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.
Elizabeth Loftus, a law professor at University of California at Irvine, said of the Zimmerman case, "There was so much media coverage that it could be that the new information that Trayvon Martin had died, that he was unarmed, the new information about the outcry and outrage, had the potential for contaminating and distorting the witnesses' memory."
Although the eyewitnesses' earlier accounts of that fateful night — descriptions that were generally less damning of Zimmerman — might be closer to the truth, they aren't what the jury will hear during the trial.
On the witness stand
According to Clifford Fishman, a law professor at the Catholic University of America, a jury tends to accept as accurate the version of events a witness describes when testifying — "even if that version is inconsistent in many significant ways with that witness's earlier statements." In other words, legally, the later versions of memories reign supreme. The burden falls to the defense attorneys to point out the weaknesses or inconsistencies in the witness's testimony at trial, and to cast doubt in the minds of the jurors about the accuracy of the witness's descriptions. [What is Reasonable Doubt?]
The prosecutors can retaliate by disputing the witness's earlier accounts; to do this, they might demonstrate that the police had initially asked leading questions or otherwise influenced the witnesses' statements in a way that made them more sympathetic of Zimmerman. In that case, the witnesses' later descriptions could be proved more trustworthy.
Furthermore, it could be that later additions to accounts, as with the witness who initially did not know whether she had seen Zimmerman or Martin on top during their fight, but later named Zimmerman, it could be that her fuller memory really had been triggered by seeing Zimmerman on television. "Appropriate cues given afterwards can retrieve additional reliable information that was not originally retrieved," Tversky noted.
Either way, Loftus said, "It looks like these witnesses are going to have some explaining to do. If they use version two of their stories in court, one attorney or the other is going have access to version one. And that can make the case against Zimmerman weaker."
This story was provided by Life's Little Mysteries, a sister site to LiveScience. Follow Natalie Wolchover on Twitter @nattyover. Follow Life's Little Mysteries on Twitter @llmysteries, then join us on Facebook.