A spatter of blood or a twist of fiber found at a crime scene is rarely the smoking gun our TV fiction promises, and evidence can be interpreted in multiple ways by different experts. But forensic science sounds like the final word, and can sometimes tip the scales in a courtroom, even though the general public—and more critically, juries—don’t always realize just how subjective its application can be.
“Juries really expect way too much of forensic sciences,” says Marcella Fierro, the state of Virginia’s chief medical examiner from 1994 until her retirement in 2007. “They’re looking for a level of certainty which the observational sciences may not be capable of.”
Take the tragic case of Michael Brown, gunned down in Ferguson, Missouri. In that instance, there were dueling interpretations of the autopsy report: Some said the pattern of Brown’s gunshot wounds supported witnesses who said the young man was charging at police officer Darren Wilson; others said the wounds clearly show Brown was surrendering. A single line of forensic evidence; multiple supported theories.
DNA evidence at least has a fundamental foothold in molecular biology, but other areas of forensics are more tenuous. Take fingerprinting: While scientists don’t seriously doubt that the ridges on your fingers are unique, fingerprint analysis can be subjective, as when an examiner is dealing with prints that are smudged or distorted. Examiners can also be influenced by outside information: In one experiment, a trio of British scientists presented experts with fingerprints they had previously (and correctly) identified as belonging to a suspect, but told them that these particular fingerprints were not a match. Despite being instructed to ignore that information, “within this new context, most of the fingerprint experts made different judgments, thus contradicting their own previous identification decisions,” the researchers wrote in a 2006 paper for the journal Forensic Science International.
Most forensic scientists don’t want to toss out fingerprinting; they want to work to strengthen its scientific foundation. The National Institute of Justice, a research wing of the U.S. Department of Justice, has called for examiners to be more open about errors, standardize the prerequisites for becoming a latent print examiner, and develop better metrics for fingerprint analysis to improve reliability, among other efforts. “I think that in the long run, the studies will show that fingerprints are good evidence,” Fierro says. “The limitations come with the problems of poor quality prints or incomplete prints.”
Other disciplines in forensics have their own problems. As forensic scientist Mechthild Prinz and Innocence Project co-founder Peter Neufeld explain in the above video (a selection from the program “The Science of Justice: A Matter of Opinion?”), fiber analysis can be severely limited by lack of context, an unfortunate byproduct of large databases. Knowing what sort of fiber is found at the crime scene is usually only useful in pinpointing a suspect if you have some idea how common the fiber is. If you find a fiber from a white cotton T-shirt at a gas-station crime scene, that doesn’t tell you as much as finding a strand of polar bear hair.
A highly critical 2009 report from the National Academy of Sciences highlighted a host of shortcomings in forensics. Outside of DNA analysis, forensic methods desperately want consistent standards, more basic research, and requirements like mandatory certification, the NAS report said. In response to the report, the DOJ formed the National Commission on Forensic Science, which is working to address these long-standing deficiencies. “They want to take forensic science out of the back rooms of police stations, and put it into laboratories that have inspection and accreditation and quality assurance measures,” Fierro says. “And I think that will significantly improve things.”
Robert Shaler, a forensic scientist who supervised the effort to identify thousands of missing 9/11 victims with DNA testing, also worries about self-appointed forensic experts who do not have backgrounds in science—sometimes ex-police officers who’ve “set out shingles,” he says—who make claims with an unjustified degree of certainty. He’s seen a case where a self-dubbed “blood spatter expert” tried to construct a pattern out of eight tiny droplets—originally assumed to be the victim’s blood but later identified as the defendant’s.
“When you have ambiguity, when the thing is not clear as a bell, that’s where these people make mistakes,” Shaler says.