Menus Subscribe Search

Follow us


Why Fingerprints Aren’t the Proof We Thought They Were

• September 20, 2012 • 4:00 AM

Fingerprint matching is a vital investigative tool. But despite its legendary aura of infallibility, courtroom claims of fingerprints’ uniqueness are slowly receding.

Scientist Nancy Knight documented snowflakes in 1988 while studying cirrus clouds for the National Center for Atmospheric Research. During a Wisconsin snowstorm she found two identical sets of snow crystals – identical under a microscope, at least – giving lie to the old belief that no two snowflakes are alike. That aura of uniqueness also surrounds the arches, loops, and whorls at the tips of our fingers, and to this day most fingerprint examiners remain steadfast that no two fingerprints are exactly alike.

“Fingerprint examiners typically testify in the language of absolute certainty,” professor Jennifer Mnookin at the University of California Los Angeles has written. But like many other claims for forensic science, the assertion that fingerprints are unique lacked a solid scientific basis and now is viewed with new caution.

“The language of certainty that examiners are forced to use hides a great deal of uncertainty,” the U.K.’s Lord Justice Leveson put it when addressing the Forensic Science Society.

Or as Penn State Dickinson School of Law professor David Kaye observed in a 2010 news release, “Fingerprint examiners and other forensic experts often testify with ‘100% confidence’ or to a ‘scientific certainty’ that a defendant is the source of a latent fingerprint or that a bullet came from a particular gun. It is time for criminalists to provide more scientifically defensible-and legally palatable-testimony.”

Meanwhile, “It’s a match!“ – the term so beloved of TV shows – is being phased out of the courtroom lexicon right along with the notion of definitive fingerprint matches that rule out everyone else on the planet.

[class name=”dont_print_this”]

Righting What’s Wrong in Criminal Justice

Wrongful convictions stem from the belated entrance of scientific rigor into the field of forensics, systemic problems, and the ubiquitous ‘human factor.’ In the coming weeks, a series of stories by crime author Sue Russell looks at why convictions go wrong, at the common reluctance to rectify error, and at innovations to better safeguard justice.

Stories so far:

A Porn Stash and a False Confession: How to Ruin Someone’s Life in the American Justice System

Red Flags: Early Warnings of Wrongful Convictions

Human Lie Detectors: The Death of the Dead Giveaway

Why Fingerprints Aren’t Proof

Litigating Lineups: Why the American Justice System Is Keeping a Close Eye on Witness Identification

The Right and Privilege of Post-Conviction DNA Testing

Seeking Second Chances Without DNA

Why Can’t Law Enforcement Admit They Blow It Sometimes?

A Prescription for Criminal Justice Errors

[/class]

The word “match” certainly troubles former NYPD street cop Nick Petraco, a veteran hair and fiber expert. Petraco, assistant professor at John Jay College of Criminal Justice at the City University of New York and a forensic consultant to the NYPD, thinks it is inherently misleading because the public is so accustomed to interpreting anything called a match – paint colors or fabric swatches, for example – to mean identical. And concerns go beyond fingerprints.

“Basically it’s a word that most people associate with ‘identical,’ meaning that it’s the exact same and that there’s no other alternative,” he says. “So if you say in relationship to a hair examination ‘the hair matched,’ to them that means that it’s from that guy’s head or from that guy’s body. But there’s no scientific basis for that, in reality, especially in cases involving things like hairs and fibers and paint chips.”

Petraco favors words or descriptions that convey that two pieces of evidence are consistent with one another; concordant. “Consistent means that they have the same properties and they could be from that location…but not necessarily,” he says.

Short of the impossible – comparing the prints of the entire world’s population, past and present – we can never know or say for sure if fingerprints are unique. More to the point, for the purposes of linking a fingerprint to a suspect or crime scene, the goal of uniqueness – or of “individualization,” as identifying something to a unique, specific source is known – may not much matter.

A 2009 National Academies of Science report on the failings and shortcomings of forsensic science recommended more research focused on probabilities – figuring out the statistical likelihood that a fingerprint could be pronounced a match erroneously.

Probabilities as reflected in those astronomic statistical odds that DNA could belong to a party other than the defendant– remember O.J.? – have been a mainstay of presenting the science as evidence to jurors.

A 2011 UCLA Law Review article, “The Need for a Research Culture in the Forensic Sciences (pdf),” whose 13 authors include Mnookin and Kaye, addressed this, raising questions such as: “How frequently might a portion of two fingerprints – or striation marks on bullets, or toolmarks, or handwriting specimens – share any given degree of similarity even if they derive from different sources?”

Meanwhile, since no scientific proof exists to “individualize” the results of fingerprint matching to the point where all other possible sources can be excluded and the language of certainty has the potential to mislead jurors, a new vocabulary can’t hurt. Many experts favor judges focusing on making sure that jurors understand that what they are really being told is that there are many similarities between a crime scene latent print and a suspect’s print and no explainable differences. And that more subtle explanation could go a long way towards demythologizing fingerprint evidence for jurors.

