Inconclusive: Iowa State University research uncovers forensic examiner bias

DES MOINES, Iowa - Any time a gun is fired, It leaves unique markings on a shell casing - sort of like its own fingerprint. Researchers at Iowa State University found that forensics researchers can correctly identify matches 99 percent of the time. But - when shown a mismatch, a third of the time they’ll mark it as “inconclusive.”

Andrew Smith, one of the researchers on the study says, the findings are concerning.

“This is extremely disconcerting because what could be happening here is innocent persons could functionally be stripped of evidence that they might need to establish their innocence,” Smith said.

Smith says forensics experts are often asked what he calls an unfair question - did the bullet come from the suspect’s gun?

“There are reasons for a mismatch other than that casing not coming from the suspect’s gun. For instance, the gun might have been altered or the evidence might be degraded,” Smith said.

That could leave examiners feeling unable to entirely rule the shell casing out. Smith says forensic examiners may experience something called ‘adversarial allegiance bias.’ In this case, that means forensic examiners could be biased toward prosecutors and law enforcement.

“When they’re detecting nonmatches, they might render inconclusive reports - they might have a bias to render inconclusive reports - because they don’t want to render a report that would hurt the side that retained them,” Smith said.

Erica Nichols Cook, the director of the Drake University Wrongful Conviction Clinic says submission forms sent to the lab often have information that could bias an examiner.

“Do they need to know it’s a murder? No. Do they need to know the race of the defendant or the victim? No,” Nichols said.

Nichols Cook says shell casings marked as inconclusive make it harder for defendants to prove their innocence.

“By choosing that third answer, by saying inconclusive, its’ basically, ‘it depends, I don’t know. It could be, it couldn’t not be.’ You’re leaving that person in that quagmire of having to defend against that without a for sure answer,” Nichols Cook said.

Nichols Cook says other forensics tests, like fingerprints and shoe prints, suffer from similar problems. Unlike DNA, which was established by the scientific community, forensics is created by law enforcement and based on recognizing patterns.

Nichols Cook says the research will help call inconclusive shell casing reports into question at trials.

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