For crimes like forcible rape, where lack of consent is an essential element of the crime, consent is, of course, an absolute defense. For all other crimes, consent of the victim is usually not a defense. There are exceptions, however, for certain (usually minor) assaults and batteries. In such cases, courts will recognize consent as a defense if: 1) the criminal act did not involve serious bodily injury or the threat of serious bodily injury, 2) there is widespread acceptance of the risk (as in a sporting event) and, 3) there is a beneficial result of the defendant’s conduct. The Model Penal Code adopts a similar approach
Even where consent can be used as a defense, it will only be valid if the consent was legally effective. In order for the consent to be legally effective, three requirements must be met. First, the consent must have been given voluntarily. Second, the consent must have been given by a person legally capable of consenting. A minor or someone with a mental impairment or someone who is intoxicated at the time he or she gave the consent is not legally capable of consenting. Therefore, consent given by any of these people is not legally effective and cannot be used as the basis for a consent defense. Third, the consent cannot have been given as a result of fraud or mistake. Consent that is given as a result of fraud or mistake is legally ineffective.
Consent can be express or implied. However, the consent is only valid to the extent that the party giving the consent intended. If the consent is implied, the consent is only valid to the extent that a reasonable person would assume under the circumstances. For example: