Insanity, madness, lunacy, and craziness are behaviors performed by certain abnormal mental or behavioral patterns. Insanity can manifest as violations of societal norms, including a person or persons becoming a danger to themselves or to other people. Conceptually, mental insanity also is associated with the biological phenomenon of contagion (that mental illness is infectious) as in the case of copycat suicides. In contemporary usage, the term insanity is an informal, un-scientific term denoting "mental instability"; thus, the term insanity defense is the legal definition of mental instability. In medicine, the general term psychosis is used to include the presence of delusions and/or hallucinations in a patient;  and psychiatric illness is "psychopathology", not mental insanity. 
In English, the word "sane" derives from the Latin adjective sanus meaning "healthy". Juvenal's phrase mens sana in corpore sano is often translated to mean a "healthy mind in a healthy body". From this perspective, insanity can be considered as poor health of the mind, not necessarily of the brain as an organ (although that can affect mental health), but rather refers to defective function of mental processes such as reasoning. Another Latin phrase related to our current concept of sanity is "compos mentis" (lit. "sound of mind"), and a euphemistic term for insanity is "non compos mentis". In law, mens rea means having had criminal intent, or a guilty mind, when the act (actus reus) was committed.
A more informal use of the term insanity is to denote something or someone considered highly unique, passionate or extreme, including in a positive sense. The term may also be used as an attempt to discredit or criticize particular ideas, beliefs, principles, desires, personal feelings, attitudes, or their proponents, such as in politics and religion.
Madness, the non-legal word for insanity, has been recognized throughout history in every known society. Some traditional cultures have turned to witch doctors or shamans to apply magic, herbal mixtures, or folk medicine to rid deranged persons of evil spirits or bizarre behavior, for example.  Archaeologists have unearthed skulls (at least 7000 years old) that have small, round holes bored in them using flint tools. It has been conjectured that the subjects may have been thought to have been possessed by spirits that the holes would allow to escape.  More recent research on the historical practice of trepanning supports the hypothesis that this procedure was medical in nature and intended as means of treating cranial trauma. 
The Greeks appeared to share something of the modern Western world's secular and holistic view, believing that afflictions of the mind did not differ from diseases of the body. Moreover, they saw mental and physical illness as a result of natural causes and an imbalance in bodily humors. Hippocrates frequently wrote that an excess of black bile resulted in irrational thinking and behavior. 
Romans made other contributions to psychiatry, in particular a precursor of some contemporary practice. They put forward the idea that strong emotions could lead to bodily ailments, the basis of today's theory of psychosomatic illness. The Romans also supported humane treatment of the mentally ill, and in so doing, codified into law the principle of insanity as a mitigation of responsibility for criminal acts,  although the criterion for insanity was sharply set as the defendant had to be found "non compos mentis", a term meaning "not sound of mind". 
The Middle Ages witnessed the end of the progressive ideas of the Greeks and Romans.[ clarification needed ]
During the 18th century, the French and the British introduced humane treatment of the clinically insane,  though the criteria for diagnosis and placement in an asylum were considerably looser than today, often including such conditions as speech disorder, speech impediments, epilepsy, and depression or being pregnant out of wedlock.
Europe's oldest asylum was the precursor of today's Bethlem Royal Hospital in London, known then as Bedlam, which began admitting the mentally ill in 1403 and is mentioned in Chaucer's Canterbury Tales . The first American asylum was built in Williamsburg, Virginia, circa 1773. Before the 19th century, these hospitals were used to isolate the mentally ill or the socially ostracized from society rather than cure them or maintain their health. Pictures from this era portrayed patients bound with rope or chains, often to beds or walls, or restrained in straitjackets.
Insanity is no longer considered a medical diagnosis but is a legal term in the United States, stemming from its original use in common law.  The disorders formerly encompassed by the term covered a wide range of mental disorders now diagnosed as bipolar disorder, organic brain syndromes, schizophrenia, and other psychotic disorders. 
In United States criminal law, insanity may serve as an affirmative defense to criminal acts and thus does not need to negate an element of the prosecution's case such as general or specific intent.  Each U.S. state differs somewhat in its definition of insanity but most follow the guidelines of the Model Penal Code. All jurisdictions require a sanity evaluation to address the question first of whether or not the defendant has a mental illness.
Most courts accept a major mental illness such as psychosis but will not accept the diagnosis of a personality disorder for the purposes of an insanity defense. The second question is whether the mental illness interfered with the defendant's ability to distinguish right from wrong. That is, did the defendant know that the alleged behavior was against the law at the time the offense was committed.
Additionally, some jurisdictions add the question of whether or not the defendant was in control of their behavior at the time of the offense. For example, if the defendant was compelled by some aspect of their mental illness to commit the illegal act, the defendant could be evaluated as not in control of their behavior at the time of the offense.
The forensic mental health specialists submit their evaluations to the court. Since the question of sanity or insanity is a legal question and not a medical one, the judge and or jury will make the final decision regarding the defendant's status regarding an insanity defense.  
