job of interviewing the witness before showing him photographs of the suspect or the crime scene. She adds that once the initial interview is complete, law enforcement, prosecutors, and defense attorneys must view the witness’ mind like a ‘crime scene’ — taking great care not to tamper with this critical piece of evidence.

“Think about memory as ‘trace evidence,’ forensic evidence in which it’s very important to make sure that it is preserved and collected correctly,” said Dysart. “You don’t want first responders running all over the crime scene.  We want to make sure we don’t contaminate the crime scene, the evidence.  For the eyewitness, that evidence is essentially in their head.  So it’s just as important that law enforcement doesn’t do anything, prosecutors don’t do anything, the media don’t do anything that potentially could contaminate the evidence.”

Jennifer Dysart, Ph.D

Jennifer Dysart, PhD, is an Associate Professor of Psychology at John Jay College of Criminal Justice in New York City. She has been conducting research on eyewitness identification for more than 14 years and has been an eyewitness expert for both state and federal courts.  She is currently serving as an eyewitness expert in the Memphis trial of Timothy McKinney, a man being tried for the third time in the murder of an off-duty police officer.  For a national perspective on the case and more about eyewitness accounts in this case, read this article published in The Nation, April 8, 2013.