Also known as: diminished capacity · diminished-capacity defense · diminished capacity defences · diminished responsibility
Written by attorneys · grounded in primary & secondary sources — see below
Evidence of mental disease or defect short of legal insanity offered to show that a defendant lacked the mental state required for the charged offense or to mitigate culpability at sentencing.
David Dunn was charged with robbery after taking property from a store clerk. Dunn offered expert testimony that his mental disease prevented formation of the required specific intent. The court admitted the evidence on the mens rea element even though Dunn did not raise an insanity defense.
Darius Dixon was charged with murder after shooting a neighbor during an argument. Dixon offered expert testimony that his PTSD prevented formation of the intent to kill. The court admitted the evidence to rebut the prosecution's proof of the required mental state even though Dixon did not claim full insanity.
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Dolores Diaz, age seventeen, was convicted of capital murder for a robbery homicide. At the penalty phase she introduced evidence of her immature brain development and limited capacity to appreciate consequences. The sentencer considered the evidence as mitigation and imposed life imprisonment rather than death.
Deanna Davenport was charged with murder. She sought to introduce evidence of schizophrenia to show she lacked intent to kill. The trial court restricted the evidence to the insanity defense only and refused to allow it on the mens rea element.
Doris Duffy was tried for assault after attacking a stranger she believed was an alien. The defense offered psychiatric testimony that her paranoid schizophrenia negated the specific intent required for the charged offense. The court permitted the evidence on the mens rea issue.
Danielle Dixon, convicted of capital murder, presented evidence of significantly subaverage intellectual functioning and adaptive deficits that began in childhood. The court held that the evidence established intellectual disability and therefore precluded imposition of the death penalty.
Diminished capacity evidence shows a mental disease or defect that negates the required mental state or mitigates culpability without establishing full legal insanity. Insanity is an affirmative defense that can result in acquittal by reason of insanity when the defendant is shown to lack criminal responsibility entirely.
Supporting sources
Yes. In capital cases a defendant may introduce evidence that a mental disease or defect substantially impaired the capacity to appreciate the wrongfulness of the conduct or to conform conduct to law. The evidence is admissible as mitigation even if it does not meet the test for legal insanity.
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Jurisdictions typically require timely written notice of intent to rely on a mental disease or defect defense. The notice must alert the prosecution that mental state evidence will be offered so the state can prepare rebuttal experts, though the notice need not specify complete versus partial impairment.
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A defendant must disclose the intent to rely on mental-state evidence in advance of trial so the prosecution may obtain its own expert evaluation. Failure to provide timely notice may result in exclusion of the defense expert or a continuance to allow rebuttal preparation.
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…charged. See Brief for American Psychiatric Association et al. as Amici Curiae 12–13; Arenella, The Diminished Capacity and Diminished Responsibility Defenses: Two Children of a Doomed Marriage, 77 Colum. L. Rev. 827, 834–835 (1977); cf. Powell v. Texas , 392 U.S. 514, 535–536 (1968) (plurality opinion) (the “doctrines of actus reus ,…