Many years ago I came across an interesting, if limited, discussion in a blog post entitled “Expert testimony in pattern evidence cases – is absolute uniqueness necessary?”1,2 That post is dated Sept 4, 2009, shortly after the publication of National Academy of Science’s report “Strengthening Forensic Science in the United States: a Path Forward”.3 But the basic question posed in it is still relevant today. I would say that most forensic practitioners today would answer the question about the necessity of ‘absolute uniqueness’ in the negative. However, their individual reason(s) for their answer will still vary.
For many people, ‘absolute uniqueness’ is mainly a ‘forbidden’ concept because of some policy they must follow, or because of a more personal recognition of an (often vague) issue relating to the ‘limits of science’. For other people the matter is a well-defined issue in science and logic dictated by the nature and limits of information (knowledge), what information can really tell us about the world, and how information can and should be used to update beliefs about the world. For the latter group (which is steadily increasing in size as awareness and understanding improves), the concept of ‘absolute uniqueness’ is neither required, nor even beneficial in forensic work.
There has been a LOT of discussion about this in recent years, but I found the blog post interesting at the time even though it focused mainly on latent print examination. I feel that not much has changed since then so, even now, it deserves recognition and consideration. Since the blog itself is no longer active, I have reposted the complete series of messages here (pulling them from archive.org).
The topic started with a post from the moderator (Barry Fisher) who wrote:
Expert testimony in pattern evidence cases – is absolute uniqueness necessary?
What information is needed to form a conclusion about an identification? Do conclusions require statistical data, as in DNA cases, to offer an opinion? Is it possible to state that two items of evidence come from a sole source? What may an expert opine when no statistical data is readily available and only experience suggests a conclusion? The National Academy report raises some profound questions and some intriguing research possibilities. But in the interim, while we wait for academics to study the multitude of pattern evidence forensic scientists encounter in their day to day work, who may report cases and testify in court? Readers are invited to speak to these issues.
This was followed by a series of 7 responses from various people. I’ll present those in sequence (with minor spelling corrections relative to the original posts):
Response #1:
Response #2:
The question can also be framed as, “is uniqueness or individuality necessary for a jury to reach a conclusion?” That’s certainly not the case in other forensic disciplines.
Response #3:
Jerry Chisum
September 21st, 2009 on 4:52 pmAbsolute uniqueness is not necessary. Look at the DNA model, they have established a probability of source. That is all fingerprints have to do. James Osterburg published a paper regarding this back about 1970. He did it without the computer. Now with AFIS, the probability of a “level 2 point” occurring as a particular location should be easy to calculate. That is all that would be necessary to meet the NAS and Court guidelines.I know I have oversimplified the process, but hope someone will go with it.
Response #4:
Jay SiegelOctober 2nd, 2009 on 7:06 am
I agree with Roy Marzioli’s assessment of the state of fingerprint science. I believe that, by extension, this applies currently to all forms of “pattern” evidence (firearms and tool marks, bite marks, handwriting, hair morphology, footwear, tire treads, etc.). Not only do I believe that a conclusion of absolute inclusion is not scientifically justified but that it is not necessary. I don’t believe that the concept of “individualization” has any place in science and is not provable in the real world. Why is it necessary to offer a nonsupportable, unprovable conclusion in court, where there is so much at stake and juries are so easily mislead? When a forensic scientists analyzes evidence that is brought to the lab by a criminal investigator, there is a reason why the focus is on that person. (This can set up a situation that invites bias on the part of the examiner, but that is another issue). The fact that there are many similarities between a latent print lifted at a crime scene and a print from a suspect, with no unexplainable differences, should be testimony enough. This would presumably be one piece of corroborating evidence in a net of evidence being offered by the prosecutor. There is no need to “guild the lily” by adding the conclusion that there is no other fingerprint in the world that could be the source for the print from the crime scene. Offering unsupportable conclusions in court reinforces the idea that forensic science really isn’t science; it is a tool of the prosecution.
Response #5:
Several issues muddy the water of discussing individualization, two of which I’ll mention briefly here.
The first is the acculturation of fingerprint examiners to the idea of individualization (sorry, two of my degrees are in physical anthropology and that’s how my mind works). The hardship is that individualization is now dogma, a belief system with true believers and proselytizers, and not science. “Uniqueness”, in the forensic sense, is not provable; this is why every other science uses statistics to address issues where the population is either so large or so intractable that sampling (and, hence, statistical analysis) is required. Much like the arguments about evolution and creationism, it is almost impossible for some fingerprint analysts to accept that individualization is not attainable in the ultimate sense (“to the exclusion of all possible humans”). We therefore, in many cases, are fighting the wrong fight. Arguing about which interpretation model to use for a latent ignores the fact that you may be arguing with someone who won’t be “converted.” Culture change is very hard and very difficult to manage.
The second muddying factor is that individualization, in its most common sense, is easily understood by anyone. I was listening to a speaker say, “Individualization does not exist and is not possible,” and I thought to myself, “Wait until his car gets stolen; it won’t be any of the thousands of Ford Taurus’ that were stolen–it was HIS.” The paradox of mass production is that, although we know that many things are analytically identical (although not numerically so, that is, if we numbered them, we could refer to “Number 3″ or “Number 16″), we know which are OURS. When the valet drops off my car, I know it’s mine and can prove it even though I haven’t memorized my VIN. The proximity of individualization to the average person’s awareness has, I feel and in part, kept fingerprints insulated from a more rigorous scientific treatment.
Response #6:
“Faith is a cop-out. If the only way you can accept an assertion is by faith, then you are conceding that it can’t be taken on its own merits.”-Dan Barker, “Losing Faith in Faith”, 1992.
The issue of ‘uniqueness’ is an enormous and unfortunate distraction. Uniqueness is neither sufficient for making absolute statements about the source of a mark, nor is it necessary for making probabilistic statements.
Any mark left by the same source is unique as well. As Wittgenstein stated: “Roughly speaking, to say of two things that they are identical is nonsense and to say of one thing that it is identical with itself is to say nothing at all” (Tractatus, 5.5303).
When considering the inference of identity of source, what should be discussed is within-source and between-source variability (see relevant literature).
Footnotes
- Original citation: http://myblogs.informa.com/forensics/2009/09/04/expert-testimony-in-pattern-evidence-cases-is-absolute-uniqueness-necessary/
- Archived version: https://web.archive.org/web/20100825230036/http://myblogs.informa.com/forensics/2009/09/04/expert-testimony-in-pattern-evidence-cases-is-absolute-uniqueness-necessary/
- National Research Council Committee on Identifying the Needs of the Forensic Science Community, Strengthening Forensic Science in the United States: a Path Forward, National Academy Press, Washington, D. C, 2009 (available here)
September 18th, 2009 on 7:15 am