Monday, April 9, 2007

Stress Effects on Eyewitness Recall

It's a recurring theme in eyewitness testimony: "My adrenaline was racing, so I'll never forget that face." Or, "I was terrified; his face is burned in my mind." As if fear is a veritable steroid for memory. Witness after witness reports the belief that there is a positive correlation between the level of stress of an event and the level of detail at which the witness recalls the identity of the perpetrator, as if the presence of stress heightens the witness's attention and, in turn, her ability to recall details later. A poll of potential jurors (PDF) conducted by the Public Defender Service for DC confirmed the prevalence of this commonly held belief.

It turns out that the opposite is true (DOC), but courts continue to consider evidence of stress in their analysis of the reliability of eyewitness identifications, as a factor that heightens reliability, rather than diminishes it. As one stark example, just last month, the U.S. District Court for the District of Massachusetts handed down this decision (PDF) in U.S. v. Loren Harty, in a firearm possession case.

In applying the Manson due process analysis for the reliability of an eyewitness ID, the court first considered the suggestivity of the identification procedure -- in that case, a field show-up. On that prong, it found "[t]hat elements of suggestiveness infected the showup in Harty’s case" to be "beyond doubt." Indeed, observed the court, "[t]he manner in which Harty was displayed beside the police cruiser braced between two uniformed officers would have made it clear to any reasonable witness that the police were convinced that they had their man." But on to the so-called "reliability factors," under Manson v. Brathwaite. In considering the witness's "opportunity to view," the court considered that the witness had been chased into a building, that the assailant had fired shots into the floor as the witness ran up the stairs and threatened to kill the witness. And how did this weigh on the reliability of the ID that sprung from this series of events? Astonishingly, the court found that "These are events that tend to focus a witness’s attention." (!)

Study after study (DOC) has confirmed a negative correlation between stress and the accuracy of eyewitness recall. That is, the more stress undergone by a witness at the time of an event, the lower her ability to recall the details of that event -- including the identity of the perpetrator.

One striking example of this negative correlation is worth bearing out in some detail. In a 2004 study by Charles Morgan, et al., researchers used a military "survival training" scenario to test the correlation between stress and accuracy of recall. Morgan et al., Accuracy of eyewitness memory for persons encountered during exposure to highly intense stress, Int'l J. of L. and Psych., 27, 265-279. The test subjects were active-duty military personnel, with an average age of 25 and average length of service of 4 years.

Each subject underwent both stressful and non-stressful interrogation room scenarios, after which the subject's ability to recall the identity of the interrogator was tested through various methods. In the stressful scenario, subjects were confronted face-to-face with an interrogator, in a well-lit room, for a full 40 minutes. When asked to select that interrogator that the subject had faced for 40 minutes in a small, well-lit room, only 34% were able to correctly identify that same interrogator from a photo lineup -- compared to 76% who were subjected to a similar, but low-stress scenario. Further, (in a different sample) 68% of the high-stress subjects selected someone other than the interrogator from the photo lineup, compared to only 12% in the low-stress scenario.

This study puts a pretty fine point on the effects of stress on eyewitness recall. If witnesses subjected to a 40-minute long interrogation session, in the most ideal, controlled viewing conditions can only accurately identify an interrogator 34% of the time, and similarly make a false identification 68% of the time, it seems abundantly clear that -- at the very least -- courts are going astray when they consider gunshots and homicidal threats as "events that tend to focus a witness's attention." These are events with a strong tendency to do quite the opposite, and the science is in on this one.

From the same paper, this sums it up:

Contrary to the popular conception that most people would never forget the face of a clearly seen individual who had physically confronted them and threatened them for more than 30 min, a large number of subjects in this study were unable to correctly identify their perpetrator. These data provide robust evidence that eyewitness memory for persons encountered during events that are personally relevant, highly stressful, and realistic in nature may be subject to substantial error.

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