Lies, on their own, are not sufficient evidence of a crime though in some situations they may themselves be a crime—making false statements, fraud, false advertising, perjury. However, lies may indicate that the defendant is aware of his own guilt, and the prosecution may rely on the fact that the defendant has lied as supplementary to other evidence.
England and Wales
In England, if the prosecution seeks to rely on the fact that the defendant lied (for example, to police), it is sometimes necessary for the judge to give a direction to the jury (known as a Lucas direction).
The direction is not used where the prosecution attempts to show that the defendant committed the crime, and, if the jury find the defendant guilty, this would mean the defendant had lied. The direction "comes into play when the prosecution say, or the judge envisages that the jury may say, that the lie is evidence against the accused; in effect using it as an implied admission of guilt .... this is quite distinct from the run of the mill case in which the defence case is contradicted by the evidence of the prosecution witnesses in such a way as to make it necessary for the prosecution to say ... that the defendant's account is untrue and indeed deliberately and knowingly false". It is appropriate for a judge to give a Lucas direction:
The direction has three parts: