Eyewitness identifications greatly sway both police and juries. As the Thomson example illustrates, an eyewitness identification can even outweigh a strong alibi supported by other testimony. This is sometimes unfortunate because eyewitness memory is highly fallible. Memory errors fall into two classes: people can 1) either completely fail to recall an event or 2) have an inaccurate recollection. People have very different attitudes about the two types of failure. Most people understand that total memory failures are common. They can introspect about occasions when they have been unable to recall an event, so failures by other people are hardly surprising. In contrast, people are overly optimistic about the accuracy of their retrieved memories, probably because most errors have little practical consequence and go unnoticed. Given the confidence in their own memory accuracy, people have too much faith in the accuracy of eyewitnesses. Memory has a multitude of quirks and inaccuracies that creep into its everyday operation. Here, I describe some basic background on memory and on the types of memory distortions that are common. Types of Memory
It is more accurate to speak of human memories rather than of human memory, since people have several distinctly different types. The basic division is among sensory, short-term and long-term memories. Each of these memories further consists of subsystems. There is a separate sensory memory for each sense, iconic (visual), echoic (auditory), etc. Some also distinguish a "working memory" consisting of separate executive, phonological loop and visuo-spatial subsystems. Most matters involving eyewitness testimony depend on accuracy of long-term memory, which has at least two subsystems, implicit and explicit memory. Implicit memory stores things that you don't consciously know, like how to peddle a bike. You just get on the thing and start peddling without conscious thought. Explicit memory stores things that you can consciously verbalize. Explicit memory further subdivides into semantic and autobiographical types. Semantic is memory for facts. For example, you know that George Washington was the first president of the United States but probably don't remember the exact circumstances when you learned that fact. Similarly, you may remember the gist of a conversation that you had a year ago, but don't remember the exact words that were used. Introspectively, semantic memory is more like "knowing" than like recalling. It's not so much that I recall George Washington being the first president as I know that George Washington was the first president. In contrast, autobiographical memory is recollection of events or episodes in you life. You remember exactly what was said and the actual, physical details. In most situations, eyewitnesses are asked to report information that is stored in autobiographical memory. Below, I discuss some of the many factors that cause unreliability in autobiographical memory. It is impossible, however, to get far in discussing memory, however, without first dispelling the homunculus fallacy. Most people intuitively imagine that perception and memory work something like this: the eyes are TV cameras that project a picture of the world to an inner screen. In the head, there is a little man, the "homunculus," who views the screen and perceives the world. Memory is simply a videotape recording of what we have seen on the screen. To remember, we simply rewind the tape, and the little man sees the pictures again. Of course, there is no little man, no screen and no videotape. The fallacy of this model should be obvious: Who is seeing the image in the head of the homunculus? There would have to be another, smaller homunculus in the head of the first homunculus and so on in infinite regress. Causes of Memory Unreliability
1. Memory is "blurred"
There are several reasons for this. One is that images in our...