Do Fingerprints Lie?

The Firm - Fingerprint Fallibility
Michael Whyte Published on Feb 4, 2012. From season 1, episode of 6 of the 'The Firm' where the fallibility of fingerprint evidence is discussed in court
How reliable is fingerprint analysis?
Fingerprinting is used by law enforcement all over the world, but it may not be as reliable as you think.
Fingerprint Evidence (Featuring Associate Professor Simon Cole)
UCISocialEcology Published on Feb 24, 2012. Fingerprint evidence was introduced in the court system over 100 years ago. But how reliable, really, is its use in identifying the actual perpetrator of the crime? Associate Professor Simon Cole, Department of Criminology, Law and Society, studies the interaction between science and the law, specializing in fingerprint identification in the justice system.
MythBusters Fingerprints Busted
How reliable is identification by fingerprint
SHIRLEY MCKIE: Apology for pain she suffered.
Brandon Mayfield and his wife, Mona, speak to reporters in 2006.
Portland News
Neoroscientist Itiel Dror

The gold standard of forensic evidence is now being challenged.

Late one afternoon in the spring of 1998, a police detective named Shirley McKie stood by the sea on the southern coast of Scotland and thought about ending her life. A promising young officer, the thirty-five-year-old McKie had become an outcast among her colleagues in the tiny hamlet of Strathclyde. A year earlier, she had been assigned to a murder case in which an old woman was stabbed through the right eye with a pair of sewing scissors. Within hours of the killing, a team of forensic specialists had begun working their way through the victim’s house. Along with blood, hair, and fibres, the detectives found some unexpected evidence: one of the prints lifted from the room where the murder took place apparently matched the left thumb of Detective McKie.

Crime scenes are often contaminated by fingerprints belonging to police officers, and investigators quickly learn to eliminate them from the pool of suspects. But McKie said that she had never entered the house. Four experts from the Scottish Criminal Record Office—the agency that stores and identifies fingerprints for Scotland’s police—insisted, however, that the print was hers. Though McKie held to her story, even her father doubted her. “I love my daughter very much,’’ Iain McKie, who served as a police officer in Scotland for more than thirty years, told me earlier this year. “But when they said the print was Shirley’s I have to admit I assumed the worst. My entire career I had heard that fingerprints never lie.”


Is fingerprint evidence really infallible?

For over 100 years most police forces and law courts have accepted finger prints as conclusive evidence. It has been assumed that as no two people have identical prints, therefore, there can be no confusion in identifying individual prints and attributing them to a particular person.

Matthew Sparkes Telegraph 2014

As we all know, fingerprints are like snowflakes: no two are alike. The arches, loops and whorls in the skin create complex and unique patterns which can be used to accurately identify us.

Well, most of the time.

In fact, it is technically possible for two people to have identical fingerprints or for two snowflakes to form the same pattern – it’s just infinitesimally unlikely.


Fair enough, but why was it that Scottish police officer Shirley McKie was wrongly accused of having been at a murder scene in 1997 after a print supposedly matching hers was found near the body?

Why did American citizen Stephan Cowans serve six years in prison before being released after fingerprint evidence was overturned by DNA?

And why was Brandon Mayfield, a Portland lawyer, who had never visited Spain linked to the horrific Madrid train bombings by FBI fingerprint experts in 2004?


Contested in the courts

Fingerprint comparison has enjoyed a long and largely successful history, and there is considerable support for the reliability of fingerprint comparisons in a majority of cases. Whether fingerprint individuality has a scientific basis has been questioned, and there have been some cases and some studies that clearly show that fingerprint comparison is not infallible. The courts however, have been very reluctant to find fingerprint  comparison inadmissible because of the support for its reliability and its lengthy track record.

Brandon Mayfield

Brandon Mayfield was held for two weeks as FBI investigators matched prints lifted from a bag containing detonating devices found at the scene with his. An independent examiner also verified the match. But Spanish National Police examiners insisted the prints did not match those of Mayfield and eventually identified another man who matched the prints.


It’s a match!

Fingerprint experts can disagree about how many points in common are needed to declare a match between two sets of fingerprints. For example, some experts will declare a match based on only 12 points in common, whereas other experts may require up to 20 points in common before declaring a match.

In England and Scotland generally 16 points are required for a match in a court of law.  Australia and France require at least twelve points and in Italy,  examiners say they have to see 16 or 17 points of similarity. In Brazil, it’s 30 and in Sweden, it’s 7. And most examiners in the United States, including those at the FBI, don’t even use a point system at all. Even the points system itself has been criticised as an entirely inadequate method of matching prints.


A report finally vindicated  Shirley McKie, who was accused of leaving a fingerprint in the home of a female murder victim. Officer McKie canvassed neighbours in the case but always swore that she never entered the victim’s home, the crime scene.

In 2012, 14 years after the event, Shirley McKie received an official apology.

Herald Scotland   Lucy Adams

THE chief constable of Scotland’s largest police force will today make an unprecedented apology to former detective Shirley McKie for the pain she has suffered as a result of the now infamous fingerprint case.

Stephen House will apologise on behalf of his force, Strathclyde, for the “pain and suffering” it caused to Ms McKie and her father Iain after meeting them last week.

She was tried for perjury in 1998 after insisting a fingerprint found in the home of murder victim Marion Ross in 1997 did not belong to her.

