fifth-amendment perjury testimony knowledge
What&#39;s stopping someone from saying &quot;I don&#39;t remember&quot;?
<p>There are (at least) two problems possibly stemming from a person testifying in court:</p>
<ul>
<li><p>If they tell the truth, the testimony may have negative consequences they potentially don't like (which is the whole point of 5th Amendment, when the testimony is by the accused or in general detrimental to person testifying).</p>
</li>
<li><p>If they lie, they can be convicted of perjury, which is a nice fallback when you don't care about actual crime but about getting the person damaged at least somehow - US history and especially politics is littered by people who were gotcha-ed by perjury charges and not main charges.</p>
</li>
</ul>
<p>If that is the case, then <strong>what prevents such a person from simply saying &quot;I do not remember&quot; when testifying</strong>? You can't prove a negative, so they can't be convicted of perjury for lying about whether they remember; they didn't testify about any fact so they can't perjure themselves; and they didn't help the prosecution (or defense) by truth.</p>

<blockquote>
<p>You can't prove a negative, so they can't be convicted of perjury for lying about whether they remember;</p>
</blockquote>
<p>Yes, they can.  The government can convict the witness of perjury by proving that they <em>did</em> remember.  This may be difficult, but it's not impossible.</p>
<p>Now you may say that we can't ever prove with certainty what a person did or didn't remember.  But the legal standard isn't <em>certainty</em>, it's <em>beyond a reasonable doubt</em>.  So a jury is allowed to draw a <a href="https://www.law.cornell.edu/wex/inference" rel="noreferrer">reasonable inference</a> about whether they remembered, based on evidence of their outward behavior and other circumstances.</p>
<p>As an extreme example, suppose Alice was overheard chatting freely about topic X an hour before the trial, but when asked about topic X on the stand, said she didn't remember.  When presented with evidence of her earlier conversation, a jury could reasonably infer that she was lying about not remembering.  Is it <em>possible</em> that she truly had a memory lapse in the intervening hour?  Sure, anything is possible.  Is it <em>reasonable</em> to believe that she did?  Probably not.</p>
<p>The <a href="https://www.justice.gov/archives/jm/criminal-resource-manual-1753-perjury-cases-special-problems-and-defenses-evasive-and" rel="noreferrer">US Department of Justice's Criminal Resource Manual</a> has this to say on the subject:</p>
<blockquote>
<p>Witnesses who claim not to remember, rather than deny a fact, may be prosecuted for perjury. However, the government must prove both that the witness at one time knew the fact and that the witness must have remembered it at the time he or she testified. United States v. Chen, 933 F.2d 793, 795 (9th Cir. 1991). If the dates of the transaction and testimony are sufficiently close, memory may be inferred. Instances in which the witness remembered other events that occurred at the same time or earlier than the event in question, or mentioned the event either immediately before or after his testimony, would be probative of the witness's memory at the time of the testimony. The two witness rule does not apply to prosecutions based on false memory lapses, and circumstantial evidence is sufficient, since there is no direct evidence possible concerning what the defendant actually believed. Gebhard v. United States, 422 F.2d 281, 287 (9th Cir. 1970).</p>
</blockquote>
<p>There's a similar issue in all laws that deal with a person's knowledge or intent.  Suppose Alice hits Bob with a stick and he dies.  To convict Alice of murder, it must be proved that by hitting Bob, she intended to kill him.  Can we ever really know what was in her heart?  Maybe not, but if there is evidence that shortly beforehand, she told someone that she was going to kill Bob, it would be a reasonable inference that this was her intention.</p>

<p>Anything you say in a court of law can be used against you in a court of law, unless the testimony is given under a grant of immunity. You would need to provide a transcript of the hypothetical testimony to get the understanding that you're looking for, but it is easy for a person to contradict themselves, claiming that you saw Jones on some date, and that you can't remember if you have ever seen Jones. Your &quot;can't remember Jones&quot; testimony can be refuted with physical evidence such as emails, journal entries, photographs or testimony such as a person testifying that they saw you talking to Jones in a manner that clearly indicates that you knew who he was.</p>
<p>You might avail yourself of the brain-surgery defense, to the effect that you may have done so but in the interim you had brain surgery and you can't remember anything from that month, but the jury will not believe that unless you provide convincing positive evidence that you had such surgery and that there is sufficient medical reason to believe that lack of recollection is possible.</p>
<p>Although this is not ideally how it is supposed to play out, if the prosecution offers some reason to think that you lied on the stand, you have to offer a reason to doubt the prosecution's position. In some jurisdictions, the finders of fact are told that they should be &quot;firmly convinced&quot; of the prosecution's claim, but there is a real tendency for juries to understand &quot;reasonable doubt&quot; as referring to &quot;a reason to doubt&quot;. That is how you &quot;prove a negative&quot;, by offering evidence that contradicts the claim.</p>

<p>Nothing stops a person from saying just that. Ronald Reagan famously did so when subpoenaed during Iran-Contra.</p>
<p><a href="https://www.chicagotribune.com/news/ct-xpm-1990-02-23-9001160156-story.html" rel="nofollow noreferrer">https://www.chicagotribune.com/news/ct-xpm-1990-02-23-9001160156-story.html</a></p>
<p>It was widely regarded as a dodge at the time. On the other hand he was diagnosed with Alzheimer's a few years later. (But on the gripping hand, his doctors say he didn't start showing signs of that until 1993. <a href="https://www.baltimoresun.com/news/bs-xpm-1997-10-05-1997278112-story.html" rel="nofollow noreferrer">https://www.baltimoresun.com/news/bs-xpm-1997-10-05-1997278112-story.html</a> So maybe it was a dodge.)</p>
<p>Other answers here have give reasons that doesn't always work. But this famous precedent seems relevant.</p>

<p>What stops them from claiming they don't remember?  Not much can stop them from doing it.  But there can be repercussions.  Another poster rightfully pointed out that someone CAN be convicted of perjury for claiming they couldn't remember something.  And even if the witness is never charged and prosecuted for perjury, the judge or jury is entitled to take everything the witness says (including claims that they can't remember something) into account when deciding whether they believe what the witness is saying.  If the facts and circumstances lead a judge or jury to doubt that the witness is telling the truth about being unable to remember, they might give the testimony of this witness (including unrelated things testified to by the witness) less weight because they doubt the witness's credibility.  And if the witness tries to later claim that they now remember the information they previously claimed to be unable to remember, that can similarly impact their credibility.</p>
