Is the science of fingerprints reliable?

I just watched a 2012 documentary on forensic science. During the show one judge they spoke to said there was no scientific basis for the belief that no two persons’ fingerprints could be identical, that no study had been made to demonstrate that theory nor could such a study be done. Fingerprinting is thus not infallible, as the FBI has often claimed in the past, but is a question of probabilities and when it comes to partial prints the ground is even less secure, with some courts demanding 16 point matches while others will accept as few as 6.

I found this astonishing. In light of the above, if in fact it’s true, how could anyone be convicted on fingerprint evidence alone, especially with a partial print? The show interviewed one Portland attorney who had been arrested by the FBI in 2004 as a result of them matching his fingerprints to that of a partial print left on a bag of detonators in the Madrid terrorist attack of that year. Four FBI analysts examined the prints and each said they were 100% certain that they matched. This poor guy, who was still visibly shaken as he recalled the events years later for the show, was heading for a probable death sentence; even his own lawyers thought he was guilty. Had not the Spanish police finally matched the latent print to an Algerian living in Spain the unfortunate attorney would have been toast. (You also have to wonder whether the Algerian was guilty!)

After watching the show I began to think that maybe fingerprinting was on the same level as lie-detecting. But before reaching a conclusion I figured I needed to get the SD on this.

Proving ‘that no two persons’ fingerprints could be identical’ is effectively asking to prove a negative, which is known as a logical impossibility. But fingerprints have been used for over a hundred years, from millions of people, without any 2 identical ones being found. So the non-identicality seems a pretty reliable conclusion. Even identical twins’ fingerprints, though very similar, can be distinguished.

Much of the problem of using them in evidence is that experts are comparing carefully taken fingerprint samples to ones accidentally left at the scene of a crime, often only partial prints or smudged or damaged ones (like the partial ones mentioned in your example). That can make it hard for any expert to make a positive identification. Just like DNA evidence: expert analysts can’t give an absolute match, but they can say they match to 99.999…% certainty – which is good enough evidence for most juries.

The other factor involved is the quality of the fingerprint analyst. Remember that in most police departments such lab workers aren’t seen as ‘cops on the streets’ by voters, so politicians tend to downplay them – thus they are usually short-staffed & real busy, and under time pressure to give results quickly. Factors that tend to undercut the quality of their results. And remember, half of them must be below average!

Fingerprints are used in courtrooms every day, but almost always as supporting evidence along with other facts. In practice, they have been proven to be quite a useful tool.

Like all evidence, fingerprints are considered in the context of other evidence. If the victim’s spouse’s fingerprints are a match, it counts a lot more than some unknown person on another continent, who never knew the victim. No evidence is 100% foolproof all the time, but matching fingerprints - even partial ones - are fairly reliable, in the context of other evidence.

On the other hand, the victim’s spouse might have a perfectly innocuous reason for their fingerprints to be on the scene. The victim’s ex with a restraining order, though, that’s probably enough to convict on.

The first part is spot on. The second part unfortunately illustrates the problem with forensics generally; not just fingerprints; the results are subject to interpretation.

There is definitely growing concern that the size of the FBI’s fingerprint database is leading to a growing number of false positives in their matches.

This paper from the National Institute of Standards and Technology about recommendations to improve the accuracy of fingerprint matching may be of interest to you. It can be found here.

Are toe prints equally good at identifying individuals?.. not that that would be very practical.

The answer to this turns out to be yes. In 1947, Stuart Palmer sent Ellery Queen’s Mystery Magazine a story called “Fingerprints Don’t Lie,” in which a criminal tried to fool the police by leaving a toe print behind. The editor wouldn’t print the story until he checked with an expert to determine if toe prints really were unique and could be traced back to an individual. The answer, as I said, was yes. It has all the same whorls and ridges and whatnot as a fingerprint.

I agree with the first part. However, without knowing the probability of two people having identical or similar fingerprints, any hard % probability you give is an educated guess at best. You can only say that the partial print found on the scene does not rule out the suspect. The prosecutor would then gloss over the fact, that the print would also not rule at least a few other people in the town.

Ah, fingerprints. Universally accepted, and never proven to be able to do what they are accepted as doing.

In 1923, in Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), the Circuit Court of Appeals held in regards to a polygraph test that expert testimony regarding scientific evidence must be based upon methods which are “sufficiently established to have gained general acceptance in the particular field in which it belongs.” The judge is the gatekeeper, and in order to avoid crackpot science from being used, should be relatively conservative in applying the standard. Because use of fingerprints to identify people already had been established as scientifically sound prior to the Frye case, courts accepted them after that standard was accepted.

In 1975, the Federal Rules of Evidence were adopted. Rule 702 stated:

This doesn’t on the face of it appear to affect the Frye standard.

