How Do We Know Every Fingerprint is Unique?
Sometimes it’s good to questions the assumed orthodoxy that we all seem to absorb by gradual osmosis from the culture and society we live and are brought up in. ‘Facts of life’ like : “Columbus discovered America*“; “Dinosaurs lived millions of years ago”; “The Medieval Ages were an age of unbelievable cruelty” and “Fingerprints are unique and of prime forensic importance in a court of law”. They’re just ‘things we know’. Sometime, somewhere along life’s journey we just picked up and subsumed all these ‘facts’.
*(For what its worth its highly likely that Columbus wasn’t the first European in the Americas it was the Vikings … but that’s another story)
In the case of last ‘fact’, How do we know they ARE unique? After all if they’re not then this would have a serious consequence for our legal and judicial system. After all fingerprints alone have been used to convict people of some very serious crimes.
The “Uniqueness” of Fingerprints
This is a rather scary example of how a belief can harden into certainty with the help of a little mathematics and a lot of bluster.
- Scientists can tell how old you are from fingerprint smudges on your phone.
- Every cat’s nosepad, like human fingerprints, is distinct and unique to that cat. No two feline noseprints are ever alike.
- In Babylon from 1885-1913 BC, fingerprints were used as substitutes for signatures in order to protect against forgery. Parties would impress their fingerprints into clay tablets on which contracts had been written. Although the ancient peoples probably did not realize that fingerprints could identify individuals, references from the age of the Babylonian king Hammurabi (1792-1750 BC) indicate that law officials fingerprinted people who had been arrested.
- The fingerprints of koala bears are virtually indistinguishable from those of humans, so much so that they can easily be confused at a crime scene even with an electron microscope.
- Ear-prints are as unique as fingerprints; burglars can be convicted on the basis of ear-prints they leave at crime scenes.
The idea that fingerprints are unique goes back centuries and seems to have been based on nothing more than the notion that something so complex cannot be replicated in every detail. The use of fingerprints as a forensic method has its origins in a monograph published in 1892 by the polymath Sir Francis Galton. With characteristic thoroughness, Galton considered all the aspects of fingerprinting needed to turn it into a science, including an estimate of the probability of two people sharing a fingerprint. To do this, he established the size of a patch of fingerprint whose pattern he could identify correctly 50% of the time. Combined with the number of such patches making up a typical fingerprint, Galton estimated that fingerprints were sufficiently different to give odds of around 1 in 64 billion of any two matching by fluke. As this comfortably exceeded the population of the world, Galton concluded that fingerprints are essentially unique. Worryingly, Galton’s experiment involved fewer than 100 fingerprints and his probabilistic argument is far from watertight. However the prospect of identifying miscreants by fingerprints proved too irresistible and by the 1920s standard texts were stating the uniqueness of fingerprints as fact.
For decades Galton’s assumption went essentially unchallenged, further enhancing its reputation. The lack of challenges is hardly proof of infallibility-merely of the ability of “scientific” evidence to subdue even the most vociferous defence lawyer. But since the late 1990s there have been several successful challenges to fingerprint evidence, though these have centred chiefly on mis-identification and planted evidence. The consensus remains that fingerprints are essentially unique, despite there being no hope of proving it beyond doubt.
Since the 1990’s though there has been some suggestive supporting evidence has emerged from studies of the biochemical processes involved in fingerprint formation. Computer models of these processes by Professor James Murray of the University of Washington have recreated the characteristic patterns of ridges. The models also show that even the smallest difference in starting conditions can utterly change the end result. As there is always a random element involved in living processes, this strongly suggests that no two people will have precisely the same fingerprints. So Galton may have been right after all-but more by luck than hard science.
So, there you have it – WE CAN’T ACTUALLY PROVE THEY ARE UNIQUE but we all seem quite happy to believe that they are!
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