Dementia As a Legal Defense
Dementia is a serious loss of cognitive function in a person, usually as a result of a disease or injury. While there is no explicit dementia defense in the legal system, there is an insanity defense that has been argued on behalf of victims of dementia. No single definition of insanity is used in all courts, and the availability of the insanity defense to a person suffering from dementia will depend on the laws of the jurisdiction.
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Legal Insanity
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Legal insanity is not the same as clinical insanity, which is diagnosed by psychologists. There are several different tests for legal insanity, but the reason it can generally be used as a defense is because it can defeat the intent element of a crime. Most crimes, as defined, must include the intent to commit the crime or at least the reasonable expectation that the act would be criminal. If this intent cannot be found because the individual was mentally incompetent to form such awareness, he is legally insane.
M'Naghten Test
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The original legal insanity test is the M'Naghten test. It simply asks the jury to decide whether the defendant knew her acts were legally wrong at the time of committing them. Insane delusions and an inability to distinguish reality from fantasy are hallmarks of the sort of insanity that is covered under the M'Naghten test. Under the M'Naghten test, someone suffering from a clinically diagnosable mental illness who nevertheless knows her acts are wrong is not able to use the insanity defense.
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Other Tests
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Over time, the M'Naghten test came to be criticized. Other tests have been developed and applied in various jurisdictions. Variations of the M'Naghten test distinguish between whether the defendant was able to tell moral right from wrong, and whether a reasonable person would believe the acts were justified if he shared the defendant's perception of reality. The Durham test asks only whether the criminal acts were themselves the product of mental illness. If so, the insanity test applies. The test applied in federal courts requires the defense to show severe, diagnosable mental disease or defect that rendered the defendant unable to appreciate the nature, quality or wrongfulness of his acts.
Burden of Proof
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Sanity is a rebuttable presumption in criminal cases. That is, a defendant is presumed sane unless he can prove otherwise. The defendant therefore has the burden of proof on the issue of insanity. He need not prove insanity beyond a reasonable doubt, however. If insanity can be shown by the balance of the evidence---that is, with greater than 50 percent likelihood---the defendant will have succeeded in establishing the insanity defense. Typically this involves expert testimony.
Dementia and Legal Insanity
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The ability to use dementia as a legal defense will depend on what insanity test is used in a particular jurisdiction. Because dementia is caused by biological pathology, there can be significant physical evidence to support a loss of cognitive function. However, loss of cognitive function alone is not sufficient to satisfy any of the legal insanity tests unless an inability to perceive the wrongfulness of particular acts can also be shown. When legal insanity is established, the defendant is not simply acquitted. Instead, she is civilly committed into a psychiatric facility.
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