insanity

Insanity is a legal concept used to describe a person’s mental state at the time an act, typically a crime, is committed. It denotes that the person was so impaired by a mental disorder that they could not understand the nature or quality of the act or could not distinguish right from wrong. Insanity is not a clinical diagnosis and does not map directly to a single mental illness.

Etymology and history: The term derives from Latin insanus, meaning “not sane.” In historical and modern law, definitions of insanity have varied by jurisdiction. Today most systems separate a legal determination of insanity from medical diagnoses.

In medicine and psychology, clinicians diagnose mental disorders using standardized criteria (for example DSM-5-TR or ICD-11). The word insanity is generally not used as a diagnostic label; instead, forensic evaluations focus on the defendant’s mental state at the time of the offense and on capacities such as understanding, appreciation of wrongfulness, and impulse control.

Legal standards: Several formulations exist. The M’Naghten Rule asks whether the person knew what they were doing or understood that it was wrong. Other standards include testing for irresistible impulse, or for whether the person lacked substantial capacity to conform behavior to the law, among others. Some jurisdictions combine criminal responsibility with civil commitment provisions. Terms such as not guilty by reason of insanity (NGRI) or guilty but mentally ill (GBMI) are used in some places.

Assessment and implications: Forensic psychiatrists or psychologists perform evaluations that consider history, behavior, and current symptoms. Insanity verdicts are relatively rare and controversial; in many places, such determinations influence detention and treatment decisions rather than exoneration alone.

See also: Mental health, Forensic psychiatry, Law and psychology.