The final consideration in every criminal case is whether any defenses exist for the conduct. In other words, the person intended to commit the act, and followed through, but cannot be held responsible due to a legally acceptable defense. Insanity is a well known example. Other examples are self-defense, and the lawful exercise of government authority. All of these situations are referred to as “affirmative defenses”, meaning that they must be proven by the defendant. A prosecutor does not have to prove that someone was sane at the time she committed a criminal act, or that she was not acting in self-defense.

In the context of a criminal trial, a person is insane if he either A) is unable to understand that his conduct was wrong, or B) understands that his conduct is wrong but is unable to control himself. It is important to note that the insanity defense is only one of several potential ways in which a defendant’s mental health can be relevant in the criminal process. A person who is found by a court to be mentally ill can be involuntarily committed to a mental health facility, and if the court finds they are insane at the time of the trial, they can be found incompetent to stand trial. Both circumstances are distinct from the insanity defense at trial.

In cases where an individual is found Not Guilty By Reason of Insanity, he will be institutionalized until a doctor certifies that he is no longer a danger to society. Many states also allow for the alternative conviction of Guilty But Mentally Ill, by which the defendant can be found guilty and sentenced for the crime, but will do the term in a mental health facility instead of a prison.

Self-defense, or the justifiable defense of another person or property is another example of a criminal affirmative defense. Under our system of law, it is permissible to use physical force to defend oneself against an imminent and unlawful attack by another. The same principle applies to certain lesser degrees when defending another person, or one’s property. Different states vary on the allowable use of force, and under what circumstances it can be used. For instance, in some states, a person has a duty to attempt to retreat from the situation before using deadly force. In other states, one has no duty to retreat when defending their home or business, but must try to retreat is the confrontation is in public. Another general rule is that self-defense cannot be used by the instigator of the situation. So for instance, if Tom breaks into John’s house late at night, and John points a gun at him and is about to shoot, Tom cannot shoot John first and claim self defense.