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Eyewitnesses

The Cardozo Law School Innocence Project Web site regularly reports new cases in which individuals who have been convicted of crimes are exonerated through DNA testing (see http://www.innocenceproject.org). The Web site reveals that about two thirds of these cases involve mistaken eyewitness identification. This is no surprise to anyone familiar with research on eyewitness reliability. Studies documenting the role of mistaken identifications in erroneous conviction date back 70 years and have revealed that mistaken identifications were involved in more than 60% of the hundreds of cases examined by researchers. This rate is especially noteworthy given that eyewitness cases probably constitute a small proportion of all cases.

Although the recent DNA exculpation cases give new emphasis to research on the pitfalls of eyewitness evidence, research on the psychological aspects of eyewitness testimony actually has a long history. Harvard professor Hugo Munsterberg, in his volume On the Witness Stand (1908), criticized the legal profession for its ignorance of research in disciplines other than law and particularly noted research on factors that influence eyewitness reliability. Munsterberg's ideas were ridiculed as premature by legal scholars, but other psychologists made further contributions to the study of eyewitness reliability during the first few decades of the century.

Despite this promising beginning, the 1940s and 1950s were unproductive, but the 1960s marked the beginning of new research activity, and by the mid1970s, scientific eyewitness research was fully rejuvenated. In the past 35 years, the volume of research has mushroomed. This research generally uses experimental methods in which witnesses are exposed to staged or videotaped events and their memory for these events and identification accuracy are tested systematically. This research has identified a wide variety of factors that can influence eyewitness perceptions, the nature of the information that is encoded into memory, the retention of this information, and the later retrieval of this information. Some notable findings are summarized here.

Perception and Estimation Problems

Although our sensory systems work well—for example, we can detect a candle flame at 30 miles on a dark and clear night—there are clear limitations to our perceptual abilities—for example, reliable identifications of faces seem to require illumination levels higher than those of an urban street with bright street lights at a distance less than 50 feet. Classic and modern studies also demonstrate that people have significant difficulty estimating the ages, heights, and weights of other people; distances; and the duration of events.

Event Characteristics

These factors include the frequency and length of exposure to a target person, the seriousness of the event, the presence of a weapon, and event stressfulness.

  • Frequency. Early research underscored how repetitive exposure to information would improve memory, and modern research has demonstrated that repeated exposure to a target person improves identification accuracy.
  • Exposure Time. Early researchers noted that witness accuracy should improve as the duration of the target viewing increased—a notion confirmed in more recent research.
  • Seriousness of the Event. There is evidence that the seriousness of events influences observers at the time of encoding—perhaps because witnesses give more serious events greater attention. Even if the seriousness of an event is not revealed to witnesses until after the fact, it can promote higher identification rates as compared to less serious events.
  • Presence of a Weapon. Many studies have confirmed the theory that the presence of a weapon attracts the attention of witnesses away from the characteristics of a perpetrator and impairs witness performance.
  • Event Stressfulness. Events clearly vary in the extent to which they are arousing or stressful, and it has long been speculated that high levels of stress would impair identification accuracy. Recent analyses confirm that heightened levels of stress can undermine witness performance.

Eyewitness Characteristics

Eyewitness characteristics can be stable witness characteristics, such as gender and race, or they can be more transient characteristics.

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