Studies have shown, for example, that if the police who conduct the identification procedures have knowledge of the case and its suspect, they will inevitably influence the eyewitness's memory of the perpetrator in the direction of identifying the suspect. A "double blind" investigation, where both the interrogator and the witness are unaware of the facts of the investigation or the identity of the suspect, is far more likely to produce a reliable identification.

The acquisition of information into memory involves a three-step process. At each stage of the process, errors are possible. During acquisition, the first step in the memory process, an event is perceived and information "bits" are initially stored in memory.
In the second stage, information is held or retained in memory. In the final stage, memory is searched and pertinent information is retrieved and communicated. In the acquisition stage, information is "encoded" into a person's memory system. However, every detail of an experience is not encoded; the human mind can only process a fraction of the rapidly incoming physical stimuli. Both consciously and unconsciously, the observer determines which details are actually encoded according to where his or her attention is focused.
The physical aspects of an event are obviously compromised by the selective nature of the acquisition stage of memory. However, matters are further complicated by the fact that acquisition also involves a social component. Thus, a witness' ability to perceive accurately is affected by both event factors—those inherent to the event itself—and witness factors—those inherent to the witness.

Several studies have been conducted on human memory and on subjects’ propensity to remember erroneously events and details that did not occur. Elizabeth Loftus performed experiments in the mid-seventies demonstrating the effect of a third party’s introducing false facts into memory.4 Subjects were shown a slide of a car at an intersection with either a yield sign or a stop sign. Experimenters asked participants questions, falsely introducing the term "stop sign" into the question instead of referring to the yield sign participants had actually seen. Similarly, experimenters falsely substituted the term "yield sign" in questions directed to participants who had actually seen the stop sign slide. The results indicated that subjects remembered seeing the false image. In the initial part of the experiment, subjects also viewed a slide showing a car accident. Some subjects were later asked how fast the cars were traveling when they "hit" each other, others were asked how fast the cars were traveling when they "smashed" into each other. Those subjects questioned using the word "smashed" were more likely to report having seen broken glass in the original slide. The introduction of false cues altered participants’ memories.

"Contrary to the popular conception that most people would never forget the face of a clearly seen individual who had physically confronted them and threatened them for more than 30 minutes, a large number of subjects in this study were unable to correctly identify their perpetrator," said Charles Morgan III, M.D., associate professor of psychiatry at Yale School of Medicine.

The study included 509 active duty military personnel enrolled in survival school training. The types of stress were modeled after experiences of military personnel who had been prisoners of war (POWs) -- food and sleep deprivation for 48 hours followed by interrogation.

There were two instructors in the room, a "guard" and an "interrogator." The high stress interrogation included physical confrontation. During the low stress interrogation, the interrogator tried to trick the subject into giving away information.

Twenty-four hours after being released from the mock POW camp, the military personnel were asked to identify the interrogator and guard in a live line up, a photo spread, and a sequential photo presentation. Regardless of the presentation, recognition was better during the low stress rather than the high stress condition. In some cases, those interrogated confused even the gender of the guard and/or interrogator.

"The present data have a number of implications for law enforcement personnel, mental health professionals, physicians, attorneys and judges," Morgan said. "All professionals would do well to remember that a large number of healthy individuals may not be able to correctly identify suspects associated with highly stressful, compared to moderately stressful, events."

The latest form of evidence to come under scrutiny is eyewitness testimony. Psychologists have long known about the fallibility of human memory. As far back as 1971, England’s Criminal Law Review Committee warned that over-reliance on eyewitness testimony could lead to false convictions. Going back even to the 1800s, famed psychologist Hermann Ebbinghaus’s memory research established the “Forgetting Curve,” which plots how human recollection fades over time, beginning within minutes of the creation of a memory.
Nevertheless, eyewitness testimony remains a vital part of the criminal justice system, and with good reason. It’s the most abundant form of evidence, and it would be nearly impossible to convict guilty people without it. The problem is that it has for far too long been used irresponsibly, without instituting proper controls to ensure that eyewitnesses aren’t prodded into false recollections, that jurors aren’t permitted to give eyewitnesses more weight than good science allows, and that jurors are made aware of the limits and fallibility of human memory.

BRAIN connections that encourage the formation of false memories have been identified. Such memories appear to be more likely in people with high-quality links between neurons in a particular brain area.

Individuals often recall the same events differently or report memories of things they should have been too young to recall. To find out if a tendency to manufacture false memories is reflected in brain structure, Lluis Fuentemilla at the University of Barcelona in Spain and colleagues induced them in 48 students in the lab.

Stress makes people much more likely to create false memories, say American researchers. It also appears to make them more certain that these false memories are correct.

The results could help explain why crime witnesses give conflicting evidence or pick the wrong man in a line up, the researchers say. But they do not account for "reconstructed memories" of childhood abuse.

Psychology student Jessica Payne of the University of Arizona, Tucson and colleagues read to 66 college students a list of 20 words. All related to sleep, such as "bed", "rest", "tired" and so on.

When asked a few minutes later if they heard the word "hat", they nearly always say no. But when asked if "sleep" was on the list, 60 per cent say yes, even though it wasn't.

Such false memories have been demonstrated before. "We wanted to know if stress would increase the rate of false memories," says Payne.

So the researchers then repeated the experiment. But before taking the test, half the students had to give an impromptu presentation under glaring light with a video camera rolling. "It was very stressful," says Payne.

The stressed group thought "sleep" was on the list nearly 80 per cent of the time. They did not pick unrelated words like "chair" as being on the list any more often then the relaxed controls.

Even more surprising was how fast the stressed students clicked the "yes" button, says Payne. Unstressed subjects usually took a tenth of a second longer before saying yes to "sleep". "It's as if they weren't quite sure," she says.

But stressed students clicked yes equally fast for all their answers, as if they were just as sure of the false memory as of the real ones.

Stress hormone
The hippocampus, a brain structure needed to form new memories, is riddled with receptors for the stress hormone cortisol. Payne believes cortisol may cause the effect by suppressing the hippocampus.

The thematic nature of the false memories suggests people might tend to let stored associations influence their recall of a stressful event. It might be possible for memories of a crime scene to be influenced by beliefs about the sorts of people who commit crimes, Payne says. But a more direct experiment is necessary to be sure.

However, the results do not deal with recovered memory syndrome, in which adults become convinced they were abused as children, Payne says. This type of false memory only involves adding related details to real events.