How do we know every fingerprint is unique? Rachel Burton, Liverpool
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Readers reply
Well, according to my mate Jimmy the Jemmy, currently serving four straight ones at HM’s pleasure, they can’t be, can they? ’Cos he was nowhere near the gaff, miles away actually, so the dabs can’t be his, they must be someone else’s, some leery geezy wot’s got identical dabs to Jimmy and did the job. And if Jimmy ever comes across that melt, well, identical fingerprints will be the least of ’is worries. wormlover
I’ve always wondered the same about snowflakes – we are told that no two are alike, but their very evanescence surely makes it impossible to justify such a statement. serapis
We now rely on computers to match fingerprints to the millions they have on file. The only thing that worries me is the number of times my iPad or iPhone tells me that it doesn’t recognise my fingerprint when I press the home button … ClareM8
It is not “proved”, but it is a mathematical certainty that the possible combinations of characteristics on the ridges (“lines”) on your fingers, their order, their presence or absence, will never plausibly repeat. There are not billions of possible combinations, but trillions upon trillions. Sir Francis Galton found a way to systematise those characteristics and to make them searchable, so they can be used. PardelLux
I occasionally use a lot of sandpaper at work. If I don’t wear gloves, my fingerprints disappear. thegreatfatsby
Are we talking fingerprints, the things on the end of your fingers, or fingerprints, the things you leave everywhere? The difference depends on the scale and accuracy of the measurement involved – of the tools being used, which will never be definitive – and on the clarity of the material being examined. The uniqueness of the swirls on your fingers depends, ultimately, on the disposition of billions of cells, assembled without any instructions to form a common, specific, species-wide pattern. How many billion-digit numbers are there? (Hint – it isn’t a billion.) It’s worth noting that identical twins don’t have identical fingerprints, although they will be similar (arches instead of circles on a given finger, say).
It’s possible, or even likely, that there are fingers out there, somewhere, that could unlock my phone, or leave marks that would put me in jail if lifted off a knife found in the back of someone I’d been seen threatening on the street earlier. This isn’t proof that the fingerprints on those fingers are the same as mine – simply that they’re similar enough to be indistinguishable, given the built-in laxness of a reader that you don’t want to have to press 40 or 50 times to open your phone, or the vague way in which information is preserved in smudged grease. iruka
The postulated uniqueness of the fingerprint finds its origin in its formation (morhogenesis) between the 13th and 17th weeks of pregnancy. Although the detailed mechanisms of fingerprint formation are still mostly unknown, it is generally accepted that the general pattern types, giving the flow to the papillary ridges, are the consequence of the confluence of patterning waves initiated at different sites on the foetus’s fingertips, as deep as the dermis. The friction ridge skin visible on the epidermis is a projection of the blueprint pattern set on the dermis. The overall random generative process is what makes fingerprints so powerful in distinguishing one person from another. In forensic science, the features taken into account when examining fingerprints (ie the features considered to reflect the uniqueness of the pattern) are usually divided into three levels of details, going from the most general to the most specific.
The first level consists in the determination of the general pattern of the ridges, which can form various patterns assigned to general classes such as (but not limited to) loops, whorls and arches. On their own, these patterns are selective (arches are shared by about 5% of the population), but these features that can be shared by several people.
The second level considers ridge characteristics such as ridge endings, bifurcations (which are called “minutiae”) and other features such as scars and wrinkles. The combination of these features add enormously to the discriminative power of fingerprints.
Finally, the third level focuses on finer details such as the intrinsic shape of pores and ridge edges. These features also contribute to the selectivity offered by the friction ridge skin.
While we’ve talked about the unique patterns of our fingerprints, it’s important to note that when investigators look at fingerprints at a crime scene, they often find marks that aren’t perfect. These marks can be partial, smeared or hard to visualise. This makes it challenging to capture the uniqueness of the fingerprint. Hence while the uniqueness of fingerprints is widely accepted because of how they form, forensic experts often work with incomplete or unclear prints from crime scenes. This means they might see only a portion of the full fingerprint pattern. Even though the chances are rare, this can sometimes lead to similarities between individuals.
So, while in theory every fingerprint is unique, the conclusions drawn by examiners are based on their expert judgment and are not absolute truths. In conclusion, while we can’t say with 100% certainty that every fingerprint is unique, it’s a reasonable assumption when looking at clear, high-quality prints. Sébastien Crot and Olivier Gianella, MSc students in forensic science at the School of Criminal Justice, University of Lausanne, Switzerland
