Wednesday, May 18, 2022

Cognitive bias in forensic analysis exposed

In February 2021, cognitive psychologist Itiel Dror set off a firestorm in the forensics community. In a paper, he suggested forensic pathologists were more likely to pronounce a child’s death a murder versus an accident if the victim was Black and brought to the hospital by the mother’s boyfriend than if they were white and brought in by the grandmother. It was the latest of Dror’s many experiments suggesting forensic scientists are subconsciously influenced by cognitive biases—biases that can put innocent people in jail, reported Science.

Dror’s work has shown that most problems with forensics do not originate with “bad apple” technicians who have infiltrated crime labs. Rather they come from the same kind of subconscious bias that affects everyone’s daily decisions—the shortcuts and generalizations our brains rely on to process reality. “We don’t actually see the environment,” Dror says. “We perceive stimuli from the environment that our brain represents to us,” shaped by feelings and past experience.

“In the span of a decade, cognitive bias went from being almost totally unheard of in forensics to common knowledge in the lab,” Brandon Garrett, a professor at the Duke University School of Law, wrote in his book Autopsy of a Crime Lab: Exposing the Flaws in Forensics. “We can especially thank Itiel Dror for helping bring about the sea change.”

Over the years Dror and other researchers have found bias just about everywhere they’ve looked—in toxicologists, forensic anthropologists, arson investigators, and others who must make judgments about often ambiguous crime scene evidence. Yet juries find forensic evidence compelling, Dror and others have found.

Many examiners feel “impervious to bias,” says Saul Kassin, a psychologist at John Jay College of Criminal Justice, “as if they’re not human like the rest of us.” In 2017, Kassin and Dror asked more than 400 forensic scientists from 21 countries about their perceptions of bias. They found that whereas nearly three-quarters of the examiners saw bias as a general problem, just over 52% saw it as a concern in their own specialty, and only 26% felt that bias might affect them personally.


Itiel Dror and his collaborators have coined various terms to describe how bias sneaks into forensic analysis—and how experts perceive and react to their biases.

TARGET-DRIVEN BIAS Subconsciously working backward from a suspect to crime scene evidence, and thus fitting the evidence to the suspect—akin to shooting an arrow at a target and drawing a bull’s-eye around where it hits

CONFIRMATION BIAS Focusing on one suspect and highlighting the evidence that supports their guilt, while ignoring or dismissing evidence to the contrary

BIAS CASCADE When bias spills from one part of the investigation to another, such as when the same person who collects evidence from a crime scene later does the laboratory analysis and is influenced by the emotional impact of the crime scene

BIAS SNOWBALL A kind of echo chamber effect in which bias gets amplified because those who become biased then bias others, and so on

BIAS BLIND SPOT The belief that although other experts are subject to bias, you certainly are not

EXPERT IMMUNITY The belief that being an expert makes a person objective and unaffected by bias

ILLUSION OF CONTROL The belief that when an expert is aware of bias, they can overcome it by a sheer act of will

BAD APPLES The belief that bias is a matter of incompetence or bad character

TECHNOLOGICAL PROTECTION The belief that the use of technology, such as computerized fingerprint matching or artificial intelligence, guards against bias.

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