Insanity defense
In criminal trials, the insanity defenses are possible defenses by excuse, by which defendants argue that they should not be held criminally liable for breaking the law, as they were legally insane during the commission of alleged crimes. It is important to note that the legal definition of "insane" in this context is quite different from psychiatric definitions of "mentally ill", also that the definition of insanity varies between jurisdictions.
The name "insanity defense" is still in use in the United States, while Australia and Canada have renamed it the mental disorder defence, as Commonwealth nations tend to shy away from it, partially due to the stigma of the word "insanity". When the insanity defense is successful, the defendant may be committed to hospital.
In the United Kingdom and the United States, use of the defense is rare and it is more common  to rely upon a state of temporary mental impairment.
In the United States a state of temporary mental impairment is not a defense. It falls under the  category of a mitigating factor referred to as "diminished capacity". A mitigating factor (which can include conditions not eligible for the insanity defense such as intoxication) can be used to attempt a  reduction of  the charges to a lesser offense or  in a reduced sentence.
The insanity defense is available in most jurisdictions that respect human rights and have a rule of law, though the extent to which it can  be applied may differ widely between jurisdictions. One example is in cases involving the battered-woman syndrome. Where self-defense is not applicable, a defendant may try to use the insanity defense, although this is almost never successful in the United States. An alternative  is the use of provocation as a defense. The battered woman syndrome can be entered as a mitigating factor  in  United States jurisdictions.
The insanity defense is based on evaluations by  forensic professionals that the defendant was incapable of distinguishing between right and wrong at the time the offense was committed. In addition, some jurisdictions require that the evaluation address the issue of whether the defendant was able to control his behavior at the time of the offense. A defendant making the insanity argument might be said to be pleading "not guilty by reason of insanity" (NGRI). A successful NGRI defense can result in an indeterminate commitment to a psychiatric facility.
Diminished responsibility (or diminished capacity) can be employed as a mitigating factor and is applicable to more circumstances than the  insanity defense in the United States.  For example, some jurisdictions accept inebriation or other drug intoxication as a mitigating factor, whereas intoxication alone is not accepted as an insanity defense.  If diminished responsibility (or capacity) is presented convincingly, the charges may be reduced to a lesser offense or  the sentence may be more lenient.
"Insane" and "insanity" are legal terms and not used by mental health practitioners to mean a psychological disorder.

## Mitigating factor
The United States Supreme Court in Penry v. Lynaugh and the United States Court of Appeals for the Fifth Circuit in Bigby v. Dretke have been clear in their decisions that  jury instructions in death penalty cases that do not ask about mitigating factors regarding the defendant's mental health violate the defendant's Eighth Amendment rights,  saying that the jury is to be instructed to consider mitigating factors when answering unrelated questions. This ruling suggests specific explanations to jury is  necessary to weigh mitigating factors.

# Psychiatric treatments
Those found to have been not guilty by reason of insanity are generally then required to undergo psychiatric treatment, except in the case of temporary insanity (see below). Defendants found not guilty by reason of insanity are generally placed in a mental institution.  Unlike defendants who are found guilty of a crime, they are not institutionalized for a fixed period, but rather held in the institution until they are determined not to be a threat. Authorities making this decision tend to be cautious and as a result, defendants can often be incarcerated for longer than they would have been in prison. In Foucha v. Louisiana (1992) the Supreme Court of the United States ruled that a person could not be held "indefinitely".
So far, in the United States, those acquitted of a federal offense by reason of insanity have not been able to challenge their psychiatric confinement through  a writ of habeas corpus or other remedies. In Archuleta v. Hedrick, 365 F.3d 644 (8th Cir. 2004), the U.S. Court of Appeals for the Eighth Circuit the court ruled against  federal insanity acquittees who want to challenge their confinement. Further, it ruled that  insanity acquittees may not attack their initial successful insanity defense.

