13 June 2009

The Fingerprint Examiner's Fallacy

There's a chap called Simon A Cole who's been calling into question the forensic validity of fingerprints.
Here's a link to one of his many papers, "Is Fingerprint Identification Valid? Rhetorics of Reliability in Fingerprint Proponents' Discourse" from his, University of California, Irvine webpage.

Part of his argument is something he calls the fingerprint examiner's fallacy.

"The fingerprint examiner’s fallacy holds that the fundamental empirical
question concerning forensic fingerprint identification is not

How accurate are latent print examiners’ attributions of source?

but rather

Are all human friction ridge arrangements unique?

At the heart of the fingerprint examiner's fallacy, therefore, is the elevation of uniqueness over accuracy.

Before going on, here are the compulsory links to Brandon Mayfield (erroneously linked to the Madrid train bombings of 2004 on the basis of fingerprint evidence) and Shirley McKie (accused of leaving her thumbprint at a murder scene). Both examples of cases where the fingerprint examiners got it spectacularly wrong: unfortunately I can't link to other cases where they got it spectacularly wrong until the people concerned prove their innocence.

However, it isn't only within the area of fingerprints that the fingerprint fallacy crops up.

In Dallagher, a ninety-four year old, deaf, arthritic woman was suffocated while she was in bed. It seemed clear that an intruder had broken into her house through a small transom window above her bed during the middle of the night: he had scrambled through the window and then suffocated the victim. The prosecution made the case that the intruder was the appellant. Ear prints were found on glass immediately below the transom window; the prints were examined by experts and compared to control prints provided by the appellant and others. The experts found that the ear prints from the scene matched those provided by the appellant who was subsequently convicted.

Later however, the conviction was found not to be safe and was overturned.

While another case, as reported by the BBC ('Ear print' burglar wins appeal), was overturned.

In this case the fingerprint examiner's fallacy was shown when "... specialist ear-print officer Cheryl McGowan told the jury that no two ears were the same [(uniqueness)] and she did not believe the ear could belong to anyone other than the defendant [(over accuracy)]."

Q. Err, Cheryl ... how accurate is your belief?
A. One-hundred Percent.
Q. Based on what?
A. ...

Anyway, here's a much subtler one from Job v Halifax [2009]. In brief, a chap has phantom withdrawals taken from his bank. He complains to the bank; they don't reimburse him; he takes his complaints to the court.

Anyway, during the proceedings Inglis J said,

"It is said that it is for the bank to show that the card did not have any flaws that would enable the key to be extracted from it; that a strong random number generator was used; that there were appropriate controls on key management; and that there were appropriate controls on the personalisation process. The only evidence is identified as being Mr Baker's assertion that there has never been a key compromise in the industry. The assertion is only a statement of the bank's ignorance."

That, "there has never been a key compromise in the industry" is the uniqueness part of the fingerprint examiner's fallacy. Again, this is presented in lieu of accuracy.

What are the accuracy rates? It is implied that they're one-hundred percent but where's the evidence? How was it measured? Is your measurement process valid? Etc.

Mr Justice Inglis's comments are quite interesting here, uniqueness in lieu of accuracy ... "a statement of ... ignorance."

As I read the Law Commission's consultation paper Expert Evidence in Criminal Trials I wonder whether or not their recommendations will end up getting rid of fingerprint evidence. I doubt it. They only mention fingerprints once in the report and that is with regard to something called judicial notice. This is where the judge says, "it's obvious to me the technique is valid, I'm not prepared to listen to any arguments otherwise, so the evidence can be admitted."


  1. Simon Cole is nothing more than a social scientist who did his research on fingerprint history. No fingerprint examiner would do a project on social science, and then call the principles invalid. He needs to study biological uniqueness in nature and quit trying to make a buck testifying against forensic science.

  2. @printman1001 "He needs to study biological uniqueness in nature ..."

    If fingerprints can be forged, it doesn't matter whether or not they are unique in nature.