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Editorial: Fingerprints put forensics on trial

Quiz: Can you spot the matching fingerprints?

HOW courts use fingerprints, the icons of forensic science, is on the cusp of a much-needed revolution. In response to claims that their use is often unscientific, statistical tools are now being designed to determine the probability that “matches” between a suspect and prints from a crime scene are spurious.

If used in court, these tools could vastly reduce the number of people convicted of crimes they didn’t commit. But their use may also face resistance from some fingerprint analysts who are reluctant to give up the long-held, but incorrect, notion that crime-scene fingerprints could only have originated from one person.

Since their first use in 1892 (see timeline), fingerprints have formed the heart of many criminal trials. But recently the realisation has dawned that two prints deemed a match by an expert may in fact come from two separate people, due to human error, coincidence, low-quality prints or a mixture of all three.

In 2005, Itiel Dror and Ailsa Peron of the University of Southampton, UK, showed that fingerprint examiners could come to different conclusions about whether two fingerprints matched when presented with the same prints under different circumstances. Fingerprints have also led to some high-profile mistakes. In 2004, they led the FBI to incorrectly identify Oregon lawyer Brandon Mayfield as a suspect in the terrorist bombings in Madrid, Spain (try our fingerprints quiz to see how well you do).

But fingerprint analysts have been slow to acknowledge the problem, and still present matches to courts with no accompanying error rate – a state of affairs that shocked the US National Academy of Sciences (NAS), which presented a report on US forensic science last year. “The NAS committee was horrified by the testimony of the fingerprint analysts,” says Thomas Bohan, who stepped down as president of the American Academy of Forensic Sciences (AAFS) last month. “[One] guy came in and said: ‘It’s infallible. It has a zero error rate. If we say it’s a match, there’s no one else in the universe it could be.'”

The NAS report, which called for new research to assess fingerprint reliability, shook forensic scientists. A year on, steps are now being taken to make the use of such evidence more scientific.

Last month the US National Institute of Justice commissioned the School of Law at the University of California, Los Angeles, to investigate error rates associated with fingerprint analysis. “Even though we have been using fingerprint evidence in court for almost a hundred years, not nearly enough is known about how often fingerprint examiners might make mistakes, or in what circumstances,” says study leader Jennifer Mnookin. The plan is for fingerprint analysts and defence lawyers to use the results of this study to give judges and juries a clearer idea of the accuracy of fingerprint evidence.

The attitudes of forensic scientists are also changing. In a 2009 survey, 75 per cent of US forensic scientists believed that innocent people may be in jail or on death row because of fingerprint errors. That’s up from 56 per cent in 2007, says Samantha Neal of West Virginia University Forensic Science Initiative in Morgantown, who presented the results at a recent AAFS meeting in Seattle. “I think the NAS report has had an impact on the perception of fingerprint analysis,” she says.

Some analysts are now taking matters into their own hands. Cedric Neumann at the UK’s Forensic Science Service in Birmingham and his colleagues have created a program that calculates the statistical weight that should be attached to a given fingerprint match. Such statistics are already used when a DNA match is presented in court.

Computer programs, such as those used by US immigration, can already match a scanned fingerprint against a database to confirm people are who they say they are. But these require good-quality prints, whereas crime-scene prints are frequently smudged or only exist in partial form. Other programs are used to pull up possible matches from a database of criminal suspects, but a human analyst ultimately decides whether there is a match.

“Saying a fingerprint is a match or not doesn’t represent the actual weight of the evidence”

This is where problems can creep in. Analysts will pick a number of features from a fingerprint and compare these with the second print, declaring it either a match, an exclusion (a non-match) or inconclusive. But not all prints are of equal quality.

“Saying something is excluded or included doesn’t represent the actual weight of the evidence,” says Glenn Langenburg, a certified fingerprint analyst with the Minnesota Bureau of Criminal Apprehension, who has been training US analysts to use Neumann’s program.

Analysts are also under no obligation to document how they reached a conclusion, so the level of accompanying information varies between labs from a little to extensive, Langenburg says.

To solve these problems, Neumann’s team asked 40 people to print their fingers 704 times. Then they created a program that uses this data, together with fingerprints taken from 12,000 people, to come up with a probability that a crime scene print matches a given suspect’s.

“It asks ‘what proportion of [features] from the suspect look like the crime scene mark, and what proportion from fingers in the general population look like the crime scene mark?'” says Neumann, who presented the program at the AAFS meeting. “Instead of going to court and saying: ‘It’s him because I say so, and I have 25 years’ experience and I’ve never been wrong’, I can say, ‘It’s him because the data supports it and here’s how I came to that conclusion’,” he says.

Justice Joseph Maltese of the New York Supreme Court agrees that if validated, such a model would be a welcome addition to the courtroom. “‘I know it’s a match because you say it’s a match’ isn’t good enough today,” he says. “If you could buttress that with some statistical data that would be very useful.”

There are still some kinks to iron out. One is that the probability given by the program depends on the number of features an analyst highlights as “of interest”, which can vary widely between analysts. “You could have very confused juries. We need a system to help us select features reliably, routinely and reproducibly,” says Langenburg.

One such tool is being developed by Virginia-based company, Noblis, in consultation with the FBI. Although the exact details are not yet public, the tool maps fingerprints for regions of high and low reliability, flagging up the areas that analysts should focus on. “It’s very promising,” says Christophe Champod of the University of Lausanne in Switzerland, who is working on a program similar to Neumann’s.

Even if fingerprint analysis can be made more consistent, there’s also the issue of what happens in court. “It will be interesting to see what jurors make of it,” says Christine Funk of the State of Minnesota Board of Public Defense. “For my whole career, it has always been definitive that a fingerprint either belongs to an individual or it doesn’t. For me as a defence lawyer it says: ‘this isn’t an absolute certainty’.”

Langenburg agrees that a statistical model may make it easier for defence lawyers to undermine fingerprint evidence, a big fear for some analysts. “They have been taught not to open the door for the defence,” he says. But if analysts are serious about transforming their field into a scientific discipline, that’s a risk they may have to take.

Quiz: Can you spot the matching fingerprints?

Editorial: Fingerprints put forensics on trial