Witnesses to crimes and other forensically relevant events sometimes describe memories in which they express confidence that, on the basis of research in perception and memory, may be inappropriately high, given the circumstances of the case. The most effective way of conveying this observation and the reasons for it to a jury is via an expert in perception and memory. In any given case, a mosaic of factors having to do with perception and memory are relevant. I briefly sketch the most common of these factors and categorize them as to the degree to which they may be unambiguously and/or quantitatively applied to a specific case. I discuss one such factor, witness-object distance, in some detail, showing how an expert might describe distance effects on perception and providing examples from actual cases.
On August 23, 1995, in Newark, New Jersey, Errich Thomas was shot and killed during a robbery of his convenience store. As the killer fled the store, he was observed by a witness, Patricia McKinnis, from the porch of her home, 271 ft away. Later, following a complex police investigation, Darrell Edwards was arrested for the crime. Despite the testimony of another eyewitness, Steven Blevins, who viewed the crime from close up and swore that the killer was not Mr. Edwards, Mr. Edwards was convicted on the basis of Ms. McKinnis's identification of him. Mr. Edwards has since languished in a prison cell, asserting his innocence, while his defense, now orchestrated by the New-York-based Innocence Project, winds through a labyrinthine appeals process.
Intuitively, Mr. Edwards's conviction seems wrong- very wrong. Even putting aside Mr. Blevins's favorable testimony, it appears obvious that Ms. McKinnis's distance from the killer, 271 ft, was too far for her to be able to see him clearly enough to be able to identify him. Why did the jury not realize this?
The reason, essentially, is that during the trial, there was no one to point out to the jury that a witness's perception of a person from that distance is extremely poor. What was emphasized to the jury, and what evidently underlay their verdict, was Ms. McKinnis's highly confident identification of Mr. Edwards, which evidently overrode any reservations they may have had about distance problems. Such a situation cries out for an expert, versed in the workings of human cognition, to provide information about (1) the perceptual consequences of seeing a person 271 ft away and (2) the reliability of a high-confidence identification following such a view. The remainder of this article concerns the degree to which such an expert can assist a jury when issues of perception and memory form a significant part of a legal case. Although I will focus on criminal cases here, the issues that I will describe apply to both criminal and civil cases.
"I'll Never Forget that Face"
A prosecutor's delight and a defense attorney's bane is an old cliché: the eyewitness who points to the defendant and utters some variant of "That's him! That's the person who I saw commit the crime! I'll never forget his face as long as I live!"
Both common sense and empirical evidence (e.g., Penrod & Cutler, 1995) tell us that a confident, in-court identification of this sort is quite persuasive to a jury. Accordingly, a central issue discussed by an eyewitness expert is that, contrary to common sense, a confident witness need not be accurate. This issue is coming to the attention of judicial authorities, as exemplified by a 2001 memo from New Jersey Attorney General John Farmer to state law enforcement agencies, in which he emphasized the importance of guarding against identification procedures (most specifically, lineup and showup procedures) that may invest a witness with a false sense of confidence. Farmer noted that "studies have established that the confidence level that witnesses demonstrate regarding their identifications is the primary determinant of whether jurors accept identifications as accurate and reliable. …