So could, if solid research is produced and eventually becomes acceptable to courts, presenting the statistical odds that a latent fingerprint (a smudge, partial or degraded print found at a crime scene) does indeed belong to a defendant. As of this moment, it’s been possible in the limited universe of an Agatha Christie mystery in which, say, 12 potential suspects are confined to a moving train, perhaps, but not in the world at large.

But new research underway is taking on the fingerprint match probability problem. And preliminary results published in February in Significance, the magazine of the American Statistical Association and the UK’s Royal Statistical Society, give a taste of what may one day be a new fingerprint evidence reality. The researchers’ report announced the creation of a statistical model that will allow fingerprint evidence to be quantified so it can be accorded appropriate weight in courtrooms.

Cedric Neumann, Pennsylvania State University assistant professor of forensic science and statistics, is its lead author. “It is unthinkable that such valuable evidence should not be reported, effectively hidden from courts on a regular basis,” he said in a statement. “Such is the importance of this wealth of data, we have devised a reliable statistical model to enable the courts to evaluate fingerprint evidence within a framework similar to that which underpins DNA evidence.”

Neumann is not suggesting anything close to overnight change here. The goals are long term and much testing remains to be done, both of the model itself and if that passes muster, of the legal system’s reaction to it. Courtroom battles surrounded DNA in its infancy and are certain to converge on any attempts to introduce a statistical approach to fingerprint evidence.

“It remains to be seen how future legal battles play out,” Neumann and his fellow researchers wrote, “but we see models such as this one as a powerful platform for change.” The full study is due to be published later this year.

Meanwhile, the FBI’s fingerprint database, the Integrated Automated Fingerprint Identification System, grows by thousands daily, with more than 70 million subjects’ prints in its criminal master file alone. Some experts, like Mnookin and cognitive neuroscientist Itiel Dror, ask if the database’s very size may make accidental matches more likely.

As Dror said in 2009, “It enables you to look at millions of prints very quickly, make quite good comparisons; it enables you to resolve cold cases and is very good in many ways. However, that same powerful technology that helps solve crimes has also increased the likelihood of finding very similar fingerprints by accidental coincidence.”

In February, the National Institute of Standards and Technology and the Department of Justice’s National Institute of Justice issued a report—three years in the making—on improving fingerprint analysis. After examining all manner of factors that can impact the process, the 34-people strong Expert Working Group on Human Factors in Latent Print Analysis offered 34 recommendations, including “Urging management at forensic science provider facilities to foster a culture in which it is understood that some human error is inevitable and that openness about errors leads to improvements in practice.”

In February 2009, the day after the release of the National Academies of Science’s sweeping report on forensics, Robert J. Garrett, then-president of the International Association for Identification, also sounded a cautionary note (pdf). “It is suggested,” he wrote, “that members not assert 100% infallibility (zero error rate) when addressing the reliability of fingerprint comparisons.”

Researchers Mark Pagea, Jane Taylor and Matt Blenkins, in their March 2011 paper Uniqueness in the Forensic Identification Sciences – Fact or Fiction, contend that “the forensic examiner should therefore not use terms such as ‘unique’ or even ‘individualization’ as they are not the issues the forensic practitioners should concern themselves with; these are issues for the judge and jury. To claim that a fingerprint or bitemark has identified someone ‘to the exclusion of all others,’ or has been ‘individualized’ to only this person, usurps the jury’s role as the rightful assessor of the evidence…”

“It’s a match!” claims also have been notoriously derailed in a couple of high-profile cases. In March, a report finally vindicated Scottish police officer Shirley McKie, who was accused of leaving a fingerprint in the home of a female murder victim. Officer McKie canvassed neighbors in the case but always swore that she never entered the victim’s home – the crime scene.

In the most notorious mistaken print case of all, Oregon resident Brandon Mayfield was erroneously fingered by several top FBI analysts as the Madrid train bomber. Fortunately for Mayfield, Spanish authorities tied the prints to their correct owner, Ouhnane Daoud. Ultimately, the FBI was compelled to admit that its bias and “circular reasoning” had led it to target an innocent man.

But the relevance and importance of probability research becomes clear when you consider that the reason such similar prints were located in the first place was as a result of the FBI database’s power to search through tens of millions of images. Reason enough for a vocabulary rethink.

Sue Russell
Journalist Sue Russell's work has appeared internationally in such publications as The Washington Post, New Scientist and The Independent. She is the author of Lethal Intent, a biography of executed serial killer Aileen Wuornos.

More From Sue Russell

A weekly roundup of the best of Pacific Standard and PSmag.com, delivered straight to your inbox.

Recent Posts

November 21 • 4:00 PM

Why Are America’s Poorest Toddlers Being Over-Prescribed ADHD Drugs?

Against all medical guidelines, children who are two and three years old are getting diagnosed with ADHD and treated with Adderall and other stimulants. It may be shocking, but it’s perfectly legal.



