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How Does Bias Affect Forensics Experts?

A new study suggests forensic anthropologists are biased by external information when performing visual examinations.
Vancouver General Hospital morgue. (Photo: vancouvercoastalhealth/Flickr)

Vancouver General Hospital morgue. (Photo: vancouvercoastalhealth/Flickr)

You’re familiar with the scene: A skeleton lies, inscrutable, on a table in a dimly lit medical examiner’s office. The M.E. pokes around a bit, the camera zooms in dramatically on the clavicle, then she rattles off a few implausibly detailed observations as detectives take notes.

The typical crime procedural depicts this as a well-honed process, but the reality is rarely so simple. A new study published in Science and Justice examines how the complex nature of forensic analysis and pressures on forensics experts could lead them to incorrect conclusions.

Studies have shown that forensics experts (such as fingerprint and DNA analysts) can be swayed by a variety of factors, including cognitive bias, time pressure, and expectations.

In recent years, researchers and legal experts have called the infallibility of scientific evidence into question. It’s an unsettling problem, considering so many jurors expect prosecutors to present scientific evidence and are willing to blindly trust forensic reports—a phenomenon known as the CSI effect, named after the popular long-running TV show.

Studies have shown that forensics experts (such as fingerprint and DNA analysts) can be swayed by a variety of factors, including cognitive bias, time pressure, and expectations.

This study focuses on forensic anthropologists, who examine bodies that are in advanced stages of decomposition. Forty-one participants with experience and qualifications in the field were asked to visually examine a skeleton, then draw conclusions about its sex, ancestry, and age at death.

Two groups were given contextual information in the form of DNA results. Group A was told the skeleton belonged to a 25- to 30-year-old Caucasian male, and group B was told the skeleton was a 50- to 55-year-old Asian woman. The third group was not given any contextual information.

The three groups returned strikingly different results despite examining the exact same skeleton (which archival data indicated was a "probable" female Caucasian between 36 and 45):

  • In the control group, 69 percent assessed the remains as female. Only 14 percent of group A—but 100 percent of group B—said the same.
  • All participants in the control group and group A concluded that the remains belonged to a Caucasian. However, only 50 percent of group B agreed with that assessment.
  • Results for age at death were even more varied. Only 38 percent of the control group said the skeleton’s age at death was 36 to 45 years. Most of group A chose the 26 to 35 age range, while group B tended toward the higher age ranges.

The researchers note that when the quality of the evidence is ambiguous, participants tend to focus on characteristics of the skeleton that validate what they've already been told—in this case, DNA. This mental shortcut is known as confirmation bias.

While DNA results may seem like an important piece of context, forensics experts are expected to "dissociate themselves from extraneous context and influences that may impact upon the collection, analysis and evaluation of evidence in order to present valid and unbiased data," writes Sherry Nakhaeizadeh, the study's author, in an email.

There's a movement underway to create standardized guidelines for forensic fields, while acknowledging their limitations. Nakhaeizadeh and her co-authors suggest that continued research is needed to create more "valid, transparent, and reliable techniques" for presenting evidence in court.