Is lying at all in a testimony considered perjury?
<p>Say Bob was murdered at a coffee shop, and Jerry was a witness at the scene. At the trial, he is under oath, and gives his testimony. In his testimony, he explicitly claims that he was watching America's Funniest Home Videos up to the point when the murderer entered the shop.</p>
<p>Later in the trial, video evidence, while first and foremost showing that the defendant did indeed murder Bob, also showed that Jerry was watching something that wasn't America's Funniest Home Videos at the point when the murderer entered the shop. For the purpose of simplicity, let's assume that what he was watching is perfectly fine legally speaking. Let's also assume that he knowingly lied about what he was watching, and didn't make a simple mistake. Even though this detail was virtually irrelevant to his testimony, he still technically lied under oath, which is the definition of perjury.</p>
<p>Could this result in Jerry being criminally prosecuted and/or his entire testimony becoming invalid?</p>

<p>Take the <a href="https://www.law.cornell.edu/uscode/text/18/1621" rel="noreferrer">US federal perjury statute</a> as an example:</p>
<blockquote>
<p>Whoever having taken an oath before a competent tribunal, officer, or person, in any case in which a law of the United States authorizes an oath to be administered, that he will testify, declare, depose, or certify truly, or that any written testimony, declaration, deposition, or certificate by him subscribed, is true, willfully and contrary to such oath states or subscribes any <strong>material</strong> matter which he does not believe to be true ...</p>
</blockquote>
<p>The key word is <em>material</em>, meaning <em>relevant</em>, <em>important</em>, <em>essential</em>.  From your description, the specific show that Jerry was watching is not material to the proceeding in which the testimony was given, so Jerry has not perjured himself.</p>
<p>Other common law jurisdictions will likewise have materiality as an element of perjury.</p>

<p><a href="/questions/tagged/england-and-wales" class="post-tag" title="show questions tagged &#39;england-and-wales&#39;" aria-label="show questions tagged &#39;england-and-wales&#39;" rel="tag" aria-labelledby="england-and-wales-container">england-and-wales</a></p>
<p><a href="https://www.legislation.gov.uk/ukpga/Geo5/1-2/6/section/1?timeline=false" rel="noreferrer">Section 1</a>
Perjury Act 1911 says that:</p>
<blockquote>
<p>(1) If any person lawfully sworn as a witness or as an interpreter in a judicial proceeding wilfully makes a statement material in that proceeding, which he knows to be false or does not believe to be true, he shall be guilty of perjury,</p>
</blockquote>
<p>Therefore, just lying under oath is not enough to be guilty of perjury. The untruth must be shown to be &quot;material&quot; (which has the same meaning given in <a href="https://law.stackexchange.com/q/88451/35069">Nate's answer</a>, i.e. <em>relevant, important, essential</em>).</p>
<p>That said, although Jerry does not appear to have committed perjury, the defence may consider using his untruths as a tactic to confuse or persuade the jury by questioning him under cross-examination as to why he lied in an attempt to discredit his testimony and allege he is not a witness of truth - and possibly also allege the police officer who took his initial statement was incompetent (or maybe corrupt) for not checking the facts. The defence argument could, with a bit of polish and refinement, go something like &quot;the witness lied and the police are incompetent, so how can you trust the video evidence.&quot; Everyone likes a conspiracy theory!</p>

<p>As discussed in <a href="https://www.pennstatelawreview.org/116/2/116%20Penn%20St.%20L.%20Rev.%20331.pdf" rel="nofollow noreferrer">this article</a>, it is well established that a conviction can be nullified if the testimony that it is based on is known by the prosecution to be false. The requirements imposed w.r.t. prosecution knowledge are not easy to satisfy. There is no evidence that the prosecution knew that the testimony was in part false, nor that it was material. Materiality is relevant, see <a href="https://supreme.justia.com/cases/federal/us/473/667/" rel="nofollow noreferrer">US v. Bagley</a>, which holds that</p>
<blockquote>
<p>the prosecutor's failure to disclose evidence that could have been
used effectively to impeach important Government witnesses requires
automatic reversal. Such nondisclosure constitutes constitutional
error and requires reversal of the conviction only if the evidence is
material in the sense that its suppression might have affected the
outcome of the trial.</p>
</blockquote>
<p>As for prosecution for perjury, he can maybe be prosecuted for perjury, though given the facts in evidence he will not be convicted (so the answer about prosecution depends on how the gaps are filled in). Perjury statutes have three essential elements. First, the statement must be false – that much could be true, depending on what he said. If he said &quot;I was watching America's Funniest Cat Videos at the time when Smith entered&quot;, and the other video showed that he was watching WTF Smackdown, the testimony would be false. If he said &quot;I had been watching America's Funniest Cat Videos at the time when Smith entered&quot;, then the statement is false if he had not been watching Cat Vid at all prior to Smith's entry. See <a href="https://supreme.justia.com/cases/federal/us/409/352/" rel="nofollow noreferrer">Bronston v. US</a> for the &quot;literal truth&quot; requirement.</p>
<p>Second, the statement has to be material, not just false. We've covered this – there is nothing in evidence indicating that the statement is material. Third, the prosecution must prove that defendant knew that the statement is false. Although you stipulate a material fact, this is not possible in the real world – the prosecution cannot stipulate that the defense knows that the statement is false. They must introduce evidence proving that he knew that the statement is literally false. It's not possible to subpoena the contents of a person's mind, and without any evidence regarding knowledge, there can be no conviction. So, knowing all of this, the prosecution cannot prosecute him for perjury, until you change the scenario.</p>

perjury