Ms McKie was later cleared of lying under oath and, in February 2006, was given £750,000 by the then Scottish Executive in an out-of-court settlement.

Justice Secretary Kenny MacAskill ordered the public inquiry, saying the controversy had “cast a shadow of uncertainty and suspicion” over the individuals involved and the criminal justice system.

Inquiry chairman Sir Anthony Campbell recommended that fingerprint evidence should be now “recognised as opinion evidence and not fact”.


So, what’s the problem

On a simple level we don’t know for certain that everybody has a unique fingerprint and even if they do, when’s a match a match? Someone has to decide and people make mistakes, they can be biased or unknowingly be influenced by events and pressures surrounding them. Maybe we should leave it to computers to decide, but could they be relied upon?

 In an ideal world, an examiner will receive a clear and well preserved fingerprint from the crime scene, or better yet, several prints left by the same person. However, this is not the world we live in. The print is much more likely to be smudged, missing a significant portion of the detail, depict only a small portion of the finger, or be found on a surface that makes preservation difficult.

Even if we assume that the print is an analyst’s dream and collected in the correct manner it still has to be interpreted. This is where the science ends and human factors begin to muddy the water.

Itiel Dror

In 2004, cognitive neoroscientist Itiel Dror, a psychologist at Southampton University, set out to examine whether the process of fingerprint analysis could  be influenced by information or expectations examiners already had when they attempted to find a match for prints from a crime scene.

Dror asked five fingerprint experts to examine what they were told were the, supposed, matched prints of Brandon Mayfield. In fact, they were re examining prints from their own past cases. Only one of the experts stuck by their previous judgments. Three reversed their previous decisions and one deemed them “inconclusive.”

Dror’s argument is that these competent and well meaning experts were swayed by what they knew (or thought they knew) about the case in front of them.


Fingerprint examiners argued that the Mayfield case was an isolated mistake, and argued that the science of fingerprint matching was fundamentally sound.


 James Randerson  The Guardian March 2007

A further study by Itiel Dror suggests otherwise. “I wanted to see if it is as objective and scientific as it claims to be,” he said. “I wanted to see if the same expert would make the same decision on the same fingerprint if it is presented in a different context.” He presented six fingerprint experts from various countries including the UK, the US and Australia with eight marks from crime scenes (called latent prints) and eight inked marks from suspects. The experts, who had 35 years experience between them, had all given judgments on the pairs of prints in previous court cases,  four as matches and four as exclusions.

But Professor Dror engineered the experiment so that none of them knew they were participating in a study, something that he says makes the study much more powerful. “If people know they are studying them they behave differently, especially if you are studying errors,” he said.

Of the 48 tests, the experts changed their decision in six cases and only two of the experts were consistent with their previous decision in all of their eight cases. They were more likely to change their decision if given contextual information, such as “the suspect has confessed”, that conflicted with their previous judgment.

“The same expert on the same fingerprint can make totally conflicting decisions, depending on the context,” said Dr Dror, who presented his results at the British Psychological Society’s annual meeting in York.


Opinion not fact

Most of us have been accused, at some time or another, of seeing what we want or expect to see and  believing what we want or expect to believe. Generally we don’t do it deliberately but even scientists have been known to interpret the results of experiments according to their beliefs or what they want or expect to find.

After the public enquiry into the case of Shirley McKie  the inquiry issued a set of key recommendations including fingerprint experts acknowledging their findings were opinion rather than fact and that new procedures should be established for complex cases.


The American Association for the Advancement of Science

Anne Q Hoy, Sept 2017

Fingerprint Source Identity Lacks Scientific Basis for Legal Certainty

Courtroom testimony and reports stating or even those implying that fingerprints collected from a crime scene belong to a single person are indefensible and lack scientific foundation, a new AAAS working group report on the quality of latent fingerprint analysis says.

“We have concluded that latent print examiners should avoid claiming that they can associate a latent print with a single source and should particularly avoid claiming or implying that they can do so infallibly, with 100% accuracy,” states the report.

Empirical tests are necessary to measure the accuracy and establish the validity of latent fingerprint examinations, states the AAAS report.

“In reality, there is not, at present, an adequate scientific basis for either claim,” the AAAS report says. “There is no basis for estimating the number of individuals who might be the source of a particular latent print.  Hence, a latent print examiner has no more basis for concluding that the pool of possible sources is probably limited to a single person than for concluding it is certainly limited to a single person.”

The report makes clear that while latent fingerprint examiners can successfully rule out most of the population from being the source of a latent fingerprint based on observed features, insufficient data exist to determine how unique fingerprint features really are, thus making it scientifically baseless to claim that an analysis has enabled examiners to narrow the pool of sources to a single person.

The short extract from the ‘The Firm’ fairly sums up the arguments of those who contest the infallibility of fingerprints as used in a court of law.




New Scientist   How far can fingerprints be trusted?

Michael Specter      The New Yorker 2002

Matthew Sparkes Telegraph 2014

James Randerson  The Guardian March 200

Lucy Adams    Herald Scotland

Anne Q Hoy, Sept 2017  The American Association for the Advancement of Science

Is Fingerprint Evidence Sound Science? By GERALDINE SEALEY Jan. 14

The New York Times


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