1. Any mental disorder severe enough that it prevents a person from having legal capacity and excuses the person from criminal or civil responsibility. * Insanity is a legal, not a medical, standard. — aka legal insanity; lunacy. 
1. An unsound, deranged, delirious, or distracted condition of mind. Manley v Staples, 62 Vt 153, 19 A 983.
A condition of mind so impaired in function, or so deranged, as to induce a deviation from normal conduct in the person so afflicted. 29 Am J Rev ed Ins Per § 2.
Such a degree of mental incapacity as renders one unable to understand and deal with the common affairs of life. Lewis v Lewis, 199 SC 490, 20 SE2d 107.
Such impairment of the mind as renders it impossible for one to understand the nature and consequences of his acts, or the character of a transaction in question. 29 Am J Rev ed Ins Per § 2.
A sickness. Robillard v Société St. Jean Baptiste De Centreville, 21 RI 348, 43 A 635.
A physical disease, consisting of a diseased or disordered condition or malformation of the organs or tissues, through which the mind receives impressions, and operates, and by which the will and judgment are impaired, and conduct rendered irrational. Blackstone v Standard Life & Acci. Ins. Co. 74 Mich 592, 42 NW 156.
Defense in criminal case. Incapacity to form a guilty intention. People v Schmidt, 216 NY 324, 110 NE 945.
Mental disease or mental defect producing the unlawful act. 21 Am J2d Crim L § 39.
Such a condition of mind and want of reason as to act from irresistible impulse. 21 Am J2d Crim L § 36.
Want of substantial capacity, as a result of mental disease or defect, to appreciate the criminality or wrongfulness of conduct or to conform conduct to the requirements of law. American Law Institute Model Penal Code, proposed ofhcial draft § 4.01( 1).
Such defect of reason, from disease of the mind, as not to know the nature and quality of the act, or if the accused did know it, that he-did not know that what he was doing was wrong; such being the definition of the celebrated M’Naghten Case, 10 Clark & F 200, 8 Eng Reprint 718.
Sometimes tested, in homicide cases, according to the ability to distinguish between right and wrong. 26 Am J1st Homi §§ 78, 79.
Inability to understand the nature and quality of the act or inability to distinguish right from wrong in respect to the act. 21 Am J2d Crim L § 34.
At time of trial for crime. The want of capacity to comprehend one’s position, to understand the nature and object of the proceedings, to conduct his defense in a rational manner, and to co-operate with his counsel to the end that any available defense may be interposed. People v Burson, 11 Ill 2d 360, 143 NE2d 939.
At the time punishment for crime to be imposed, particularly the execution of the death penalty. The want of sufficient intelligence to understand the nature of the proceedings, what the trial was for, the purpose of punishment, the impending fate, or of sufficient understanding to know any fact which might exist making punishment unjust or unlawful, or of intelligence requisite to convey such information to attorneys or the court. 21 Am J2d Crim L § 76.
As a condition which operates to prevent or suspend the running of a statute of limitations. Such a mental impairment as renders one incompetent to manage his own affairs and endangers his person or property. Brown v Smith, 119 Colo 469, 205 P2d 239, 9 ALR2d 961.
As a matter of avoidance of the effect of an exception in a life insurance policy of death by suicide. An impairment of the reasoning faculties so far that one is not able to understand the moral character, the general nature, the consequences and effect of an act he is about to commit, or is impelled to the act by an impulse which he is not able to resist. 29A Am J Rev ed Ins § 1144. 
“The lawyers refer to ‘insanity.’ This is a legal term only, and one that is not used by the psychiatrist; the latter prefers to speak of mental disorder, mental illness, or of psychosis or neurosis.” Winfred Overholser, Psychiatry and the Law, 38 Mental Hygiene 243, 244 (1954).
“The word ‘insanity’ is commonly used in discussions of this problem although some other term would seem to be preferable such as ‘mental disease or defect,’ — which may be shortened to ‘mental disorder’ in general discussions if this is clearly understood to include disease of the mind, con~ genital lack, and damage resulting from traumatic injury, but to exclude excitement or stupefaction resulting from liquor or drugs. Apart from its uses in the law ‘insanity’ is usually employed to indicate mental disorder resulting from deterioration or damage as distinguished from cont genital deficiency. Criminal incapacity may result as readily from one as from the other, but while the earlier authorities spoke of the ‘idiot’ and the ‘madman,’ . . . the more recent tendency in the law has been to include both under the ‘insanity’ label.” Rollin M. Perkins & Ronald N. Boyce, Criminal Law 952 (3d ed. 1982).
