4 ms·

Polygraphs, fingerprints, DNA, face matching, forensic ballistics, forensic psychology, witness testimony (especially police testimony), ...

boomboomsubban 4y ago
DNA has a fair amount more credibility than the others. While it's not the silver bullet people think it is, and it can be used to falsely incriminate someone, there is a scientifically supported method of showing two samples of DNA belong to the same person.
- djleni 4y ago
  Maybe you know more about this than me, something that makes me uneasy about DNA evidence: Say you can confirm a sample is 100% a certain person. How do investigators have any idea how it got there? How do you know it’s from a criminal and not the guy that stocked the convenience store shelves? I tried to look into this and maybe I was searching the wrong thing but found nothing.
  - boomboomsubban 4y ago
    >How do investigators have any idea how it got there? They often can't be certain, and I'm sure somebody is in prison because they're DNA was in an inconvenient place, but it's a far superior form of evidence than the other methods mentioned. Unless a crime is widely watched in person with supporting recordings and DNA evidence, it's basically impossible to be a 100% certain of what happened. That's why court only attempts to prove guilt beyond reasonable doubt.
  - thaumasiotes 4y ago
    The relevant concept is called "chain of custody"; essentially you're trusting every person who ever handled the sample to have correctly documented where they got it and what they did with it. This is the same way we guarantee the integrity of elections.
  - jessaustin 4y ago
    One catchphrase is "means, motive, opportunity". DNA can indicate a possible opportunity, in that a suspect's DNA can be shown to have appeared in the same place a crime occurred. That doesn't address means or motive, i.e. whether the suspect had the ability or inclination to commit the crime. If I were on a jury and a DNA sample was all the prosecution had, my verdict would be not guilty. Keeping a massive DNA database and using it to convict someone, anyone because a crime occurred near something they touched before would be fallacious and unjust. However, if we already had other reasons to suspect someone, and DNA gives us another, that's much more powerful.
- gus_massa 4y ago
  IMHO, the real DNA analysis is credible. The problem is that there are some attempts to make weaker test and try to confuse people so they think it's the real test. For example: https://en.wikipedia.org/wiki/DNA_profiling#Partial_matches https://en.wikipedia.org/wiki/DNA_profiling#Partial_matches Quoting the Wikipedia article: > We can see no reason why partial profile DNA evidence should not be admissible provided that the jury are made aware of its inherent limitations and are given a sufficient explanation to enable them to evaluate it. That sound good, but every time I read it again it looks more and more fishy.
MerelyMortal 4y ago
I'm willing to believe that fingerprints are unique until proven otherwise, though I would need to see the actual finger prints and not some computer report claiming they "match". I also know that fingerprints can be faked.
- throwaway74829 4y ago
  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).
  - MerelyMortal 4y ago
    I think this is the case you're refering to: https://www.theregister.com/2004/05/26/fbi_madrid_blunder/ https://www.theregister.com/2004/05/26/fbi_madrid_blunder/
ars 4y ago
> witness testimony There are two kinds of witness testimony - the most common is where a witness sees a stranger and testifies about it. Those have little credibility. But there's another kind, where the witness actually knows the person, those are far more credible. Historically only this type was permitted, but over time things seem to have shifted to the first type.

boomboomsubban 4y ago

DNA has a fair amount more credibility than the others. While it's not the silver bullet people think it is, and it can be used to falsely incriminate someone, there is a scientifically supported method of showing two samples of DNA belong to the same person.

djleni 4y ago
Maybe you know more about this than me, something that makes me uneasy about DNA evidence: Say you can confirm a sample is 100% a certain person. How do investigators have any idea how it got there? How do you know it’s from a criminal and not the guy that stocked the convenience store shelves? I tried to look into this and maybe I was searching the wrong thing but found nothing.
- boomboomsubban 4y ago
  >How do investigators have any idea how it got there? They often can't be certain, and I'm sure somebody is in prison because they're DNA was in an inconvenient place, but it's a far superior form of evidence than the other methods mentioned. Unless a crime is widely watched in person with supporting recordings and DNA evidence, it's basically impossible to be a 100% certain of what happened. That's why court only attempts to prove guilt beyond reasonable doubt.
- thaumasiotes 4y ago
  The relevant concept is called "chain of custody"; essentially you're trusting every person who ever handled the sample to have correctly documented where they got it and what they did with it. This is the same way we guarantee the integrity of elections.
- jessaustin 4y ago
  One catchphrase is "means, motive, opportunity". DNA can indicate a possible opportunity, in that a suspect's DNA can be shown to have appeared in the same place a crime occurred. That doesn't address means or motive, i.e. whether the suspect had the ability or inclination to commit the crime. If I were on a jury and a DNA sample was all the prosecution had, my verdict would be not guilty. Keeping a massive DNA database and using it to convict someone, anyone because a crime occurred near something they touched before would be fallacious and unjust. However, if we already had other reasons to suspect someone, and DNA gives us another, that's much more powerful.
gus_massa 4y ago
IMHO, the real DNA analysis is credible. The problem is that there are some attempts to make weaker test and try to confuse people so they think it's the real test. For example: https://en.wikipedia.org/wiki/DNA_profiling#Partial_matches https://en.wikipedia.org/wiki/DNA_profiling#Partial_matches Quoting the Wikipedia article: > We can see no reason why partial profile DNA evidence should not be admissible provided that the jury are made aware of its inherent limitations and are given a sufficient explanation to enable them to evaluate it. That sound good, but every time I read it again it looks more and more fishy.

MerelyMortal 4y ago

I'm willing to believe that fingerprints are unique until proven otherwise, though I would need to see the actual finger prints and not some computer report claiming they "match". I also know that fingerprints can be faked.

throwaway74829 4y ago
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).
- MerelyMortal 4y ago
  I think this is the case you're refering to: https://www.theregister.com/2004/05/26/fbi_madrid_blunder/ https://www.theregister.com/2004/05/26/fbi_madrid_blunder/

ars 4y ago

> witness testimony There are two kinds of witness testimony - the most common is where a witness sees a stranger and testifies about it. Those have little credibility. But there's another kind, where the witness actually knows the person, those are far more credible. Historically only this type was permitted, but over time things seem to have shifted to the first type.