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Fingerprints are unique in the technical sense (i.e. no one has the exact same set of fingerprints) -- but matching is a completely different issue. I don't know how far forensic science has come since a decade ago when I learned some of it, but at that point they were running "feature matching" instead of full-fingerprint matching. In other words, they looked for aberrancies (e.g. "islands," "deltas," "bifurcations" etc. at specific spots), and then checked those against a finger-to-person database. Then a list of possible matches would be found, if they also contained some set amount of same features in the same places. There are two main issues here. First, while taking an optical scan can provide very high-resolution images of a fingerprint -- field data is never like that. Fingerprints that are collected usually are not uniform, and dependent on pressure applied by the finger(s) in question and the amount of oil left behind (which powder sticks to "reveal"). Because of this, you're never going to get a perfect fingerprint in the field. Some areas will be faint, misleadingly separated (and even have false artificial aberrancies not present on the original finger), or outright missing -- leading to less "features" to check against. Others will be completely "blotched" out by a high amount of oil, removing vital information. Second, you will almost never get an exact match because of this -- you will be getting a list of possible matches. I can remember one notable case of a terrorist's prints being wrongly attributed to someone who could never have been at the scene, much less anywhere near the country -- but was still whisked away by (I believe?) Interpol. Forensic evidence is just a piece of numerous possible stories that can be spun up any which way. There's something to be said here about human biases, incentives, and so on -- but I think it's intuitive enough (and frankly, I don't want to write more). |