In most jurisdictions within the United States, if the insanity plea is accepted, the defendant is committed to a psychiatric institution for at least 60 days for further evaluation, and then reevaluated at least yearly after that.
Insanity is generally no defense in a civil lawsuit, but an insane plaintiff can toll the statute of limitations for filing a suit until gaining sanity, or until a statute of repose has run.
Feigned insanity is the simulation of mental illness in order to deceive. Amongst other purposes, insanity is feigned in order to avoid or lessen the consequences of a confrontation or conviction for an alleged crime. A number of treatises on medical jurisprudence were written during the nineteenth century, the most famous of which was Isaac Ray in 1838 (fifth edition 1871); others include Ryan (1832), Taylor (1845), Wharton and Stille (1855), Ordronaux (1869), Meymott (1882). The typical techniques as outlined in these works are the background for Dr. Neil S. Kaye's widely recognized guidelines that indicate an attempt to feign insanity. 
One famous example of someone feigning insanity is Mafia boss Vincent Gigante, who pretended for years to be suffering from dementia, and was often seen wandering aimlessly around his neighborhood in his pajamas muttering to himself. Testimony from informants and surveillance showed that Gigante was in full control of his faculties the whole time, and ruled over his Mafia family with an iron fist. 
Today feigned insanity is considered malingering. In a 2005 court case, United States v. Binion , the defendant was prosecuted and convicted for obstruction of justice (adding to his original sentence) because he feigned insanity in a Competency to Stand Trial evaluation.
In modern times, labeling someone as insane often carries little or no medical meaning and is rather used as an insult or as a reaction to behavior perceived to be outside the bounds of accepted norms. For instance, the definition of insanity is sometimes colloquially purported to be "doing the same thing over and over again and expecting a different result."  However, this does not match the legal definition of insanity. 
The insanity defense, also known as the mental disorder defense, is an affirmative defense by excuse in a criminal case, arguing that the defendant is not responsible for their actions due to an psychiatric disease at the time of the criminal act. This is contrasted with an excuse of provocation, in which the defendant is responsible, but the responsibility is lessened due to a temporary mental state. It is also contrasted with a finding that a defendant cannot stand trial in a criminal case because a mental disease prevents them from effectively assisting counsel, from a civil finding in trusts and estates where a will is nullified because it was made when a mental disorder prevented a testator from recognizing the natural objects of their bounty, and from involuntary civil commitment to a mental institution, when anyone is found to be gravely disabled or to be a danger to themself or to others.
The M'Naghten rule is any variant of the 1840s jury instruction in a criminal case when there is a defence of insanity:
that every man is to be presumed to be sane, and ... that to establish a defence on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party accused was labouring under such a defect of reason, from 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 he was doing what was wrong.
Sanity refers to the soundness, rationality, and health of the human mind, as opposed to insanity. A person is sane if they are rational. In modern society, the term has become exclusively synonymous with compos mentis, in contrast with non compos mentis, or insanity, meaning troubled conscience. A sane mind is nowadays considered healthy both from its analytical - once called rational - and emotional aspects. According to the writer G. K. Chesterton, sanity involves wholeness, whereas insanity implies narrowness and brokenness.
Anti-psychiatry is a movement based on the view that psychiatric treatment is often more damaging than helpful to patients, highlighting controversies about psychiatry. Objections include the reliability of psychiatric diagnosis, the questionable effectiveness and harm associated with psychiatric medications, the failure of psychiatry to demonstrate any disease treatment mechanism for psychiatric medication effects, and legal concerns about equal human rights and civil freedom being nullified by the presence of diagnosis. Historically critiques of psychiatry came to light after focus on the extreme harms associated with electroconvulsive treatment or insulin shock therapy. The term "anti-psychiatry" is in dispute and often used to dismiss all critics of psychiatry, many of who agree that a specialized role of helper for people in emotional distress may at times be appropriate, and allow for individual choice around treatment decisions.
Thomas Stephen Szasz was a Hungarian-American academic and psychiatrist. He served for most of his career as professor of psychiatry at the State University of New York Upstate Medical University in Syracuse, New York. A distinguished lifetime fellow of the American Psychiatric Association and a life member of the American Psychoanalytic Association, he was best known as a social critic of the moral and scientific foundations of psychiatry, as what he saw as the social control aims of medicine in modern society, as well as scientism. His books The Myth of Mental Illness (1961) and The Manufacture of Madness (1970) set out some of the arguments most associated with him.
"Feigned madness" is a phrase used in popular culture to describe the assumption of a mental disorder for the purposes of evasion, deceit or the diversion of suspicion. In some cases, feigned madness may be a strategy—in the case of court jesters, an institutionalised one—by which a person acquires a privilege to violate taboos on speaking unpleasant, socially unacceptable, or dangerous truths.
