Suppose that Person A is accused of a crime. Person B (unbeknownst to anybody but themselves) actually committed the crime. Suppose further that Person B is called as a witness in the case of Person A.

Would there be a way for Person B to make it apparent to the court that any conviction of Person A for the crime would be invalid, without simultaneously putting themselves on the hook for the crime?

I am asking this primarily in the context of the right to avoid self-incrimination. Could Person B say something along the lines of "I know for a fact that Person A is not guilty of this crime, but I refuse to explain why I know that, on grounds that it would incriminate me". Or, is there no way for the court to accept that Person A's conviction would be unsafe, whilst simultaneously respecting Person B's fifth amendment rights?

  • what is the jurisdiction? What kind of case is it?
    – Trish
    Aug 12, 2019 at 21:31
  • @Trish A hypothetical one. I'd be interested in answers from any jurisdiction, but I suppose the US is the obvious one.
    – Alex
    Aug 12, 2019 at 22:44
  • 2
    even a hypothetical jurisdiction and case needs this, as for example in Germany or Greece or the UK the rights for self incrimination are very different than in the US, and even there, some jurisdictons have bylaws that impact the answer.
    – Trish
    Aug 13, 2019 at 7:31

2 Answers 2


Person B, while on the witness stand, could only respond to valid questions. It is not likely that any question would permit the response "I know A is not guilty, but I can't tell you how I know because it would incriminate me." If B did say something of this sort, and was believed, the authorities would no doubt have strong suspicion that B was at least involved, and would investigate B. Since B chose to say that much, such an investigation would not be barred by the "fruit of the poison tree" rule. B would be taking a serious risk by doing this.

And of course, B might not be believed, and A might be convicted anyway.

  • That makes sense, thank you. I'm surprised, though. Apparently there was an 1895 U.S. Supreme Court case which stated, “it is better to let the crime of a guilty person go unpunished than to condemn the innocent.” Does the court really have no mechanism for B to highlight A's innocence without condemning himself (based on the idea that if B were not assured of that, he would not make such a testimony)? Surely, this violates the statement above, since if B is given such an opportunity, a guilty man goes free, whereas, if B is not, an innocent man is imprisoned.
    – Alex
    Aug 8, 2019 at 22:27
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    @Alex the problem with that is that if such a mechanism existed then there would be a market for stooges who would do just that for money - "I know they are innocent, but I cannot give you enough information to corroborate my claim". Yes, it would be perjury, but that would be just as difficult to prove if the court had to accept the claim at face value. Does the claim "I know something but Im not telling you enough to validate it" really introduce reasonable doubt? Thats for the judge and jury to decide.
    – user4210
    Aug 8, 2019 at 22:44
  • 1
    @Moo, that's a very good point, I hadn't considered that... Thank you! :)
    – Alex
    Aug 8, 2019 at 22:45
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    @Alex: "Does the court really have no mechanism for B to highlight A's innocence without condemning himself (based on the idea that if B were not assured of that, he would not make such a testimony)?" B can (prior to being called as a witness) ask for immunity in exchange for his testimony. If the person with appropriate authority (prosecutor, judge, governor, etc.) agrees to this in writing, and then B reveals Bself as the true perpetrator, then that authority figure will be publicly embarrassed, so they usually tend to be quite careful in handling immunity deals. Jan 6, 2023 at 21:33

There was a similar case in the USA (but reported on a computer site in the context of facial recognition, not because any law aspects).

Mrs. A stole things in a store. She was almost caught by two store detectives, but managed to escape. Shortly later, Mrs. B matching the description was arrested, the two store detectives swore she was the thief, and she was convicted.

A local newspaper reported about the case, including a photo of Mrs. B. Mrs. A saw the report, realised what had happened, and went to the police confessing the theft. Mrs. B was released. Unfortunately it was not reported what happened to Mrs. A.

The newspaper then also published photos of Mrs. A and Mrs. B. They looked absolutely identical. So the two store detectives couldn't be blamed one bit for identifying Mrs. B.

I would hope that a judge would give Mrs. A the most lenient sentence possible, because she (1) confessed without being a suspect at all, and (2) through her confession kept an innocent person out of jail.

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