Pressure, especially actual or threatened physical force, put on a person to act in a particular way. Acts carried out under duress usually have no legal effect; for example, a contract obtained by duress is voidable (see also economic duress; undue influence). In criminal law, when the defendant's power to resist is destroyed by a threat of death or serious personal injury or by circumstances, he will have a defence to a criminal charge, although he has the * mens rea for the crime and knows that what he is doing is wrong (R v Graham  1 WLR 294). Duress is not a defence to a charge of murder as a principal (i.e. to someone who actually carries out the murder himself), nor is it a defence to aiding and abetting murder (R v Howe  AC 417 (HL). The threat need not be immediate; it is sufficient that it is effective; for example a threat in court to kill a witness may constitute duress and thus be a defence to a charge of perjury, even though it cannot be carried out in the courtroom (Hudson and Taylor  2 QB 202). However, the defence is unavailable to someone who failed to take available alternative action to avoid the threat. See also coercion; excuse; necessity; self-defence.