Although every wrongful conviction is unique, organizations such as the Innocence Project and the Michigan Innocence Clinic that work to overturn wrongful convictions have identified types of criminal justice system errors seen again and again in cases where innocent individuals are convicted. Some of the most common causes of wrongful conviction are listed below. Any given wrongful conviction may have resulted from a combination of these and other, less common, causes:
FAULTY EYEWITNESS IDENTIFICATION
The most common cause of wrongful convictions in the U.S. is that eyewitnesses misidentified the suspect. This occurs because the human mind does not remember events and people perfectly or consistently. Also, those already imperfect memories can be contaminated by events afterwards, causing altered and incorrect memories to be fixed in our minds. The problem of people's imprecise and inaccurate memories is made worse when police manipulate lineups of suspects to try to make eyewitnesses identify a suspect favored by police. For example, when police detectives encourage witnesses or victims to believe they have identified the correct suspect, then they may begin to believe their incorrect memory is the truth. Similarly, if detectives suggest to witnesses or victims that they may have erred in remembering some details, then they may begin to develop a new, supposedly corrected version of their memory. The result is that witnesses and victims can be convinced they have correctly identified the suspect, when in reality they are identifying an innocent person.
JUNK SCIENCE (unvalidated or improper forensics).
Many forensic disciplines apply techniques and methods that have not been approved by the scientific community. Because of this, forensic analysts sometimes give testimony without a proper scientific basis for their conclusions. Forensic analysts also sometimes engage in outright misconduct by fabricating and falsifying evidence. Examples of faulty science include: bite mark and fingerprint analysis as well as hair and fiber comparisons in which an analyst claims there is a match to a particular person; and improper use of DNA evidence, such as by claiming a person contributed, or did not contribute, to a DNA sample when the data are actually inconclusive.
Many innocent defendants from all walks of life will make incriminating statements, provide confessions, or plead guilty to crimes when they are put under severe stress. This can happen when innocent people are pressured, misled, threatened, or abused by police officers during interrogations until the innocent suspects come to believe confessing will lead to a better outcome than continuing to maintain their innocence. Looking at the 330 people who were exonerated by DNA evidence between 1989 to 2015, it was found that around 25% gave false confessions after long interrogations. These innocent people often recanted afterwards, but their coerced or pressured confessions were still used to convict them.
INFORMANTS & JAILHOUSE SNITCHES
People who have been given an incentive to testify, such as payment or favors like reduction or dismissal of their own charges, may make false statements that lead to an innocent person's conviction. Such statements are especially damaging when the jury is not informed of the incentives that these witnesses were given to testify
GOVERNMENT MISCONDUCT (bad police work and prosecutorial misconduct)
While most police officers and prosecutors are hard-working, well-intentioned individuals, sometimes police investigators, prosecutors, and other government officials work to ensure that a defendant is convicted even when the evidence is weak or clear proof of innocence exists. Detectives and prosecutors may believe a suspect is guilty and then succumb to tunnel vision, selectively promoting evidence that appears to support guilt, and ignoring or hiding evidence that supports innocence. In other cases, officials knowingly frame innocent individuals to gain greater status, job promotions, or other benefits. Police officers have caused wrongful convictions by hiding, altering, or fabricating evidence, coercing confessions, manipulating eyewitness identifications, pressuring witnesses, making deals with snitches so they provide false testimony, and lying while testifying at trials. Prosecutors have also caused wrongful convictions by ignoring or hiding exculpatory evidence that would help suspects prove their innocence, encouraging witnesses to lie on the stand, using "expert" testimony from people who are not experts, and lying to jurors, defense lawyers, and judges.
The failure of lawyers to investigate a case sufficiently, call witnesses to testify (such as witnesses who could provide an alibi), and prepare for the trial can lead to the conviction of the innocent. Lawyers may be unable to handle complex cases, which can be overwhelming. When innocent people lack funding for private attorneys and are granted a court-appointed defense attorney, their public defenders may have little experience or be so overworked that they cannot represent their clients effectively.
UNFAIR, BIASED JUDGES
Judges contribute to wrongful convictions when they fail to make fair and reasonable decisions about the evidence and testimony allowed in court. As a result, juries are exposed to false information that biases them against a suspect. Judges may fail to exclude a suspect's confession obtained through pressure and unsupported by physical evidence. Judges may allow testimony from felons whose motives are questionable. Judges may fail to question a supposed expert's credentials and testimony outside of a jury's hearing. Judges also may fail to require prosecutors to turn over exculpatory evidence. Knowledge is power. Understanding these causes of wrongful conviction can help our society more readily recognize and fix flaws in the criminal justice system so that the innocent remain free.