However, the Supreme Court of the United States disagreed. In Daubert v. Merrill Dow Pharmaceuticals, 500 US 579 (1993), the Court held that Rule 702 did NOT incorporate the Frye standard for admissibility of scientific expert opinion. Instead, the Court opined that the judge is tasked with determining that:

a) the testimony is both relevant and reliable;

b) if the testimony is based upon science, it must be demonstrated that it is the product of sound “scientific methodology”;

c) sound scientific methodology can be demonstrated by, among other factors:

  1. Whether the theory or technique employed by the expert is generally accepted in the scientific community;
  2. Whether it has been subjected to peer review and publication;
  3. Whether it can be and has been tested;
  4. Whether the known or potential rate of error is acceptable; and
  5. Whether the research was conducted independent of the particular litigation or dependent on an intention to provide the proposed testimony.

Rule 702 has been twice amended following the Daubert case to more fully reflect the opinion of the Court. It now reads:

Not every jurisdiction has adopted the Daubert standard (which only explicitly applies to federal courts). California continues to use the Frye standard, for example.

Which brings us back to fingerprints. I recall having attended a seminar shortly after Daubert issued, in which the question of fingerprint identification was raised. At the time, there were no actual scientific studies extant which attempted to establish the potential error rate on fingerprint analysis. Thus, the use of expert testimony to identify people via fingerprint analysis remained acceptable almost wholly because of the fact that it always had BEEN acceptable. I’ve been waiting to see if that continues to be the case now for over twenty years. And despite the fact that there have been some noted mis-identifications from fingerprints (mostly because of faulty application of the proper procedures), as well as studies that call the analysis into question as being reliable, as far as I know, no court has yet opined that fingerprint analysis for purposes of identification of a person is unacceptable as a basis for expert testimony.

Wait until you hear about eyewitness testimony!

Responses such as this are why I love this place. Thank you, DSYoungEsq, for that detailed and fascinating answer. And thanks to you, Pixel_Dent, for the link. It certainly is of interest to me.

And some jurisdictions are split. In Florida, the legislature has authority over the evidence code, to the extent that it is substantive law. But the Florida Supreme Court controls the procedural aspects of the evidence code. In 2013, the legislature adopted the Daubert standard. Last week, the FSC declined to adopt the standard citing constitutional issues. So Daubert is the law in Florida, but - for now - only for administrative hearings.

I always assumed any CSI-like database checking was followed up by an actual human comparing the whole partial print to the suspect’s actual print of record. Matching the rest of the ridges, not just the “points”. Is this not actually done?

Especially with the case of the Portland attorney, shouldn’t have a full comparison been enough to show the FBI’s error? That he wasn’t The Guy?

Or are they so rigid in their thinking, so blind to the status quo, that they are incapable of actually thinking anymore? They assume everyone is guilty (except fellow law enforcement, of course, who are never guilty, even if they are, but I digress…)?

Couldn’t the defense go into court and dispute a questionable fingerprint match by making a giant blown up photo of the defendant’s print and the evidence print, and show the jury all the places that don’t agree? And if not, why not?

Because sometimes you have a partial print, and you have six points of match in the parts you have, and most people will extrapolate to a fuller match, but maybe there are actually no other points of match at all.

Also, when you blow up a picture, you make it very unclear. Try it with a photocopy machine sometime.

I am not a forensic scientist, but from what I have read on the subject I am sure that the rest of the visible print is examined, but a match depends on the location and type of the points. Perhaps the rest of the ridges looked similar. Who knows, the standard is the points. Since we don’t have any of the prints nor the expertise to read them, it is impossible know.

Incidents such as this are powerful tools to keep the procedure honest-while the consequences to the accused are huge, the consequences to the system of a proven false positive are also huge. Lawyers and judges will be aware of this case forever. Fortunately for the accused, it worked out. No one knows how many false positives there are, but the number is clearly nonzero.

Gotta get out the good ol’ pantograph for that one.

Looking at the wiki article about the Portland attorney, it seems to contradict the documentary in several key points.

“20 possible matches for one of the fingerprints were found in the FBI database and one of the possible matches was Brandon Mayfield.”

“The FBI described the fingerprint match as “100% verified”. According to the court documents in judge Ann Aiken’s decision, this information was largely “fabricated and concocted by the FBI and DOJ”.”

"As was discovered during the court case, even the FBI’s own records show that this fingerprint, despite the sworn testimony of FBI and DOJ agents, was in all reality not an exact match but only one of 20 prints “similar” to the ones retrieved from Madrid. "

"Before his arrest, Spanish authorities informed the FBI in a letter from April 13, that they reviewed the fingerprint on the bag as a negative match of Mayfield’s fingerprint,[4] though this letter was not communicated to Mayfield’s attorneys. "

If the article is correct, it does not support the argument that fingerprints are not unique.

This is an older but interesting article that makes the case that it is training that is the problem.

not a forensic scientist, myself, but i dated one for a while, and she often complained that the tv shows were no where near the reality.

according to her, its actually always done by humans. like most things on tv, the “fingerprint database” doesnt work like on csi; there’s no blinking computer screen with the words “match found” on it. if the local forensic team even has access to the fbi database (i.a.f.i.s.) what they get is a list of several, if not scores, of possible matches. its then up to the humans to do the old fashioned, side-by-side comparison to come up with the match. and often these are not exact matches, because, as stated above, the quality of the print taken from the crime scene is usually lacking.