Non compos mentis is a Latin legal phrase that translates to "of unsound mind": nōn ("not") prefaces compos mentis, meaning "having control of one's mind." This phrase was first used in thirteenth-century English law to describe people afflicted by madness, the loss of memory or ability to reason.
Historically, mental disorders have had three major explanations, namely, the supernatural, biological and psychological models. For much of recorded history, deviant behavior has been considered supernatural and a reflection of the battle between good and evil. When confronted with unexplainable, irrational behavior and by suffering and upheaval, people have perceived evil. In fact, in the Persian Empire from 550 to 330 B.C., all physical and mental disorders were considered the work of the devil. Physical causes of mental disorders have been sought in history. Hippocrates was important in this tradition as he identified syphilis as a disease and was, therefore, an early proponent of the idea that psychological disorders are biologically caused. This was a precursor to modern psycho-social treatment approaches to the causation of psychopathology, with the focus on psychological, social and cultural factors. Well known philosophers like Plato, Aristotle, etc., wrote about the importance of fantasies, dreams, and thus anticipated, to some extent, the fields of psychoanalytic thought and cognitive science that were later developed. They were also some of the first to advocate for humane and responsible care for individuals with psychological disturbances.
Forensic psychiatry is a subspeciality of psychiatry and is related to criminology. It encompasses the interface between law and psychiatry. According to the American Academy of Psychiatry and the Law, it is defined as "a subspecialty of psychiatry in which scientific and clinical expertise is applied in legal contexts involving civil, criminal, correctional, regulatory, or legislative matters, and in specialized clinical consultations in areas such as risk assessment or employment." A forensic psychiatrist provides services – such as determination of competency to stand trial – to a court of law to facilitate the adjudicative process and provide treatment, such as medications and psychotherapy, to criminals.
Hervey Milton Cleckley was an American psychiatrist and pioneer in the field of psychopathy. His book, The Mask of Sanity, originally published in 1941 and revised in new editions until the 1980s, provided the most influential clinical description of psychopathy in the twentieth century. The term "mask of sanity" derived from Cleckley's belief that a psychopath can appear normal and even engaging, but that the "mask" conceals a mental disorder. By the time of his death, Cleckley was better remembered for a vivid case study of a female patient, published as a book in 1956 and turned into a movie, The Three Faces of Eve, in 1957. His report of the case (re)popularized in America the controversial diagnosis of multiple personality disorder. The concept of psychopathy continues to be influential through forming parts of the diagnosis of antisocial personality disorder, the Psychopathy Checklist, and public perception.
Moral insanity referred to a type of mental disorder consisting of abnormal emotions and behaviours in the apparent absence of intellectual impairments, delusions, or hallucinations. It was an accepted diagnosis in Europe and America through the second half of the 19th century.
Clark v. Arizona, 548 U.S. 735 (2006), is a United States Supreme Court case in which the Court upheld the constitutionality of the insanity defense used by Arizona.
United States v. Binion, 132 F. App'x 89, is a case in which the United States Court of Appeals for the Eighth Circuit applied two recent U.S. Supreme Court decisions, United States v. Booker and United States v. Fanfan, in upholding the sentencing decision by the trial court, the United States District Court for the Eastern District of Missouri.
In criminal law, a mitigating factor, also known as an extenuating circumstance, is any information or evidence presented to the court regarding the defendant or the circumstances of the crime that might result in reduced charges or a lesser sentence. Unlike a legal defense, the presentation of mitigating factors will not result in the acquittal of a defendant. The opposite of a mitigating factor is an aggravating factor.
Settled insanity is defined as a permanent or "settled" condition caused by long-term substance abuse and differs from the temporary state of intoxication. In some United States jurisdictions "settled insanity" can be used as a basis for an insanity defense, even though voluntary intoxication cannot, if the "settled insanity" negates one of the required elements of the crime such as malice aforethought. However, U.S. federal and state courts have differed in their interpretations of when the use of "settled insanity" is acceptable as an insanity defense and also over what is included in the concept of "settled insanity".
An ultimate issue in criminal law is a legal issue at stake in the prosecution of a crime for which an expert witness is providing testimony.
Frendak v. United States, 408 A.2d 364 is a landmark case in which District of Columbia Court of Appeals decided that a judge could not impose an insanity defense over the defendant's objections.
Insanity in English law is a defence to criminal charges based on the idea that the defendant was unable to understand what he was doing, or, that he was unable to understand that what he was doing was wrong.
The following outline is provided as an overview of and topical guide to psychiatry:
Psychopathy, from psych and pathy, was coined by German psychiatrists in the 19th century and originally just meant what would today be called mental disorder, the study of which is still known as psychopathology. By the turn of the century 'psychopathic inferiority' referred to the type of mental disorder that might now be termed personality disorder, along with a wide variety of other conditions now otherwise classified. Through the early 20th century this and other terms such as 'constitutional (inborn) psychopaths' or 'psychopathic personalities', were used very broadly to cover anyone who violated legal or moral expectations or was considered inherently socially undesirable in some way.