Eyewitness Misidentification
Far and away the most common contributing factor, eyewitness misidentifications have played a role in approximately 70% of the over 300 DNA exonerations that have occurred since the early 1990s.
Memory is malleable and fallible; the human brain does not record memories like a video camera. Over 100 years of scientific studies has called into question the validity and reliability of eyewitness testimony, particularly when it comes to identifying perpetrators. In the last several decades, law enforcement agencies across the county have adopted procedures that help guard against mistaken identifications. These revised procedures include conducting live and photo lineups in a sequential (vs. simultaneous) manner and making sure that the lineup is “double-blind” that is, conducted by an individual who does not know where the suspect falls in the lineup sequence.
http://www.innocenceproject.org/causes/eyewitness-misidentification/
Unreliable or Improper Forensic Science
Unlike DNA analysis, most forensic science techniques were developed solely with an eye towards solving crimes. Even though many of these disciplines are cloaked in air of scientific authority, they typically lack the key hallmarks of sound science. A 2009 report prepared by the National Academy of Sciences examined a number of forensic disciplines – including microscopic hair comparison, toolmark and impression evidence, and bite mark analysis – and concluded that, with the exception of DNA evidence, every discipline lacked sufficient indicia of validity (ability of the method to produce an accurate result) and reliability (ability of the method to produce consistent results). Certain arson investigation techniques are also faulty, as this report and video by the Arson Research Project explain.
In addition to the inherent problems with forensic science itself, experts in these fields will sometimes testify to conclusions beyond even what the limited science on their subject allows, by indicating that two samples were a “match” or “consistent with each other” when there is no way to prove the sample could not be matched or be consistent with samples from other individuals. Finally, although most forensic experts—even those in undeveloped forensic disciplines—are honest, hardworking individuals just trying to do their jobs, there have been some instances of outright fraud and misconduct. In West Virginia, the State Supreme Court was forced to reexamine hundreds of convictions over a span of more than a decade when it was uncovered that the forensic laboratory technician, Fred Zain, had falsified numerous test findings. Other instances of falsified forensic evidence has been uncovered in Massachusetts, Washington D.C., and Texas.
False Confessions
In 25% of DNA exonerations, defendants made incriminating statements, confessed, or pleaded guilty to crimes they did not commit. Typically the result of coercive or suggestive interrogation techniques, false confessions may be given due to the threats or promises made during the interrogation; extended periods of time without food, drink, or sleep; diminished capacity or mental impairment; youth; ignorance of the law; denial of Fifth Amendment right to counsel during interrogation; simple misunderstanding; suggestive lines of questioning; and torture. Like eyewitness identifications, procedures and guidelines have been developed for interrogations—such as video recording all interrogations completely—to reduce the instances of false confession and insure proper interrogation techniques are used.
Unreliable Informant Testimony
In more than 15% of DNA exonerations, an informant testified against the defendant. These informants often have incentives to take the stand, which may not be disclosed to the jury—such as payment, avoiding criminal prosecution, or reducing a sentence. They may testify in multiple cases and use nefarious means to gather information necessary to support their false testimony. A 2005 study showed how such testimony was used to put 38 innocent individuals on death row.
Bad Lawyering
Last, but not least, ineffective, incompetent, and overburdened defense counsel often allows the various other factors leading to wrongful convictions to go unchallenged in court. Failing to investigate alibis or challenge shoddy forensic evidence, failing to file pretrial motions, failing to prepare opening statement or closing argument, and sleeping during trial are all examples ineffective assistance of counsel that led the convictions of innocent defendants later exonerated by DNA evidence.