Under this defense, the lawyer argues that the defendant lacked the capacity to distinguish right from wrong or to understand the likely consequences of his/her actions Examples of incapacity include:
The insanity defense is an affirmative defense. An affirmative defense means that the burden is on the defendant to present evidence to rebut the presumption of sanity. Under this defense, the defense lawyer does not deny that the defendant committed the elements of the alleged crime but still attempts to prove the innocence of the defendant because, generally speaking, he did not appreciate the wrongfulness of his conduct or was unable to conform his conduct to the law.
The insanity defense is based on the principle that the government can justify punishment only if the accused is capable of controlling his behavior and was capable at the time the crime was committed of understanding that his actions were wrongful. If the defendant was incapable of distinguishing right from wrong when he committed the crime, he cannot be held criminally responsible. An insanity defense generally rests on the testimony of a psychiatrist who examined the defendant, his history, and the facts of the case. In the United States, the court will appoint a psychiatrist at the government's expense to examine the defendant if he cannot afford to hire his own psychiatrist.
The insanity defense is a controversial and complicated topic and a full discussion is beyond the scope of this resource. There are various definitions and standards for insanity.
If a defendant is acquitted as a result of the insanity defense, he is released. Because of the controversial nature of the plea, and the obvious concern about releasing a person who has committed a crime, many jurisdictions have adopted a "guilty but mentally ill" or "guilty by reason of insanity" plea. Under this plea, a defendant admits guilt and is sentenced to a mental health institution until he is judged able to return safely to society.
The defendant was too young to be held criminally liable for the crime
The defendant was mentally incapable of the intent required for the crime