Reports from eyewitnesses play an important role in the development and propagation of both religious and paranormal beliefs. People are often ready to believe the personal reports of what others say that they have seen and experienced. Thus, it is important to consider just how reliable people's memory and their testimony can be.
Eyewitness Testimony & Criminal Trials
Perhaps the most important thing to note is that, even though there is a popular perception of eyewitness testimony being among the most reliable forms of evidence available, the criminal justice system treats such testimony as being among the most fragile and even unreliable available. Consider the following quote from Levin and Cramer's Problems and Materials on Trial Advocacy:
Eyewitness testimony is, at best, evidence of what the witness believes to have occurred. It may or may not tell what actually happened. The familiar problems of perception, of gauging time, speed, height, weight, of accurate identification of persons accused of crime all contribute to making honest testimony something less than completely credible. (emphasis added)
Prosecutors recognize that eyewitness testimony, even when given in all honesty and sincerity, isn't necessarily credible. Merely because a person claims to have seen something does not mean that what they remember seeing really happened - one reason why is that not all eyewitnesses are the same. To simply be a competent witness (competent, which is not the same as credible), a person must have adequate powers of perception, must be able to remember and report well, and must be able and willing to tell the truth.
Critiquing Eyewitness Testimony
Eyewitness testimony can thus be critiqued on several grounds: having impaired perception, having impaired memory, having inconsistent testimony, having bias or prejudice, and not having a reputation for telling the truth. If any of those characteristics can be demonstrated, then the competency of a witness is questionable. Even if none of them apply, though, that does not automatically mean that the testimony is credible. The fact of the matter is, eyewitness testimony from competent and sincere people has put innocent people in jail.
How can eyewitness testimony become inaccurate? Many factors can come into play: age, health, personal bias and expectations, viewing conditions, perception problems, later discussions with other witnesses, stress, etc. Even a poor sense of self can play a role - studies indicate that people with a poor sense of self have greater trouble remembering events in the past.
All of these things can undermine the accuracy of testimony, including that given by expert witnesses who were trying to pay attention and remember what happened. The more common situation is of an average person who wasn't making any effort to remember important details, and that sort of testimony is even more susceptible to error.
Eyewitness Testimony & Human Memory
The most important foundation for eyewitness testimony is a person's memory - after all, whatever testimony is being reported is coming from what a person remembers. To evaluate the reliability of memory, it is once again instructive to look to the criminal justice system. Police and prosecutors go to great lengths to keep a person's testimony "pure" by not allowing it to be tainted by outside information or the reports of others.
If prosecutors don't make every effort to retain the integrity of such testimony, it will become an easy target for a clever defense attorney. How can the integrity of memory and testimony be undermined? Very easily, in fact - there is a popular perception of memory being something like a tape-recording of events when the truth is anything but.
As Elizabeth Loftus describes in her book Memory: Surprising New Insights into How We Remember and Why We Forget:
Memory is imperfect. This is because we often do not see things accurately in the first place. But even if we take in a reasonably accurate picture of some experience, it does not necessarily stay perfectly intact in memory. Another force is at work. The memory traces can actually undergo distortion. With the passage of time, with proper motivation, with the introduction of special kinds of interfering facts, the memory traces seem sometimes to change or become transformed. These distortions can be quite frightening, for they can cause us to have memories of things that never happened. Even in the most intelligent among us is memory thus malleable.
Memory is not so much a static state as it is an ongoing process - and one which never happens in quite the same way twice. This is why we should have a skeptical, critical attitude towards all eyewitness testimony and all reports from memory - even our own and no matter what the subject, however mundane