If there were a piece of scientific evidence that had a 30% failure rate, we would not let it into court. Polygraphs, which have been shown to be accurate between 90 and 95% of the time, are inadmissible. The justice system has decided that juries will give too much weight to the information. Yet eyewitness testimony, in study after study, has shown to have high error rates, but is not only let into court, it is frequently the determining factor in a jury’s decision.
The rules of evidence are quite complicated. The central rule is that the evidence has to be relevant. After that, most rules are designed to keep juries from letting the wrong factors determine their decision. In other words, the evidence is relevant, but due to human nature, a jury either will not hear it at all or a judge must carefully limit the evidence in order to prevent that particular piece of evidence from being the only thing a jury considers.
Studies have shown there is no correlation between how confident a witness is and how accurate their memory actually is. But a confident witnesses is a more believable witness, so prosecutors encourage witnesses to state a high degree of certainty to a jury.
Most people overestimate their own ability to remember things, and likewise overestimate other people’s memories. False memories tend to accord to our philosophy and world view. As a recent slate article put it, “The scary part is that your memories have already been altered. Much of what you recall about your life never happened, or it happened in a very different way. Sometimes our false memories have done terrible things. They have sent innocent people to jail.”