In a court of law, the defendant can tell as much lies as she wants in order to vindicate herself of the charges. But if the court finds them guilty, the false explanations that the person had given are not revisited. Why is this so? Even if a person is doing so to save her skin, shouldn't giving false testimonies in court be regarded as a crime after the person has been proven guilty?

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    How broad the right to not incriminate yourself is varies by jurisdiction. So you should include one in the question. Jul 27 '18 at 10:41
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    What jurisdiction is it?
    – ohwilleke
    Jul 27 '18 at 14:22

In a court of law, the defendant can tell as much lies as she wants in order to vindicate herself of the charges. But if the court finds them guilty, the false explanations that the person had given are not revisited. Why is this so? Even if a person is doing so to save her skin, shouldn't giving false testimonies in court be regarded as a crime after the person has been proven guilty?

It isn't clear which country you are talking about as the rules are not uniform.

In the U.S. there is absolute immunity from civil liability (i.e. lawsuits) for testimony given in court, to prevent collateral litigation.

Also, in the U.S., it is very common for a defendant not to testify at all in a criminal case, indeed that probably happens in most criminal trials, because if they do, their testimony can be impeached with evidence of their prior criminal record, and most criminal defendants think that the harm of revealing a prior criminal record to a jury is greater than the benefit of testifying (empirically, this is a somewhat doubtful question).

In theory, in the U.S. a defendant who does testify under oath in a criminal case against him could be prosecuted for perjury if he lies. But, proving perjury beyond a reasonable doubt is often more difficult than proving guilty of an underlying crime beyond a reasonable doubt. A jury verdict of guilty does not always even mean that the jury found the defendant's testimony to be not credible, and even if it did, it could have found that the defendant's testimony was not credible for reasons other than a lie, such as failure of memory, misperception of the events, differences of opinion or characterization, etc. Also, not all lies are about material facts and in a perjury prosecution you have to prove that the fact lied about was material.

In the entire State of Colorado there are a dozen or two instances of perjury that are prosecuted (some of which do not involve court testimony) and people actually commit perjury many times a day in courts across the state. This is very typical of other U.S. states and of the federal judicial system.

In England and Wales, the compromise is handled differently, to allow more criminal defendant testimony. Criminal defendants, unlike other witnesses in a criminal case, are not under oath and cannot be impeached with their criminal records. And, since they are not under oath they cannot be prosecuted for perjury either. The jury can consider what a criminal defendant says, but knows that it is not sworn which (theoretically) might make it less credible evidence.

Also, keep in mind that in England and Wales there is a formal sentencing penalty for taking a case to trial and in the U.S. there is a large disparity in practice between sentences of people who went to trial and sentences of people who did not go to trial, so there is already an implicit penalty for denying guilt at trial when you are found to be guilty beyond a reasonable doubt.

The main priority of prosecutors and law enforcement is to establish a record of proven criminal conduct and punish someone for it. If that is accomplished, a lie that ultimately wasn't believed by the jury really didn't change much. By analogy, in a civil case for fraud, you can only prevail if someone actually believed your lie about a material point, which is almost never the case when the jury convicts someone about the charge relevant to the lie. No harm, no foul.

This is weighed against a fairly modest expense caused by going to trial. The cost of the criminal justice system is quite low, in part due to low salaries and in part because of the way that these cases are handled. In a typical U.S. jurisdiction the combined amount prosecutors and public defenders are paid per felony case on average is under $100, which isn't a huge amount per trial even after plea bargains are omitted, so the harm associated with taking the case to trial knowing you are guilty and lying to attempt to disprove your guilt, isn't huge when someone is convicted anyway.

Acquittals at trial are not very common. About 1-2% of all criminal cases that are not dismissed by the prosecution without any conviction result in an acquittal, about 4-8% result in a conviction following a trial and 90%-95% result in a plea bargained conviction in the U.S. Also, at least half of those acquittals at trial are in case where the defendant doesn't testify, acquitted defendants who testify at trial aren't always lying, juries are only about 90% accurate in assessing credibility, and prosecuting someone for lying in a case where there is an acquittal would create an appearance of not trusting the jury to do their job of assessing credibility which is institutionally problematic for prosecutors.