November 21 • 2:00 PM

The Best Moms Let Mess Happen

That’s the message of a Bounty commercial that reminds this sociologist of Sharon Hays’ work on “the ideology of intensive motherhood.”


November 21 • 12:00 PM

Eating Disorders Are Not Just for Women

Men, like women, are affected by our cultural preoccupation with thinness. And refusing to recognize that only makes things worse.


November 21 • 10:00 AM

Queens of the South

Inside Asheville, North Carolina’s 7th annual Miss Gay Latina pageant.


November 21 • 9:12 AM

‘Shirtstorm’ and Sexism in Science

Following the recent T-shirt controversy, it’s clear that sexism in science persists. But the forces driving the gender gap are still being debated.


November 21 • 8:00 AM

What Makes a Film Successful in 2014?

Domestic box office earnings are no longer a reliable metric.



November 21 • 6:00 AM

What Makes a City Unhappy?

According to the National Bureau of Economic Research, Dana McMahan splits time between two of the country’s unhappiest cities. She set out to explore the causes of the happiness deficits.


November 21 • 5:04 AM

Sufferers of Social Anxiety Disorder, Your Friends Like You

The first study of friends’ perceptions suggest they know something’s off with their pals but like them just the same.


November 21 • 4:00 AM

In 2001 Study, Black Celebrities Judged Harshly in Rape Cases

When accused of rape, black celebrities were viewed more negatively than non-celebrities. The opposite was true of whites.


November 20 • 4:00 PM

Women, Kink, and Sex Addiction: It’s Not Like the Movies

The popular view is that if a woman is into BDSM she’s probably a sex addict, and vice versa. In fact, most kinky women are perfectly happy—and possibly healthier than their vanilla counterparts.


November 20 • 2:00 PM

A Majority of Middle-Class Black Children Will Be Poorer as Adults

The disturbing findings of a new study.


November 20 • 12:00 PM

Standing Up for My Group by Kicking Yours

Members of a minority ethnic group are less likely to express support for gay equality if they believe their own group suffers from discrimination.


November 20 • 10:00 AM

For Juvenile Records, It’s ‘Justice by Geography’

A new study finds an inconsistent patchwork of policies across states for how juvenile records are sealed and expunged.


November 20 • 8:00 AM

Surviving the Secret Childhood Trauma of a Parent’s Drug Addiction

As a young girl, Alana Levinson struggled with the shame of her father’s substance abuse. But when she looked more deeply into the research on children of drug-addicted parents, she realized society’s “conspiracy of silence” was keeping her—and possibly millions of others—from adequately dealing with the experience.



November 20 • 6:00 AM

Extreme Weather, Caused by Climate Change, Is Here. Can Nike Prepare You?

Following the approach we often see from companies marketing products before big storms, Nike focuses on climate change science in the promotion of its latest line of base-layer apparel. Is it a sign that more Americans are taking climate change seriously? Don’t get your hopes up.


November 20 • 5:00 AM

How Old Brains Learn New Tricks

A new study shows that the neural plasticity needed for learning doesn’t vanish as we age—it just moves.


November 20 • 4:00 AM

The FBI’s Dangerous Misrepresentation of Encryption Law

The FBI no more deserves a direct line to your data than it deserves to intercept your mail at the post office. But it doesn’t want you to know that.


November 20 • 2:00 AM

Brain Drain Is Economic Development

It may be hard to see unless you shift your focus from places to people, but both destination and source can benefit from “brain drain.”


November 19 • 9:00 PM

Gays Rights Are Great, but Ixnay on the PDAs

New research suggests both heterosexuals and gay men are uncomfortable with public same-sex kissing.


November 19 • 4:00 PM

The Red Cross’ Own Employees Doubt the Charity’s Ethics

Survey results obtained by ProPublica also show a crisis of trust in the charity’s senior leadership.



November 19 • 2:00 PM

Egg Freezing Isn’t the Feminist Issue You Think It Is

New benefits being offered by Apple and Facebook probably aren’t about discouraging women from becoming mothers at a “natural” age.


Follow us


Sufferers of Social Anxiety Disorder, Your Friends Like You

The first study of friends' perceptions suggest they know something's off with their pals but like them just the same.

Standing Up for My Group by Kicking Yours

Members of a minority ethnic group are less likely to express support for gay equality if they believe their own group suffers from discrimination.

How Old Brains Learn New Tricks

A new study shows that the neural plasticity needed for learning doesn't vanish as we age—it just moves.

Ethnic Diversity Deflates Market Bubbles

But it's not in the rainbow and sing-along way you'd hope for. We just don't trust outsiders' judgments.

Online Brain Exercises Are Probably Useless

Even under the guidance of a specialist trainer, computer-based brain exercises have only modest benefits, a new analysis shows.

The Big One

One company, Comcast, will control up to 40 percent of Internet service coverage in the U.S., and 19 of the top 20 cable markets, if a proposed merger with Time Warner Cable is approved by regulators. November/December 2014

Copyright © 2014 by Pacific Standard and The Miller-McCune Center for Research, Media, and Public Policy. All Rights Reserved.