There are three main legal measures used to determine whether or not a defendant was legally insane at the time of the crime he or she committed. The measure used depends upon the jurisdiction. The three are as follows:
M?NAGHTEN RULE
1. At the time of commission of offense the defendant was laboring under:
a. A mental illness.
b. Causing a defect of reason.
c. Defendant not know the nature and quality of his act OR not know what he was doing was wrong (cognitive impairment).
2. Loss of control is no defense.
3. Defendant with delusions must be determined whether, if the facts were as he believed, his actions would have been criminal.
IRRESISTIBLE IMPULSE TEST
1. As a result of mental disease or defect defendant lacked substantial capacity to control actions or conform conduct to law (volitional impairment).
AMERICAN LAW INSTITUTE (ALI) TEST
1. Used in most jurisdictions.
2. Defendant cannot be held accountable if he cannot appreciate the wrongfulness of his conduct OR conform his conduct to the requirements of the law.
3. Combines M?Naghten and Irresistible Impulse tests allowing either cognitive impairment or volitional impairment.
PROCEDURAL CONSIDERATIONS
1. Presumption of sanity.
2. Burden upon defendant to prove insanity.
3. Sanity may not be tried if defendant legally incompetent.
DIMINISHED CAPACITY
1. Recognized as defense in some states.
2. Defendant lacked the state of mind required for a specific crime as the result of a mental defect.
3. Differs from insanity because recognizes varying degrees of mental stability or instability.