Illustrating the dangers of prosecuting people whose testimony is not believed for lying is a recent case where a rape victim was not believed by police and prosecuted and convicted of making a false statement to police, only to have her statement vindicated two decades later by DNA testing that resulted in the person she accused being convicted of rape. (The dispute was over whether the two people had sex, not over whether there was consent to sex that admittedly took place.)

All this said, it is very frustrating that lying in court is very common (although it isn't just defendants who lie, police and other witnesses also often lie in court), but it isn't very easy to design a good and workable way to discourage this from happening. If it was easy to be sure when someone is lying and when they are telling the truth, we'd scarcely need evidentiary trials at all.

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    I may update this with some citations if I have time.
    – ohwilleke
    Jul 27 '18 at 20:30
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    Important to note: bringing up the prior record isn't just for defendants, but witnesses in general. Also, it can't just be any crime. Judges weigh, in considering whether to admit a prior conviction, the type of crime and several other factors to ultimately decide whether the evidence would be helpful to a jury in determining the person's credibility vs whether it would unfairly prejudice the jury against him. Also, more likely that crimes of dishonesty will be admitted than other types, as that is what speaks to the credibility issue when impeaching a witness.
    – A.fm.
    Jul 29 '18 at 3:45
  • @A.fm. The relevant rule of evidence actually leaves judges with very little discretion. All felonies almost always come in if they aren't more the a certain number of years old, any prior dishonesty conviction almost always comes in.
    – ohwilleke
    Jul 29 '18 at 22:28
  • You're narrowing my statement but leaving yourself (necessary) outs with, eg, "all felonies almost always." That's simply not the rule. This changes by jurisdiction, of course. For ex, many have a timeframe within which convictions must have occurred (eg, w/in past 10 years in Illinois). If the prior conviction is for the same crime (not same occurrence) as current charges, likely to be held highly prejudicial against defendant and not allowed, esp if alt way to impeach exists. Some states bar prior conviction evidence unless the defense has attempted to establish the defendants credibility.
    – A.fm.
    Jul 31 '18 at 1:26
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    Your comments about English criminal procedure are very out of date. The possibility of a defendant giving unsworn evidence was abolished in the early 80s (legislation.gov.uk/ukpga/1982/48/section/72) and the usual practice was for a defendant to give sworn evidence since the late 1890s. It took me a while to dig up the reference, but my experience (at trial) is that defendants are sworn/affirmed. Please delete the E&W material as it is very misleading. Mar 6 at 16:04

I don't believe your premise is necessarily true. As the Supreme Court has held, a defendant can be prosecuted for perjury after being convicted for another offense at trial:

The conviction of Williams, at a former trial, for beating certain victims is not former or double jeopardy. Obviously perjury at a former trial is not the same offense as the substantive offense, under 18 U.S.C. § 242, of depriving a person of constitutional rights under color of law. . . . It would be no service to the administration of justice to enlarge the conception of former jeopardy to afford a defendant immunity from prosecution for perjury while giving testimony in his own defense.

United States v. Williams, 341 U.S. 58, 62 (1951) (emphasis added).

Now as a practical matter, trying a convicted person for perjury is most likely rare, since the prosecutor has already obtained a conviction for the underlying offense. Starting another prosecution for perjury might be excessive or unnecessary. But that doesn't mean it's categorically prohibited.

Note also that perjury by the defendant can be considered by the judge to enhance the defendant's sentence. United States v. Dunnigan, 507 U.S. 87, 96 (1993).

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    So the convicted for something else perjurer is being punished, especially considering the maximum for punishment (in US federal court anyway) is only 5 years. There is also the issue of concurrent vs consecutive punishments, which means that the convict may not even serve any more time if they did receive a separate conviction for perjury.
    – sharur
    Jul 27 '18 at 16:24
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    On the other hand, prosecuting a person for lying to a federal agent under 18 USC 1001 is incredibly common (but usually the false statements in question were made long before the trial began).
    – Kevin
    Jul 28 '18 at 1:04
  • @sharur Let's put it this way: once a defendant is convicted and sentenced for murder, nobody really cares or has the time to hash out the details of what point or counterpoint made during a trial was a lie or not. Everyone got what they came for and it's on to the next one.
    – A.fm.
    Jul 29 '18 at 3:48

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