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Chapter 4


Introductory Note: This chapter considers several defenses which the defendant may raise regarding his lack of mental responsibility for the alleged offense. These include: (1) the insanity defense (including the “XYY chromosome” defense); (2) the defense of diminished responsibility (which can negate the existence of the required mens rea); (3) automatism (the doing of acts while in an unconscious state); (4) the defense of intoxication; and (5) infancy.


A. General purpose: If the defendant can show that he was insane at the time he committed a criminal act, he may be entitled to the verdict “not guilty by reason of insanity.” This defense has been recognized in Anglo-American law for several hundred years. Its principal justification is that where the defendant’s mental disease has prevented him from distinguishing between “right” and “wrong,” or from controlling his conduct (depending on the test employed in the particular jurisdiction) the punishment and deterrence objects of the criminal law would not be served by convicting. It is felt that it would be inappropriate and unfair to punish the defendant for something that he could not help, and futile to attempt to deter him from similar misconduct by convicting him.

1. Incarceration as objective: But another significant reason for the defense has also been noted. Most serious crimes are defined in terms of intent; thus in most states, first-degree murder may be committed only by causing the death of another with intent to do so. If no insanity defense existed, an insane defendant might very well be able to show that his insanity prevented him from forming the intent to kill; this would be the case, for instance, in the frequently-cited hypothetical of the man who strangles his wife believing that he is squeezing a lemon. (See M.P.C., Comment 2 to § 4.01). The strangler might therefore go free.

a. Limits use of mental disease: But in many (perhaps most) states, the insanity defense is coupled with a rule that no evidence relating to mental disease or defect may be introduced except as part of an insanity defense. This means that the strangler must either plead insanity, or not be allowed to show that his mental disease prevented him from forming an intent to kill. Coupled with the fact that in virtually every state, an insanity acquittal leads almost inevitably to the defendant’s involuntary commitment to a mental institution (see infra, p. 86), this means that the insanity defense serves as a means of avoiding the outright release of certain defendants who would otherwise be acquitted for lack of the necessary mens rea. See L, p. 324-25.

2. Not constitutionally required: Virtually every state recognizes some form of the insanity defense. Johnson, p. 280. However, probably the federal Constitution does not require the states to recognize insanity as a complete defense. Id.

B. Tests for insanity: Several different formulations exist for determining whether a defendant was insane, in a way entitling him to acquittal. The principal ones are as follows:

C. M’Naghten “right-wrong” rule: At least half of the states apply, as their sole criterion for application of the insanity defense, a rule first set forth in M’Naghten’s case, 8 Eng. Rep. 718 (1843). In that case, the defendant shot and killed Edward Drummond, private secretary to Sir Robert Peel, Prime Minister of England. The defendant believed that Peel had been conspiring to murder him, and shot Drummond thinking him to be Peel. A jury found him not guilty by reason of insanity.

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