Consent by the injured party can negate the existence of a tort as is apparent in the Latin phrase volenti non fit injuria – to one who is willing, no wrong is done. Consent is judged by its objective manifestation: if a reasonable person would believe that the injured party consented there will be no tort, even if there was no willingness in fact. In certain circumstances silence or inaction will constitute consent.
Even where there is consent, the defendant cannot exceed the scope of the consent. For example, a football player can expect to be tackled and a serious injury may result with no tort; however, if he were tackled during a stop in play or on the sidelines, he would probably have a cause of action against the tackler – the action lay outside the customs of the game and therefore exceeded the scope of consent. Likewise, if the defendant performs a substantially different act than what was consented to, a tort will lie.
Consent can be revoked before the act or even during the act. For example, one could consent to sexual contact and then revoke that consent during the act, and the defendant could be liable for battery if he continues with the contact. Furthermore, consent can be vitiated by an affirmative misrepresentation. So, if the defendant knew or reasonably should have known that the plaintiff was mistaken as to what she was agreeing to, then he can still be guilty of an action. But, the mistake on the plaintiff's part must extend to the “essential character” of the act itself rather than some collateral matter. There are several interesting cases involving consensual sex between a husband and wife where the husband was found guilty of battery for not disclosing STDs contracted in an extramarital affair.
A majority of states also hold that one cannot consent to an illegal act, although a minority of states and the Restatement hold that the consent will defeat a civil action except where the force exceeds the consent. One must also be legally capable of consenting; therefore, an infant or an incapacitated person cannot legally consent (this rule usually extend to intoxicated people as well, though the level of intoxication necessary to vitiate the consent varies). There is one exception to this rule, known as the “emergency exception.” In the emergency exception the person must be unconscious and time must be of the essence. In these situations the court will look at whether a reasonable person would have consented had they been conscious.