Insanity is not a mental health term. It is a legal term describing the state of mind of a person who, because of mental disease or defect, is excused from an otherwise criminal act. Insanity in criminal law is most typically associated with the so-called insanity defense, which has its roots in the English common law of the seventeenth and eighteenth centuries. By entering a plea of not guilty by reason of insanity (NGRI), the defendant may admit to all the acts charged but deny having the requisite mental state to be legally guilty.

The insanity defense is one of many points at which criminal law intersects with mental illness. According to statistics from the U.S. Bureau of Prisons, approximately 16 percent of all incarcerated inmates ...