Shadings Of Eyewitness Testimony:
Not All Weak Evidence Is Created Equal
17 February 1999

    Of all the classes of weak evidence, eyewitness testimony is probably the most abundant and the most cited. Therefore, it seems important to me to point out that it is not all equally weak, and that certain types of eyewitness testimony may be much stronger than other types. This brief essay explores some of the gradations of such testimony.
    None of the material here is original. All of it comes from the 1979 book "Eyewitness Testimony" by the dean of American experts on the subject, Elizabeth F. Loftus. This little essay simply summarizes a few of her main points.
    Loftus begins by noting that the mind does not record events in the neutral way that a tape recorder does. She describes in detail the three stages of recording events as acquisition, retention, and retrieval, each of which can be colored by conditions. Here are a few of her introductory paragraphs, with literature references omitted.

    "When we experience an important event, we do not simply record that event in memory as a videotape recorder would. The situation is much more complex. Nearly all of the theoretical analyses of the process divide it into three stages. First, there is the acquisition stage—the perception of the original event—in which information is encoded, laid down, or entered into a person's memory system. Second, there is the retention stage, the period of time that passes between the event and the eventual recollection of a particular piece of information. Third, there is the retrieval stage during which a person recalls stored information. This three-stage analysis is so central to the concept of the human memory that it is virtually universally accepted among psychologists.
    "When a complex event is experienced, some of the features of that experience are extracted first to be stored and later to be utilized in arriving at action decisions. Early on, in the acquisition state, the observer must decide to which aspects of the visual stimulus he should attend. Our visual environment typically contains a vast amount of information, and the proportion of information that is actually perceived is very small. The process of deciding what to attend to is broken down into an even finer series of decisions, each corresponding to where a person will make his next eye fixation.
    "Once the information associated with an event has been encoded or stored in memory, some of it may remain there unchanged while some may not. Many things can happen to a witness during this crucial retention stage. The witness may engage in conversations about the event, or overhear conversations, or read a newspaper story—all of these can bring about powerful and unexpected changes in the witness's testimony.
    "Finally, at any time after an event a witness may be asked questions about it. At this point the witness must re-create from long-term memory that portion of the event needed to answer a specific question. This re-creation may be based both on information acquired during the original experience and on information acquired subsequently. In other words, both the acquisition and the retention stages are crucial to what happens during retrieval. The answer that a person gives is based on this re-creation.
    "Any thorough analysis of the memory process must account for events during each of the three stages. One of the most critical problems in research on human memory is to account for a person's inability to retrieve information accurately. Events at any one or several of the stages can be the cause of this retrieval failure. The information may simply not have been perceived in the first place—failure at the acquisition stage. The information might be accurately perceived, but is then forgotten or interfered with during the retention stage. And finally, information may have been accurately perceived in the first place but may become inaccessible during questioning—a failure at the retrieval stage. It is usually a very difficult task to determine which stage is the source of failure."

Factors that influence perception are exposure time, frequency, salience of detail, type of fact, violence of the event, stress to witness, and expectations of the witness.

Exposure time—the longer the exposure, the better the event can be remembered.
Frequency—the more the event is repeated, the better it can be remembered.
Salience of detail—the more a detail catches our attention, the better it can be remembered.
Type of fact—some aspects of events are easier to perceive correctly than others. (Matters of time are particularly hard to remember accurately.)
Violence of event—the more violent the event, the harder it is to remember correctly.
Stress to witness—increasing stress first helps perception, then hinders it. The crossover point varies with the difficulty of the task.
Expectation of the witness—to a large extent, we perceive what we are conditioned to perceive.

Factors that affect retention include length of time after the event, postevent information, and intervening thoughts of witnesses.

Time since the event—the longer the time, the less reliable the memory.
Postevent information—hearing someone else discuss an event can plant false memories in a witness; learning of information that conflicts with a witness's memory can create a compromise memory or even introduce nonexistent objects into it; salient details are harder to affect than peripheral details; and false information can be deliberately introduced most effectively well after the event.
Intervening thoughts of witnesses—witnesses can inadvertently bend their memories in directions that are most advantageous for them; information that is verbally labeled is better remembered than information not so labeled; events guessed at early in the process are remembered more poorly than those never guessed at; events recalled early are retained better than those not thought of until later.

Factors that affect retrieval include the environment of retrieving, the type of retrieval, the wording of the questions, who is asking the questions and how, and hypnosis and recall.

The environment of retrieving—new environments inhibit recognition.
Type of retrieval—narrative reports have fewer errors but are less thorough; answering questions leads to more errors than providing details freely.
Wording of the question—leading questions result in more errors than neutral questions.
Who is asking the questions and how—persons of authority get longer answers but with the same accuracy; praise for a witness gets the same results as confrontation; the more confident a witness is about a response, the more accurate it usually is.
Hypnosis and recall—hypnosis often makes it easier for witnesses to fabricate information in a desire to please the questioner.

With so many things that can go wrong with memories, it's a wonder that anyone ever remembers anything right!