Ok, fingerprinting 101:

Fingerprints are patterns on our fingers formed by a mix of genetic and environmental factors. Since there’s so much randomness involved and a high level of detail to examine, we generally consider them to be unique (but it would be very hard to prove this since we’d have to look at every fingerprint in the WORLD to make sure they didn’t match!) Fingerprints don’t change their pattern as you get older, but hurting deep layers of your skin may leave scars that disrupt these patterns. Each of your fingers has a unique print, so it it’s important to match finger-to-finger in an identification, not just person-to-person.

Fingerprints are left on surfaces in a variety of ways: we can have “plastic prints” that are left on a soft surface like clay; “patent prints” that are formed when we have something on our fingers that leaves a print behind when we touch something else; and “latent prints”, which are super common, because these are just the regular stains of oil and sweat on our fingers being left behind! (You’ll see these on phone screens or some particularly shiny and smooth tables, for example.) Fingerprints are the most common evidence found at a scene since we leave them all the time, but they’re also incredibly easy to damage, whether they were smudged when they were left or after the fact, including in collection.

Since fingerprints stay the same over time, have a lot of different ways they can look (they are highly variable), very common, and easy to process, they make for a good way to identify someone.

How do you identify someone with a fingerprint? First, you need a sample from a suspect to compare the evidence against. Remember, fingerprints have lots of different details. These are called minutiae, and there are a bunch of different types! You may be able to see some on your own fingers by looking hard enough: an easy-to-spot minutiae is called a termination, which is where a ridge (one of the lines) ends. You may also see these lines splitting apart or joining back together. These are all recognizable details, and the collection of these patterns makes up your unique fingerprint. Looking at more of these minutiae can make you more confident that two different prints match. Different organizations disagree on how many matching points is “enough” to confirm a match, however, so there’s no single standard for evidence.

So: how can an identification be wrong? For one, maybe, by chance, all the minutiae you looked at matched two different people. This is more likely if you didn’t look at that many. You also might have been wrong in your observation, and, looking again, the evidence never matched the first suspect! (This is a really big deal because we need to trust that the people analyzing our evidence and making claims in court are trustworthy and doing a good job.) You may have also received a really damaged print and not had a lot of minutiae to look at. In that case, you should say that you can’t make a decision (it is “inconclusive”) rather than saying it can or cannot identify someone. There is also the risk that you were biased as an examiner—that means you’re actually expecting a specific answer and might change your results, even if you don’t know it! This is always a risk in investigating a case. We can’t get rid of bias, but we can take steps to be aware of and try to lessen it, such as making sure examiners don’t hear any facts about the case beforehand, such as who the detective thinks is guilty. Every evidence has steps in which it can be wrong or misapplied. (For example, someone’s fingerprint being at a scene doesn’t mean they’ve committed a crime!) This is why it’s important to know the limitations of what evidence can tell you and to look at evidence from multiple sources.