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Regarding US law, my general understanding is that if evidence was obtained in an illegitimate way (e.g. under false pretenses), then it may be inadmissible as evidence to support a conviction in a criminal case. The basic rationale for that is somewhat clear, procedurally limiting the power of the government to punish people for crimes to only use the tools that the law allows for.

However, that kind of rationale doesn't really hold up when a defendant in a criminal case obtains evidence that demonstrates the accusation isn't true, but may have been obtained by the defendant in some illegitimate way - if the point is to restrict the power of the government to punish private persons, then a private person defending themselves against punishment by the government doesn't fall under that. As an example, let's say that in a given jurisdiction you don't have the right to film someone on your property without their consent, but you have hidden cameras on your premises anyway, and then you'd like to use those recordings to defend yourself when they show you did not commit a crime you are being accused of.

TLDR: In a criminal trial, when can evidence in favor of the defendant be thrown out on the grounds that the evidence was obtained illegally/illegitimately? What is the rationale for that?

I'm happy with answers for US law, any commonwealth nation's law, or any European nation's law.

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    Are we talking about a scenario where the defendant procures evidence to show he is not guilty through illigitiment means OR the prosecution illegitimately obtains exonerating evidence that is thrown out because it was obtained as part of fruit of the poisonous tree?
    – hszmv
    Mar 20 at 20:29
  • @hszmv Let's say the defendant procures the evidence; I hadn't considered that poisonous tree might be relevant, probably because of what I assume the rationale for that to be.
    – G. Bach
    Mar 20 at 22:16
  • Just curious — is there any "jurisdiction you don't have the right to film someone on your property without their consent" — barring of course places where privacy is expected e.g. bathroom?
    – Greendrake
    Mar 21 at 1:56
  • @Greendrake Germany for example is relatively restrictive on this, German data privacy law gives everyone pretty wide-reaching control over whether they can be recorded. I'm not sure about the specifics, but a cursory search suggests that if it's at all allowed to film people e.g. in your flat (not talking about the bathroom here, just the living room area or the hallway) without informing them, then you are probably required to delete the recordings within 48 hours if no legal need arises in that time frame.
    – G. Bach
    Mar 21 at 2:47
  • @G.Bach, we're talking about the US here, but since you mentioned Germany: A key requirement would be a sign outside that the premises are monitored by video, why that happens, and where to file complaints.
    – o.m.
    Mar 21 at 7:18
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Typically, the prosecution will go for the "bigger fish" of the two cases and has the discretion not to charge a defendant with a crime if they so choose. It might be that, if a Peeping Tom was to turn in evidence that a neighbor was killing his wife, the prosecutor could strike a deal with the Tom to not charge him with the crime in exchange for his testimony at trial. The rational is that of the two, the murderer is more of a threat to society than the Tom, and if the Peeping Tom must go unpunished. Nothing in the deal needs to immunize the Tom from any future crimes, and police will certainly be knocking on his door if new crimes in the neighborhood come to light. If a witness is granted immunity from prosecution, they cannot invoke their 5th amendment rights on the witness stand, since everything they testify to is never going to be prosecuted.

Another example is that Bob is on trial for the Murder of Alice. The cornor testifies that Alice died no later than 9:30 pm on the night of the 10th. Bob's defense is that he couldn't have done it, because the 10th was on a Friday and Friday's are "Blackjack and Hookers" night, which typically starts at 8 pm and goes well into Saturday morning. Bob not only does this, but produces pictures of him and the girls playing Black Jack with a date stamp of the 10th and all the Hookers in picture testify that Bob was definately there that night and didn't leave for any gap of time long enough to kill Alice. Now, Bob may have some legal problems stemming from the illegal gambling and the solicitation of prostitution, sure... but they are comparitively minor crimes to murdering a human! Bob may even have an explination that will help catch the real killer (such as the coat with his name and blood were left at the scene, which Bob could realize that he cut himself making snacks for the girls in prep for Black Jack and Hookers night, and his brother Charlie offered to clean the coat for him... which tracks with some of the DNA partially matching Bob's DNA... The prosecution could only prove a case against Charlie if Bob testifies and might offer a deal that they will grant him immunity from prosecution for Black Jack and Hookers night if Bob testifies against his brother. Bob can agree and testify, or Bob can refuse and be charged for his crimes (but not the crime he didn't commit). The evidence against Bob entered into the record at his trial for his Friday tradition is now a matter of public record and can be used to link Charlie to Alice without Bob's testimony. The deal is advantageous to Bob because he isn't going to jail on a murder charge and it's advantageous to the prosecution because it means they have to successfully convict Bob before they can even move on Charlie.

In the event that the prosecution discovered the exculpatory evidence through an unconstitutional search and seizure or "Fruit of the Poisonous Tree" (evidence that, while possibley be legally obtained, could only have been done so as a result of evidence obtained by an illegal search) would be barred from introduction by the Prosecution, but not the defendant, since the defendant is not bound by the Constitution (As a general rule, the Bill of Rights are the people's Negative Rights against the Government, which means that the government may not violate those rights unless given permission by the the individual. This comes up in trials from time to time. For example, a defendant may "waive" their right to a trial by jury and instead be granted a bench trial (the Judge alone makes the verdict). One can "waive" their right to Free Speech by not speaking (although U.S. Free Speech Laws are so broad, this is only a technicallity as no speech is a protected form of speech. It's also protected while under oath in that the defendant need not say anything at all and have the presumption of innocence. Or they can waive that by taking the stand and testifying.

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  • From what I can tell, the first two paragraphs don't really fit my question; the first one is about incriminating evidence, the second one about evidence that was gathered legally, but also incriminates the defendant of a lesser crime than he is charged with The third paragraph however is interesting; as far as I understand it, specifically the fruit of the poisonous tree doctrine doesn't bar the defendant from using exculpatory evidence that is such fruit, the doctrine only bars the prosecution from using it as evidence supporting a conviction?
    – G. Bach
    Mar 22 at 1:19
  • If that's correct about evidence gathered by the government, that while "fruit of the poisonous tree" doctrine, or more generally the exclusionary rule, only bar the prosecution from using evidence gathered illegally by the government, then that covers a good lot of cases. Do you have some thoughts on exculpatory evidence illegally gathered by private persons? Say e.g. the Peeping Tom snuck a camera into his neighbor's bedroom and recorded recorded that it wasn't X who killed the neighbor's wife, but Y; can X use that as exculpatory evidence?
    – G. Bach
    Mar 22 at 1:24
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    There's a few stories where criminals will hand over evidence to a crime they committed because they witnessed a more serious crime. Generally, I feel that anything the prosecution needs to do to get the criminal witness to co-operate would be helpful. In the case you mentioned, video evidence supposedly exists so the Tom may not get a sweetheart deal... you don't need him to explain the tape to use it in court. However, eyewitness testimony is also evidence. And yes, X may use this as evidence though "how" they found it needs to be discussed.
    – hszmv
    Mar 22 at 11:54
  • Additionally, at trial there are tools that, if new evidence comes to light, can allow for the trial to go away. The prosecutor may drop the charges at any time prior to the jury getting the case. Additionally, during trial the defense can at two points motion to dismiss charges to the judge (first when the prosecution rests, and second after closing statements but before the judge hands the case over to the jury). Finally, the judge has to power to issue a directed verdict, ordering the jury to find not guilty. The Judge cannot do this for guilt.
    – hszmv
    Mar 22 at 12:00

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