## Incompetency and Mental Illness
An important distinction to be made is the difference between competency and criminal responsibility.
- The issue of competency is whether a defendant is able to adequately assist his attorney in preparing a defense, make informed decisions about trial strategy and whether or not to plead guilty or accept a plea agreement. This issue is dealt with in UK law as "fitness to plead". *Criminal responsibility, however, deals with whether a defendant can be held legally responsible for his criminal behavior.
Competency largely deals with the defendant's present condition, while criminal responsibility addresses the condition at the time the crime was committed.
In the United States, a trial in which the insanity defense is invoked typically involves the testimony of psychiatrists or psychologists who will present opinions on the defendants state of mind at the time of the offense. Mental health practitioners are restrained from making a judgment on the issue of whether the defendant is or is not insane or what is known as the "ultimate issue".
Insanity is a legal concept, not a psychiatric concept of mental illness.  If a person has a diagnosed mental disorder is not sufficient reason, from the court's point of view, to relieve them from all responsibility for illegal acts they may commit. A person may have a mental disorder and be a competent person in many other ways,  able to write checks, handle his personal affairs, hold a job and carry on a variety of behaviors despite the mental disorder. Likewise, a person may commit a criminal act, independent of the fact that he has a mental disorder. Depending on the jurisdiction, other elements need to be proven, for the court to accept that the mental disorder was responsible for the criminal act, that is,  it must be shown that the defendant committed the crime because of the mental disorder. For example, the mental disorder interfered with his ability to determine right from wrong at the time the offense was committed.
Since it would unduly stigmatize  a person with a diagnosed mental illness to say that because of the mental illness he is not responsible for his behavior. Therefore, persons whose mental disorder is not in dispute will be determined sane as the court will decide that despite a "mental illness" the defendant was responsible for the acts he committed and he will be treated in court as a normal defendant. If the person has a mental illness and it is determined that the mental illness interfered with the person's ability to determine right from wrong, and other associated criteria a jurisdiction may have, and if the person is willing to plead guilty or is proven guilty in a court of law, some jurisdiction have an alternative option known as either a Guilty but Mentally Ill (GBMI) or a Guilty but Insane verdict. Michigan (1975) was the first state to create a GBMI verdict. Sometimes a person without mental illness can be found to be insane; for example, a person who is acting under the influence of a drug that was involuntarily administered (though voluntary intoxication has been rejected by most jurisdictions as a defense to crime). (See: Intoxication defense)

# History of the insanity defense
The concept of defense by insanity has existed since ancient Greece and Rome. Edward II, under English Common law, declared that a person was insane if their mental capacity was no more than that of a "Wild Beast".  The first complete transcript of an insanity trial dates to 1724. The fate of insane defendants was uncertain in the United Kingdom until the Criminal Lunatics Act 1800, following the acquittal of James Hadfield, provided for their indefinite detention.
The insanity plea was codified in English law with the M'Naghten Rules, which came as a direct result of the attempted assassination of British Prime Minister Robert Peel in 1843(1). The rules define the defense as "at the time of the commission of the acts constituting the offense, the defendant as a result of a severe mental disease or defect, was unable to appreciate the nature and quality of the wrongfulness of his acts."  The key is that the defendant could not appreciate the nature of his actions during the commission of the crime.
However, in the United States, the insanity defense could be credited to New Hampshire Chief Justice Charles Cogswell Doe.
In Ford v. Wainwright 477 U.S. 399 (1986), the US Supreme Court upheld the common law rule that the insane cannot be executed. It further stated that a person under the death penalty is entitled to a competency evaluation and to an evidentiary hearing in court on the question of his competency to be executed.
In Wainwright v. Greenfield, the Court ruled that it was fundamentally unfair for the prosecutor to comment during the court proceedings on the petitioner's silence invoked as a result of a Miranda warning. The prosecutor had argued that the respondent's silence after receiving Miranda warnings was evidence of his sanity. 

