An affirmative defense is a legal strategy in which a defendant acknowledges facts surrounding a criminal allegation but argues that specific circumstances justify or excuse their actions, making them not legally responsible. Unlike other defenses that aim to prove the defendant did not commit the crime, an affirmative defense concedes the act but presents a legal justification or excuse. Key characteristics of affirmative defenses include admission of the act, the presentation of facts that legally justify or excuse the conduct, and the defendant’s burden of proof to establish the defense.

Common affirmative defenses include self defense, where a defendant argues they used necessary and proportional force to protect themselves from imminent harm, and defense of others, which extends the justification to protecting another person. The necessity defense applies when the defendant committed the crime to prevent a greater harm, while duress occurs when the defendant was forced to commit the act under an immediate threat of death or serious harm. Entrapment is used when law enforcement induces a person to commit a crime they would not otherwise have committed. The insanity defense is based on mental illness that prevented the defendant from understanding the nature of their actions or distinguishing between right and wrong. Mistake of fact applies when the defendant acted under a genuine misunderstanding of facts that, if true, would have made their actions legal.

To determine whether an affirmative defense applies, a defendant should first review the elements of the crime and understand what the prosecution must prove. They should then analyze their actions to assess whether a justification or excuse exists, such as self-defense, duress, or necessity. Gathering supportive evidence, such as witness statements, medical records, or expert testimony, is crucial in establishing an affirmative defense.