“Another objection to the word ‘insanity’ is_ the unwarranted assumption that it refers to a very definite mental condition, seldom put into words but apparent in many discussions of the problem.” Id.
Various Forms of Insanity:
emotional insanity – (1872) Insanity produced by a violent excitement of the emotions or passions, although reasoning faculties may remain unimpaired; a passion that for a period creates complete derangement of intellect. * Emotional insanity is sometimes described as an irresistible impulse to do an act. 
1. Passion or frenzy produced by anger, jealously, or other emotion, sometimes of overwhelming force. 21 Am J2d Crim L § 37.
The sate of mind of one who, while in possession of his ordinary ffaculties and unaffected by any mental disease, gives way to his passions to such an extent as to become a temporary maniac. 29 Am J Rev ed Ins Per § 3. 
diminished capacity – (1912) 1. An impaired mental condition — short of insanity — that is caused by intoxication, trauma, or disease and that prevents a person from having the mental state necessary to be held responsible for a crime. * In some jurisdictions, a defendant’s diminished capacity can be used to determine the degree of the offense or the severity of the punishment. 2. A failure-of-proof defense or a partial-excuse defense based on the defendant’s diminished capacity at the time of the offense. — aka diminished responsibility; partial insanity; partial responsibility.
“Diminished capacity refers to two distinct doctrines: the use of evidence of mental abnormality to negate a mens rea required by the definition of the crime charged (the mens rea variant) and the use of mental abnormality evidence to establish some type of partial affirmative defense of excuse (the partial excuse variant). Courts have used various other terms, such as diminished responsibility, to refer to one or both of these distinct doctrines, but the term used is unimportant. Confusion arises, however, when the two types of doctrine are not clearly distinguished.” Stephen J. Morse, “Diminished Capacity,” in Encyclopedia of Crime & Justice 528, 528 (Joshua Dressler ed., 2d ed. 2002).
testamentary capacity – (1819) The mental ability that a person must have to prepare a valid will. * This capacity is often described as the ability to recognize the natural objects of one’s bounty, the nature and extent of one’s estate, and the fact that one is making a plan to dispose of the estate after death. Traditionally, the phrase “of legal age and sound mind” refers to the testator’s capacity. — aka disposing capacity; disposing mind; sound mind; sound disposing mind. See age of capacity under AGE.
temporary insanity – (18c) Insanity that exists only at the time of a criminal act. 
black-rage insanity defense – (1995) An insanity defense based on an African-American’s violent eruption of an er induced at least partly by racial tensions. * This defense was first used in the mid-1990s.
See idiocy; imbecility; inquest in lunacy; non compos mentis; total mental disability; wild beast test. 
Various Tests used to Determine Sanity:
appreciation test – requires clear, convincing evidence that at the time of the crime, the defendant suffered from a severe mental disease or defect preventing them from appreciating the wrongfulness of the conduct. Established via the Insanity Defense Reform Act of 1984: 18 USCA § 17. — aka Insanity Defense Reform Act of 1984 test.
Currens test – requires no more than the jury must be satisfied that as a result of mental disease or defect, the accused lacked substantial capacity to conform his conduct to the requirements of the law.
Durham test – holds that a defendant is not criminally responsible for an act that was the product of mental disease or defect. — aka Durham rule; product test.
McNaghten test – the “right and wrong test” to determine a person’s sanity. — aka McNaghten rule; right-and-wrong test; right-wrong test.
substantial-capacity test – the Model Penal Code’s test for the insanity defense; determines if the person lacks substantial capacity to appreciate the criminality of the conduct or to conform their conduct to the law. Model Penal Code § 4.01. — aka Model Penal Code test; MPC test; American Law Institute test; ALI test.
Disclaimer: All material throughout this website is compiled in accordance with Fair Use.
: Black’s Law Dictionary Deluxe Tenth Edition by Henry Campbell Black & Editor in Chief Bryan A. Garner. ISBN: 978-0-314-62130-6
: Ballantine’s Law Dictionary with Pronunciations
Third Edition by James A. Ballantine (James Arthur 1871-1949). Edited by William S. Anderson. © 1969 by THE LAWYER’S CO-OPERATIVE PUBLISHING COMPANY. Library of Congress Catalog Card No. 68-30931
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