# Controversy over the insanity defense
There are many different interpretations of "insane" and many different notions of how to deal with insane individuals.
Some opponents of the insanity defense, including Thomas Szasz, believe that psychiatry itself emerged as a way to justify mercy, of making persons "exceptional" and thus not deserving of the harsh punishment we would as a society wish to dole out to people who had extremely selfish or widely shared rationales for their actions. Since extreme selfishness ("self-absorption") or broadly shared resentments (e.g. envy of the rich, hatred of another ethnic group) are somewhat infectious behaviors, some argue that schizophrenia and other "mental illness" were defined into existence to protect those whose motives and behaviors were not so infectious, and whose offenses were thus unlikely to be repeated by others. The cost of this system of mercy, however, was to classify the psychiatrist and patient in an ongoing unequal-power relationship (See anti-psychiatry).
In ancient Rome, Latin tribes held various religious beliefs that included considering the insane to be divinely blessed, and therefore beyond the reach of human jurisdiction. It is alleged   that insanity as an excuse was introduced in the ancient Roman legal system based upon this tradition. Some modern critics claim that this precedent precludes the insanity defense's validity in a modern secular state like the United States.
The public tends to believe that the insanity defense is used more often than it actually is, possibly because insanity defense cases tend to be of a high-profile nature. The insanity plea is used in the U.S Criminal Justice System in less than 1% of all criminal cases. Only one fourth of those defendants are found "not guilty by reason of insanity".
Some U.S. states have begun to ban the use of the insanity defense and a 1994 Supreme Court ruling upheld the right of Montana to do so. Idaho and Utah have also banned the defense.  In 2006, the Supreme Court decided Clark v. Arizona, reaffirming the prerogative of the states to deviate from or even totally abolish the insanity defense.

# Rules of appreciation
In this section, various rules applied in United States jurisdiction with respect to insanity defenses are discussed.

## The M'Naghten Rules
The M'Naghten Rules (1843) 10 C & F 200, state, inter alia, that a person may be "insane" if "...at the time of the committing of the act, the party 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." During the mid- 20th Century the M'Naghten Rules were gradually adapted in most jurisdictions in the United States by other tests listed below. Following the attempted assassination of another prominent political figure (president Ronald Reagan), however, the M'Naghten Rules underwent a major revival such that they apply in the majority of states. In the UK, however, their application has been largely unaltered.

## Irresistible Impulse
There is also an idea of irresistible impulse, which argues that a person may have known an act was illegal but due to mental impairment lost control of their actions. This is a more liberal test than that set by the M'Naghten Rules because it applies to defendants who are fully aware of their actions. The defense was first approved in the U.S. in Ohio in 1834 and emphasized the inability to control one's actions. Since then it has been adopted by other States, but is open to criticism since there is no way to identify impulses which could be resisted or controlled, and each case must therefore turn upon its own facts. In 1994, Lorena Bobbitt was found not guilty of the felony of malicious wounding when it was argued that an irresistible impulse led her to cut off her husband's penis. The principle has not been applied in the U.K.

## The Durham rule
The Durham rule or "product test" was set forth by the United States Court of Appeals for the District of Columbia Circuit in 1954 and states that "... an accused is not criminally responsible if his unlawful act was the product of mental disease or defect". After the 1970s, US jurisdictions have tended not to recognize this argument as it places emphasis on "mental disease or defect" and thus on testimony by psychiatrists and is argued to be somewhat ambiguous.

## The Brawner rule
The Brawner Rule, from the case of United States v. Brawner, 471 F.2d 969 (1972) by the United States Court of Appeals for the District of Columbia Circuit set aside the Durham ruling arguing the rulings requirement that a crime must be a product of mental disease or defect placed the question guilt on expert witnesses and diminished the jurys role in determining guilt. Under this proposal, juries are allowed to decide the "insanity question" as they see fit. Basing its ruling on the American Law Institutes (ALI) Model Penal Code, the court ruled that for a defendant not to be criminally guilty for a crime the defendant, (i) lacks substantial capacity to appreciate that his conduct is wrongful, or (ii) lacks substantial capacity to conform his conduct to the law.
It is noteworthy that this case was (1) decided by the U.S. Court of Appeals for the District of Columbia circuit and not the United States Supreme Court, and is thus not a national precedent, and (2) not based on constitutional arguments and was thus superseded by Congress in 1984 with the Insanity Defense Reform Act of 1984.

