Are Eyewitnesses in the Zimmerman Trial Reliable?

Newly released court documents in the second-degree-murder case against neighborhood watch volunteer George Zimmerman reveal that, in the month following his fatal shooting of Trayvon Martin on Feb. 26, four key witnesses significantly changed their accounts of what they saw and heard that night. The more recent versions of their memories tend to be more damning of Zimmerman than their initial statements.

Which raises the question: How reliable are eyewitnesses?

For example, days after the shooting, one woman told police she had seen two men running through the streets and then engaging in a fistfight. Less than one month later, she told investigators she saw just one person running, and couldn't describe his or her appearance as she hadn't had her contact lenses in at the time.

Another witness, who was initially interviewed March 20, said she saw two people on the ground immediately after the shooting but was not sure which one was on top. In another interview with investigators six days later, she said it had definitely been Zimmerman on top, explaining that she was sure because she had been able to compare Zimmerman's and Martin's sizes after seeing them on TV.

A third witness originally said he saw a black man (presumably Martin) pinning down and punching a lighter-skinned man (Zimmerman) who was calling for help. Later, the witness wasn't sure any punches were thrown or that he had heard distress calls.

And a man who initially described Zimmerman as looking bloody and in shock after the shooting later implied the shooter had been calm, cool and collected.

Why do people's memories change over time? And which versions of these witnesses' stories are to be believed ? the earlier ones untarnished by time, or the later ones, perhaps less sullied by what may have been false impressions imbued early on by news reports or police interviewers?

The reliability of witness testimony is a vastly complex subject, but legal scholars and forensic psychologists say it's possible to extract the truth from contradictory accounts and evolving memories. According to Barbara Tversky, professor emerita of psychology at Stanford University, the bottom line is this: "All other things equal, earlier recountings are more likely to be accurate than later ones. The longer the delay, the more likely that subsequent information will get confused with the target memory."

However, in some cases not all other things are equal.

How we remember

Memory is a reconstructive process, says Richard Wise, a forensic psychologist at the University of North Dakota. "When an eyewitness recalls a crime, he or she must reconstruct his or her memory of the crime." This, he says, is an unconscious process. To reconstruct a memory, the eyewitness draws upon several sources of information, only one being his or her actual recollection.

"To fill in gaps in memory, the eyewitness relies upon his or her expectation, attitudes, prejudices, bias, and prior knowledge. Furthermore, information supplied to an eyewitness after a crime (i.e., post-event information) by the police, prosecutor, other eyewitnesses, media, etc., can alter an eyewitness's memory of the crime," Wise said in an email. [How Are Memories Stored in the Brain?]

That external input is what makes eyewitness testimony so unreliable. Eyewitnesses are generally unaware that their memory has been altered by post-event information, and feel convinced they're recalling only the incident itself. "Once an eyewitness's memory of the crime has been altered by post-event information, it is difficult or impossible to restore the eyewitness's original memory of the crime," Wise told Life's Little Mysteries.

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