Diminished Capacity Defense

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Diminished capacity is an affirmative defense meaning that although the accused was not insane, due to emotional distress, physical conditions, or other psychological factors, he could not fully comprehend the nature of the criminal act he was committing. Unlike the Insanity Defense, a diminished capacity defense does not exculpate the accused of all criminal liability. Instead, the defense serves to remove the element of premeditation or criminal intent from the defendant’s actions, thus warranting conviction of a lesser crime, such as manslaughter instead of murder.[1] The rationale for diminished capacity defenses is that while a mentally abnormal defendant might not meet the test for legal insanity, he may nonetheless suffer from a serious rationality impairment that compromises his judgment and responsibility for his actions. Typically, courts are reluctant to create too broad a diminished capacity defense for fear that too many dangerous defendants would go free earlier than concerns of public safety require.[2]

International Law

The Rome Statute of the International Criminal Court provides for an insanity defense. Article 31(1)(a) provides that a person shall not be criminally responsible if, at the time of that person's conduct:

"The person suffers from a mental disease or defect that destroys that person's capacity to appreciate the unlawfulness or nature of his or her conduct, or capacity to control his or her conduct to conform to the requirements of law."[3]

The Trial Chamber of the International Criminal Tribunal for Yugoslavia has held that the mental disease defense is only admissible in two circumstances: (1) in case of impairment of the accused's capacity to appreciate the unlawfulness of or the nature of his conduct; or (2) in case of an impairment to control his conduct in order to conform to the requirements of the law.[4]

Specific Country Applications

Sri Lanka

Sri Lanka does not make specific provision for a diminished capacity defense, but Penal Code § 77 provides that ”[n]othing is an offence which is done by a person who, at the time of doing it, by reason of unsoundness of mind, is incapable of knowing the nature of the act, or that he is doing what is either wrong or contrary to law.”[5]

When the “unsound mind” defense is raised, the court conducts a medical examination of the defendant and further engages in a preliminary inquiry to determine the defendant’s mental status. If convicted, the verdict must list whether or not the person of unsound mind actually committed the act in question and will be examined at least once every sixth months while being held in custody of a prison or mental institution to ascertain his mental state. A person of unsound mind can also be remanded to the custody of a caretaker home instead of the prison if approved by the Prison Commissioner.[6]

Additionally, the Court of Appeals may quash a guilty verdict against a person it feels to be of unsound mind per the Penal Code § 77 definition and order that person into safe custody instead of prison.[7]

United States

Diminished capacity, referring to a defendant’s abnormal mental condition short of insanity, is a recognized legal defense in some courts in the United States. There are two forms of diminished capacity defenses: mens rea and partial responsibility.[8]

While some states bar the diminished capacity defense in its entirety, others allow the defense to put forward evidence of a mental abnormality to negate an element of the crime, usually mens rea.[9] The Model Penal Code also provides for a partial responsibility defense, wherein a homicide that would otherwise constitute murder is instead considered manslaughter if committed as the result of an “extreme mental or emotional disturbance for which there is a reasonable explanation or cause.” The reasonableness of this extreme mental or emotional disturbrance is determined from the viewpoint of a person in the defendant’s situation under the circumstances as he believes them to be true.[10] At least two states appear to recognize the partial responsibility defense.[11]

A famous diminished capacity case is People v. White,[12] where the defendant Daniel James White was convicted of two counts of voluntary manslaughter instead of murder for the assassinations of San Francisco Mayer George Moscone and San Francisco Supervisor Harvey Milk. White openly walked into San Francisco City Hall and shot both victims in broad daylight, but at his trial put forth a successful diminished capacity defense. A series of psychologists testified for the defense that White was suffering from severe depression rendering him unable to premeditate or deliberate before assassinating Moscone and Milk. The jury convicted White on two counts of voluntary manslaughter as a result.[13]

See Defenses


  1. http://legal-dictionary.thefreedictionary.com/Diminished+Capacity
  2. http://law.jrank.org/pages/979/Diminished-Capacity-partial-excuse-variant.html
  3. http://untreaty.un.org/cod/icc/statute/99_corr/cstatute.htm
  4. Geert-Jan G. J. Knoops, Defenses in Contemporary International Criminal Law, p. 140-142, available at http://books.google.lk/books?id=3LMvleMMdmUC&printsec=frontcover#v=onepage&q&f=false, citing Prosecutor v. Delalic, Case No. IT-96-21, ICTY Trial Chamber Judgment, para 283 (Nov. 16, 1998)
  5. Penal Code § 77 (No. 2 of 1883), available at http://www.lawnet.lk/process.php?st=2001Y1V19C&hword=%27%27&path=5
  6. Code of Criminal Pocedure § 374 – 386 (No. 2 of 1979), available at http://www.lawnet.lk/process.php?st=1981Y2V26C&hword=%27%27&path=5
  7. Code of Criminal Procedure § 338
  8. http://www.lexisnexis.com/lawschool/study/outlines/html/crim/crim16.htm
  9. Model Penal Code § 4.02(1), available at http://law.fordham.edu/assets/Faculty/model_penal_code_selected_sections(1).pdf
  10. Model Penal Code § 210.3(1)(b), available at http://law.fordham.edu/assets/Faculty/model_penal_code_selected_sections(1).pdf
  11. http://www.lexisnexis.com/lawschool/study/outlines/html/crim/crim16.htm
  12. People v. White, 117 Cal. App. 3d 270, 172 Cal. Rptr. 612 (Cal. Ct. App. 1981), available at http://www.lawlink.com/research/CaseLevel3/57613#B0422
  13. For additional discussion of the Moscone-Milk Assassinations, please see http://en.wikipedia.org/wiki/Moscone%E2%80%93Milk_assassinations#Trial_and_its_aftermath