## The Insanity Defense Reform Act of 1984 (U.S.)
There was widespread public outcry over John Hinckley Jr.'s successful use of the insanity defense in his trial for the attempted assassination of Ronald Reagan. The Insanity Defense Reform Act of 1984, enacted by Congress in 1984 in response to the verdict in the Hinckley trial, and codified at Title 18, U.S. Code, Section 17,  states that a person accused of a crime can be judged not guilty by reason of insanity if "the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts."
The Insanity Defense Reform Act mirrors the M'Naughten rules except that it requires the defendant suffer from severe mental defect, and places the burden on the defendant to prove by clear and convincing evidence (higher than the preponderance of the evidence standard required of the defendant by most states following the M'Naughten Rules).

## The substantial capacity test
The substantial capacity test was defined by the American Law Institute, in its Model Penal Code. This argues that insanity should be defined as a lack of substantial capacity to control one's behavior. Substantial capacity is defined as: "the mental capacity needed to understand the wrongfulness of  act, or to conform...behavior to the...law." This is related to the M'Naghten Rule and the idea of 'irresistible impulse'.

## Temporary insanity
The notion of temporary insanity argues that a defendant was insane, but is now sane. A defendant found to have been temporarily insane will often be released without any requirements of psychiatric treatment. This defense was first used by U.S. Congressman Daniel Sickles of New York in 1859 after he had killed his wife's lover, Philip Barton Key, but was most used during the 1940s and 1950s. Since then, it has not been as  successful.

# Scottish law
The Scottish Law Commission in its  Discussion Paper No 122 on Insanity and Diminished Responsibility (2003)  pp.16/18 confirms that the law has not substantially changed from the position stated in Hume's Commentaries:
The phrase "absolute alienation of reason" is still regarded as at the core of the defense in the modern law (see HM Advocate v Kidd 1960 JC 61 and Brennan v HM Advocate (1977) JC 38).

# Insanity Defense Usage and Success Rate
Media coverage in the United States tends to dictate how situations are perceived by the public.  A case using the insanity defense usually receives a lot more media attention because it is considered unusual or dramatic.  This increased coverage gives the impression that the defense is widely used but this is not the case.  According to an eight-state study the insanity defense is used in less than 1% of all court cases and is only successful in 26% of cases. Of those cases that were successful, 90% of defendants had been previously diagnosed with mental illness.
Angie Cannon wrote in Sniper Insanity
The recent cases of Lee Boyd Malvo and Andrea Yates are examples of high-profile use of the insanity defense; both are characterised by their dramatic circumstances.

# Footnotes
-  "Federal Insanity Acquittees - Person Found Not Guilty by Reason of Insanity May Not Attack His Successful Insanity Defense in Habeas Petition". Journal of the American Academey of Psychiatry and the Law. Retrieved 2007-10-11..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
-  Shapiro, David L. (1991). Forensic Psychological Assessment: An Integrative Approach. Needham Heights, MA: Simon & Schuster. pp. p. 69. ISBN 0-205-12521-2.CS1 maint: Extra text (link)
-  "Ford v. Wainwright 477 U.S. 399". Cornell Law School. Retrieved 2007-10-04.
-  "The American Dictionary of Criminal Justice: Key Terms and Major Court Cases". Scarecrow Press. Retrieved 2007-10-06.
-  "The Advocacy Handbook - FAQ". Retrieved 2007-10-12.
-  State v. Thompson, 90 Wright Ohio Rep. 622 (1834)
Schmalleger, Frank (2001). Criminal Justice:  A Brief Introduction. Prentice Hall. ISBN 0130887293.