Eyewitness Accuracy
As a result of human information processing limitations, a person may be mistakenly identified as a criminal suspect by an eyewitness. The crime location may have been too dark, or the encounter may have been too brief for the eyewitness to accurately perceive the perpetrator. In addition, people tend to overestimate the duration of brief events, while they underestimate the duration of prolonged events (Penrod, Loftus, & Winkler, 1982). Other psychological research has demonstrated that the presence of a weapon also reduces the accuracy of eyewitness accounts (Loftus, Loftus, & Messo, 1987). This "weapons focus effect" appears to be the result of the observer's attention being directed toward the weapon, thereby diverting attention away from situational aspects and the perpetrator (Kramer, Buckhout, & Eugenio, 1990).
In addition to perceptual errors, eyewitness accuracy may be reduced by errors that occur within the memory process. Memory can be divided into three stages: encoding, storage, and retrieval. Errors may occurs at any of these stages. The acquisition of information is referred to as encoding by cognitive psychologists. According to the Yerkes-Dodson principle, information is encoded best when a person is moderately aroused. At this level of arousal, a person's attention is focused and information is acquired well. Extreme arousal, or stress, however, causes information to be lost or encoded inaccurately.
At the second stage of memory, storage, acquired stimulus information is stored. However, both interference and decay can reduce the accuracy of stored information. Decay refers to the loss of stimulus information due to the passage of time. The amount of time that has lapsed between the witnessing of the crime and the subsequent questioning of the eyewitness can determine the amount of information that the eyewitness recalls (Lipton, 1977). Secondly, interference, which refers to the loss of old stimulus information due to interference caused by new stimulus information, can also reduce the accuracy of eyewitness accounts. Gorenstein and Ellsworth (1980) found that after viewing mugshots, the accuracy of eyewitnesses' ability to recognize faces viewed before the mugshots was reduced. Moreover, questions that eyewitnesses are asked after the crime have been found to interfere with information that eyewitnesses acquired during the crime. In one study, subjects were shown a film of an automobile accident and then asked a series of follow-up questions about the accident (Loftus, 1979). A misleading follow-up question contained information about a barn, which was actually not present in the film. One week later, 17% of subjects erroneously reported seeing a barn in the film. In effect, the misleading question incorporated new and inaccurate stimulus information in the memory store.
Retrieval, the final stage in the memory process, involves the recall of information from the memory store. Yet, recall may be influenced by the types of questions that eyewitnesses are asked. One study found that language can influence the retrieval of stimulus information (Loftus & Palmer, 1974). In this study, subjects were shown a film of an automobile accident and then asked a series of follow-up questions. Participants who were asked how fast the cars were going when the cars "smashed", reported an average speed of 40.8 mph. However, participants who were asked how fast the cars were going when the cars "contacted" reported an average speed of 31.8 mph. Suggestive questioning procedures, therefore, should be eliminated as much as possible to minimize their effect on eyewitness accuracy. Another retrieval phenomenon, known as unconscious transference, refers to the generation of memory that is related to an incident, but, is not relevant to the issue being considered. For example, a bank teller who has been robbed, may mistakenly identify a one of his regular customers as a suspect. This phenomenon was demonstrated by Robert Buckhout (1974) when he staged a mock assault in front of 141 unsuspecting college students. Seven weeks later, these students were asked to pick the perpetrator from a group of 6 photographs. Of the 60% who did not correctly identify the assailant, 2/3 incorrectly chose an innocent bystander who was at the crime scene.
There are several other phenomenon that have been examined for their effect on eyewitness memory. Some of these include exposure to misleading post event information, lighting conditions, stress, weapon focus, individual differences, cross-race effects, exposure duration and more. For further reading, I reccomend Mistaken Identification, The Eyewitness, Psychology and the Law (Cutler & Penrod, 1995) and Adult Eyewitness Testimony (1994).
References
Buckhout, R. (1974). Eyewitness testimony. Scientific American, 231, 23-31.
Gorenstein, G.W., & Ellsworth, P.C. (1980). Effect of choosing an incorrect photograph on a later identification by an eyewitness. Journal of Applied Psychology, 65, 616-622.
Kramer, T.H., Buckhout, R., & Eugenio, P. (1990). Weapons focus, arousal, and eyewitness memory: Attention must be paid. Law and Human Behavior, 14, 167-184.
Lipton, J.P. (1977). On the psychology of eyewitness testimony. Journal of Applied Psychology, 62, 90-95.
Loftus, E.F. (1979). Eyewitness testimony. Cambridge, MA: Harvard University Press.
Loftus, E.F., Loftus, G.R., & Messo, J. (1987). Some facts about "weapons focus." Law and Human Behavior, 11, 55-62.
Loftus E.F., & Palmer, J.C. (1974). Reconstruction of automobile destruction: An example of the interaction between language and memory. Journal of Learning and Verbal Behavior, 13, 585-589.
Penrod, S.D., Loftus, E.F., & Winkler, J. (1982). The reliability of eyewitness testimony: A psychological perspective. In N.L. Kerr & R.M. Bray (Eds.), The psychology of the courtroom. Orlando, FL: Academic Press.
Wrightsman, L.S., Nietzel, M.T., Fortune, W.H. (1994).
Psychology
and the legal system. Belmont, CA: Wadsworth.