In order to be convicted of most crimes, it needs to be found that the actor had the appropriate mens rea — that is, the “state of mind” — to commit the crime. The state of mind required depends on both the crime and the jurisdiction, but certain mental defects may negate the existence of that particular state of mind. There are two broad categories of mental disorder defense that are typically asserted. First is the insanity defense that depends on a particular defect of the defendant’s mind. The other category of mental disorder defense is intoxication, which may be broken down into voluntary and involuntary intoxication.
In order to assert a mental disorder defense, the defendant must show that there was a defect of some kind that prevented him or her from forming the necessary state of mind to commit the crime. For example, most jurisdictions require that anyone convicted of murder must have had “malice aforethought” in committing their actions that led to the death of the victim. Malice aforethought generally implies that the person had specifically intended to cause at least serious bodily harm to the victim before committing the act that killed him or her. An appropriate mental disorder defense would tend to show that the defendant did not have the mental capability to intend to kill or seriously harm the victim at the time of the offending action. If this mental disorder defense is successful, it would not necessarily result in the innocence of the defendant, but may have the crime reduced to a less serious charge such as manslaughter.
There are several types of tests for the insanity mental disorder defense that are recognized in various jurisdictions. However, there are two in particular that are most commonly recognized. The majority rule is the “M’Naghten” rule, which may be asserted if the defendant did not know his or her act would be wrong or did not understand the nature and quality of his or her actions. The other is the “irresistible impulse” rule, which requires a showing that the defendant was unable to control his or her actions or conform his or her conduct to the law.
Intoxication may be raised as a mental disorder defense if the intoxication would have put the defendant in a state in which he or she could not form the requisite intent to committing the particular crime for which he or she is charged. The one line to be drawn is if the intoxication was voluntary or involuntary. Involuntary intoxication occurs when a person ingests an intoxicating substance without knowledge of its nature, under threat of serious bodily harm or pursuant to medical advice. Conversely, voluntary intoxication occurs when a person purposefully takes an intoxicating substance with the knowledge of its intoxicating nature and is a much
