Insanity is a legal concept, not a medical diagnosis. Historically, the Sharia, in Moslem law, may be the earliest example of a lesser punishment of a person who commits a homicide while in an altered mental state. Both children and “lunatics” are considered unable to intend to kill another and so a homicide by either was deemed unintentional, punishable only by a fine. Intentional or unintentional homicide is punishable by death.

With the rise of humanism, the enlightenment, and the acceptance of the scientific method in Western cultures, a greater acceptance arose that a verdict of murder required both the act of killing another human being (actus reus) and the ability to have a guilty mind (mens rea). This is more or less the state of affairs today.

Insanity is defined differently in different jurisdictions and the evaluator of the accused’s mental state must be familiar with the applicable law. Some jurisdictions, such as Idaho, Michigan, Montana and Utah, have no provision for completely exculpating a “lunatic” but instead provide for a “guilty but mentally ill” verdict, thus legally criminalizing the act. Most states use the M’Naghten (referred in some later texts as “McNaugton”) Rule or a variant of it.

The original M’Naghten Rule arose out of a notorious homicide in England in 1843 by Daniel M’Naghten, when he shot and killed Edward Drummond, the secretary of the Prime Minister of England, instead of his intended victim, the Prime Minister himself. M’Naghten did not flee and upon his arrest clearly exhibited a paranoid delusion to the doctors at Bethlehem (Bedlam) Hospital who later testified at his trial. Because of the gravity of the case, Lord Chief Justice Tindal presided, and the jury ruled unanimously M’Naghten was not guilty, by reason of insanity. An uproar ensued and an explanation demanded of the 15 Law Lords by Parliament. The explanation of the court’s reasoning is found at 8 Eng. Rep. 718, 8 Eng. Rep. 722 (1843). This case is the first time insanity was defined at an appellate level.

Lord Chief Justice Tindal framed the issue to the jury:

“The question to be determined is, whether at the time the act was committed, the prisoner had or had not the use of his understanding, so as to know that he was doing a wrong or wicked act. If the jurors should be of the opinion that the prisoner was not sensible, at the time he committed it, that he was violating the laws both of God and man, then he would be entitled to a verdict in his favor of insanity.”

However, even if the accused suffered from a delusion which influenced his actions, if he knew the act was contrary to law, he is punishable and not insane.

In a circumstance where a man is under the influence of a delusion whereby he irrationally believes the victim is going to kill him and so he acts in self-defense, then he is insane. On the other hand, if he believes the victim merely slandered him or otherwise injured his character, then he is not insane because the killing is viewed as revenge.

Today, the M’Naghten Rule states that a person “may be insane if at the time of committing of the act, the accused was laboring under such a defect of reason, arising from a disease of the mind, as not to know the nature and quality of the act he was doing, or, if he did know it, that he did not know what he was doing was wrong.”

Following John Hinckley’s acquittal by reason of insanity (under the Durham Rule) of charges in his assassination attempt of President Reagan, the Insanity Defense Reform Act of 1984 (U.S.) was enacted. Insanity was re-defined to occur if “the defendant, as a result of severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts.” For the defendant, this is an affirmative defense. The Act differs significantly from M’Naghten in two ways:

• The mental disease or defect must be severe.
• The defendant has the burden of proof and must prove his case by “clear and convincing” evidence, not merely by the “preponderance” of evidence, which is what most states require.

Other legal descriptions of insanity are not discussed here, but may be relevant in certain jurisdictions:
Model Penal Code from ALI
Irresistible Impulse

In my practice, most successful insanity defenses have involved a person suffering from paranoid delusions, usually schizophrenia, often with command hallucinations. The presence of delusions and/or hallucinations seem to a jury to be more transparently a “mental disease or defect” than, say, a homicidal mother suffering from a post-partum depression with psychosis who kills her child because her delusion includes killing herself and her child or children.

Since M’Naghten involves “knowledge” as a component, the existence of delusions and other misperceptions of reality and misunderstanding about the meanings of the actions of others are understandable to the jury as to why the person’s mental state makes him insane.

Obsessional stalking behavior resulting in a homicide of the person stalked is usually too goal directed, organized, and logical to be deemed insane unless the stalking is the result of a delusional system. Even then, the stalking may appear “sane” to the jury because the psychiatrist must explain why the defendant was sneaking around if he didn’t know what he was doing was wrong.

“>Note: My evaluations of murderers and alleged murderers (approximately 100) in capital cases have been fascinating. Elsewhere on this site I discuss my examination of David Carpenter, before he was dubbed the “Trailside Killer” in connection with his murder of 11 women.