In jurisprudence, duress or coercion refers to a situation whereby a person performs an act as a result of violence, threat, or other pressure against the person. Black's Law Dictionary (6th edition) defines duress as "any unlawful threat or coercion used... to induce another to act  in a manner  otherwise would not ". Duress is pressure exerted upon a person to coerce that person to perform an act they ordinarily would not perform. The notion of duress must be distinguished both from undue influence in the civil law. In criminal law, duress and necessity are different defenses.

Duress has two aspects. One is that it negates the person's consent to an act, such as sexual activity or the entering into a contract; or, secondly, as a possible legal defense or justification to an otherwise unlawful act. Defendants utilizing the duress defense admit to breaking the law but claim that they are not liable because, even though the act broke the law, it was only performed because of extreme, unlawful pressure. In criminal law, a duress defense is similar to a plea of guilty, admitting partial culpability, so that if the defense is not accepted then the criminal act is admitted.

Duress or coercion can also be raised in an allegation of rape or other sexual assault to negate a defense of consent on the part of the person making the allegation.

Discussion.

A defendant who raises a defense of duress has actually done everything to constitute the actus reus of the crime, and has the mens rea because they intended to do it to avoid some threatened or actual harm. Thus, some degree of culpability already attaches to the defendant for what was done.

In criminal law, the defendant's motive for breaking the law is generally irrelevant unless a defendant is raising an affirmative defense allowed for by law. (Duress may or may not be allowed as an affirmative defense for some particular charge -- in particular, it is generally forbidden for murder, and many jurisdictions also forbid it for sexual assault. Malum in se offences, generally, are less likely to recognize duress as a defense than malum prohibitum offences.)

A successful affirmative defense means not that a criminal act was justified, but that the act was not criminal at all. But if no affirmative defense of duress is available, then the duress may be considered as justifying a lighter sentence, typically in proportion to the degree of duress. If the duress is extreme enough, for example, the defendant might be found guilty of murder but given a minimal, or even trivial, sentence.

In some rare cases, a successful argument of duress -- even when not an affirmative defense -- might result in the jury nullifying the charge by refusing to convict.

The basis of the defense is that the duress actually overwhelmed the defendant's will and would also have overwhelmed the will of a person of ordinary courage (a hybrid test requiring both subjective evidence of the accused's state of mind, and an objective confirmation that the failure to resist the threats was reasonable), thus rendering the entire behavior involuntary. Thus, the liability should be reduced or discharged, making the defense one of exculpation.