insanity and diminished capacity

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Insanity and diminished capacity are two related, but distinct legal terms that are relevant in the field of Criminal Law. Both concepts can be used by individuals as defenses against a wide variety of charged crimes. However, the two concepts also contain important differences and are by no means synonymous. 

Insanity, also known as the insanity defense, refers to a legal defense in which the defendant argues that they should not be held criminally responsible for their actions due to a mental illness or defect. Therefore, insanity is an affirmative defense. Insanity defense asserts that the defendant lacked the mental capacity to understand the nature of their actions or to distinguish right from wrong at the time of the offense. Successful insanity defense can result in a finding of "not guilty by reason of insanity," which may lead to the defendant being committed to a mental health facility rather than being incarcerated. States dictate how the insanity defense may be invoked in their own courts while the federal government does so for the federal court system. Insanity is covered in Section 4.01 of the Model Penal Code. In 1984, the Insanity Defense Reform Act (18 U.S.C. § 17) was passed. This statute placed the burden of proving insanity on the defendant. Today, insanity is rarely invoked and only approximately a quarter of insanity defenses succeed.

Diminished capacity, on the other hand, is only a partial defense because it is not presented as an excuse or justification for a crime but rather as an attempt to prove that the defendant was incapable of forming the requisite intent of the crime charged and, therefore, is innocent of that crime but likely still guilty of a lesser offense. Therefore, diminished capacity acknowledges that the defendant may have had some mental impairment but does not necessarily claim a lack of criminal responsibility, as is the case with the insanity defense.

Related Legal Issues:

  • The law of evidence: admissibility of evidence
  • Competence to Stand Trial (from James R. Elkins, West Virginia University)


  • The case against Daniel M’Naghten: First legal test (M’Naghten Rule) derived for criminal insanity.
  • People v. William Freeman (1847): the first case in the United States where a defendant used an insanity defense. See also: People v. William Freeman Wex entry. 
  • The UNABOM case: the law of evidence and sentencing; admissibility of evidence; the constitutional law of double jeopardy.
  • The Hinckley case: the assassination attempt of President Reagan.

Supreme Court Cases on the Insanity Defense and Diminished Capacity:

  • Pate v. Robinson, 383 U.S. 375 (1966)
  • Drope v. Missouri, 420 U.S. 162 (1972)
  • Faretta v. California, 422 U.S. 806 (1975)
  • McKaskle v. Wiggins, 465 U.S. 168 (1984)

In the News:

  • Supreme Court allows state bans on insanity defense (1994)
  • John Salvi III case (1996)
  • DuPont murder (1997)
  • Theodore J. Kaczynski (1998)

Additional Resources: 

  • Insanity Defense: Past, Present, and Future

[Last updated in June of 2023 by the Wex Definitions Team]