Insanity, madness, and craziness are terms that describe a spectrum of individual and group behaviors that are characterized by certain abnormal mental or behavioral patterns. Insanity can be 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 either of delusions or of hallucinations or both in a patient; and psychiatric illness is "psychopathology", not mental insanity.

A copycat suicide is defined as an emulation of another suicide that the person attempting suicide knows about either from local knowledge or due to accounts or depictions of the original suicide on television and in other media. The publicized suicide serves as a trigger, in the absence of protective factors, for the next suicide by a susceptible or suggestible person. This is referred to as suicide contagion.

A spike of emulation suicides after a widely publicized suicide is known as the Werther effect, following Goethe's novel The Sorrows of Young Werther [少年维特的烦恼].

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 which the holes would allow to escape. However, 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.

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