IV. Causes of Wrongful Convictions
Studies reveal several factors related to miscarriages of justice, labeled “causes,” although they are not so in a scientific sense. Typically, more than one factor is found in each wrongful conviction. Although a few wrongful convictions are caused only by honest witness error, most involve some level of negligence or malfeasance by criminal justice officers or defense lawyers. A troubling minority of cases involve perjury or knowingly dishonest action by forensic examiners, prosecutors, and police. Brief descriptions of the major factors related to wrongful convictions follow. Note that the list that follows is not comprehensive.
A. Eyewitness Identification
Mistaken eyewitness identification is the leading cause of wrongful convictions. It was involved in 79% of the first 200 DNA exonerations. Although an overall error rate for eyewitnesses is not well established, some experts place it at about 25%. This figure is the same as the proportion of DNA tests in rape cases conducted by the Federal Bureau of Investigation laboratory in which the DNA did not match the mistakenly identified suspect. The human memory does not record all information like a video recorder; it drops most information out of short-term memory and stores the central, but not peripheral, elements of those events in long-term memory. This makes facial recall somewhat uncertain. Events during a crime, such as extreme stress or focus on a weapon, decreases facial recall by victims and witnesses. In addition, unconscious transference can lead witnesses to superimpose the face of someone previously observed but not well-known onto the memory of the perpetrator. Memory is dynamic and can change during the recall stage from what was observed. Memory is also malleable and can change under the influence of suggestion. These and other factors show that eyewitness identification should be received with caution, and yet police, prosecutors, and especially jurors tend to rarely disbelieve eyewitness evidence.
Problems with eyewitness identification are made worse by flawed police procedures. Police have relied on “showups,” which is showing the suspect or the suspect’s photograph alone to the witness without a lineup. Courts rule that showups are suggestive and will exclude showup evidence unless one of several easy-to-produce factors is present. The showup exception factors are whether the witness paid attention, had a good opportunity to view the perpetrator, gave an accurate description, was certain, and viewed the showup shortly after the crime.
Even lineups are often flawed. Police are not always scrupulous in ensuring that the suspect does not stand out from the lineup fillers. Laboratory research shows that more errors occur when fillers are selected on the basis of their similarity to the suspect than on the basis of the victim’s description of the perpetrator. Police or prosecutors have at times suppressed the uncertain identification or nonidentification by one lineup witness while promoting the testimony of another. These and other elements often make lineups a less-than-optimal method of an accurate identification.
B. Forensic Science Error or Misconduct
Problems with expert evidence presented by forensic scientists or forensic examiners is the second leading cause of wrongful convictions; erroneous forensic evidence supported the convictions of 57% of the first 200 DNA exonerations. Forensic error and misconduct take a variety of forms, including problems inherent in the method, incompetent or untruthful experts, and substandard forensic laboratories.
Some expert evidence is based not on scientific testing but on comparisons that rely ultimately on the experts’ subjective evaluations. Some of these methods, such as fingerprints, bullet and tool mark examinations, and footprint and tire impressions, are relatively credible and accurate, but known errors have nevertheless occurred. If such expert evidence goes unchallenged by defense attorneys (by having other experts evaluate it), it is possible that honest but mistaken conclusions will lead to false convictions. Other kinds of expert comparison, such as handwriting analysis, are more subjective and require closer scrutiny. Even lower on the reliability scale are comparison methods that are so tentative that some label it “junk science.” Two such methods, microscopic hair analysis and bite mark impressions on skin, have caused numerous wrongful convictions. Hair analysis has now been largely replaced by DNA analysis, and bite mark evidence, although accepted in courts, has been subject to strong criticism.
Examiners have been known to err even where evidence is based on forensic science, which includes blood analysis (serology, which has been replaced by DNA analysis), drug analysis, forensic toxicology (the science of poisons), and organic and inorganic analysis of crime scene trace evidence. Even worse, in a few notorious cases forensic examiners have been exposed as pathological liars who always testified to benefit the prosecution, even when no tests were conducted. In addition to outright falsification, forensic experts can mislead courts and juries by overstating the strength of their findings, reporting inconclusive reports as conclusive, failing to report conflicting results, and the like. When expert witness perjury has been exposed, state criminal justice systems have had to reopen hundreds of cases to ensure that they did not result in wrongful convictions. Some specialized arson investigators have relied on incorrect or outdated fire science to report that fire and burn patterns were evidence of arson when this was not true.
Finally, even the most reliable methods can produce incorrect results if the forensic laboratories are substandard. As DNA testing becomes more sensitive, the risks of contamination rises unless the laboratories are in pristine condition. Testing in some inferior laboratories has even led to several people being wrongly convicted on the basis of erroneous DNA analysis. Among the worst cases was the Houston, Texas, police laboratory. A few years ago conditions were so poor that the laboratory’s roof leaked, contaminating samples with excess moisture.