Guilty until proven innocent: police perceptions jeopardize investigations

October 22, 2018 by Ayleen Barbel Fattal, Florida International University

An investigator's belief that a person is guilty may be the reason innocent people get convicted, according to a recent study.

Research by Florida International University legal psychologist Steve Charman found that detectives and who believe a suspect is guilty—based on their knowledge of certain evidence—tend to view additional evidence as more incriminating than it actually is, further increasing their belief in the suspect's guilt.

"We found officers' evaluations of evidence were related to their initial beliefs in a suspect's guilt," Charman said. "The more likely they were to believe the suspect was guilty, the more incriminating they perceived subsequent ambiguous evidence to be, creating a bias snowball effect."

This is the first study to examine this bias snowball effect on the evaluation of evidence among experienced police officers.

Participants read about a crime that contained either incriminating, exonerating or neutral evidence against a suspect. They were asked how likely the suspect was to be guilty and then evaluated four pieces of evidence: the suspect's alibi, a comparison of handwriting samples, an informant's testimony, and a comparison between a facial composite and the suspect.

Results showed the more strongly the police officers initially believed the suspect to be guilty:

  • The more they believed an informant's testimony;
  • the more similar they thought the suspect looked to a facial composite of the criminal;
  • the more similar they thought the 's handwriting was to a handwriting sample left at the crime scene.

According to the National Registry of Exonerations, there have been 2,285 exonerations since 1989. Of those, 52 percent were due to misconduct by police, prosecutors or other government officials while 24 percent were due to false or misleading forensic evidence.

Steven Avery is one of those exonerated.

Featured in the Netflix documentary Making a Murderer, Avery's story captured viewers in 2015 with the first installment of the series. After serving 18 years in prison for sexual assault and attempted murder, Avery was exonerated by DNA evidence in 2003. Two years later, he and his nephew Brendan Dassey were arrested and later convicted of the murder of photographer Teresa Halbach. Part one of the series suggested serious misconduct by investigators and raised questions about the validity of the convictions. Part two is now available on Netflix. It challenges the prosecution's case by pointing out suspects who were overlooked, questioning evidence that may have been planted and meticulously reviewing forensic .

Charman's study supports prior research which suggests that certain people in the legal system, such as forensic examiners, should be kept blind or unaware of certain case details in order to eliminate bias. However, his study showed this also exists in police officers and detectives—investigators who cannot reasonably be kept blind to case information. Charman says more research is needed to develop strategies that may mitigate these biases instead of just identifying them, especially for police officers and detectives.

In total, 89 with an average of 20 years of experience from police and sheriff's departments across the United States participated in the study via an online survey. A comparison sample of 227 college students also participated. The study was published in the Journal of Applied Research in Memory and Cognition.

Explore further: Feeling sleepy? You might be at risk of falsely confessing to a crime you did not commit

More information: Steve D. Charman et al. Cognitive Bias in the Legal System: Police Officers Evaluate Ambiguous Evidence in a Belief-Consistent Manner, Journal of Applied Research in Memory and Cognition (2017). DOI: 10.1016/j.jarmac.2017.02.001

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