An Affirmative Defense limits, excuses or voids a defendant's criminal or civil liability. An Affirmative Defense can be valid, even if the allegations against the defendant are admitted or proven. In criminal law, Affirmative Defenses can be broadly categorized as excuse or justification defenses. Justification defenses are allowed when the defendant did not actually violate the law. In other words, it was a difficult situation and the defendant acted lawfully, because his actions were justified.
Justification Defenses include: Self Defense, Defense of Others, Defense of Property, Law Enforcement Defense and Necessity. A criminal defense attorney will assert an excuse defense when a Defendant is incapable of controlling his behavior. Excuse Defenses include: Duress, Intoxication, Insanity, Diminished Capacity, Entrapment and Infancy. An Affirmative Defense is different than a Negating Defense.
A Negating Defense attacks an essential element of the prosecutor's case. For example, if a defendant is arrested for intentional marijuana possession, but the defendant actually thought that she was buying Italian spices, the essential element of intent is potentially missing from the prosecution's case. Unlike an Affirmative Defense, where in most jurisdictions the defendant has the burden of persuasion, when putting forth a Negating Defense, the defendant needs only supply sufficient evidence to raise the issue.