Concept

Proof and Evidence

M'Naghten Rule

Proof and Evidence

The earlier sections taught what a defendant must show to satisfy the test. This section concerns who must show it, at what moment the mental condition has to exist, and what evidence may be used to prove it. The defendant carries the burden, and the traditional rule frames the elements accordingly: he must prove a disease of the mind, a resulting defect of reason, and, as a consequence, a lack of capacity at the time of the charged conduct to know the nature and quality of the act or that it was wrong.¹ The judges' answer in M'Naghten's Case supplies the starting point, because it tells jurors that every person is presumed sane and sufficiently rational to be responsible for his crimes until the contrary is proved to their satisfaction, and that the defense must be "clearly proved."² How much proof that language demands in a modern prosecution depends on the jurisdiction's own rule, so an exam answer should apply the standard the stem supplies and should not add one it omits.

The presumption of sanity and the burden on the defendant fit together for reasons that can be read off the structure of the defense. Most people who commit crimes are responsible for them, so a rule that begins by presuming responsibility matches the ordinary case and spares the prosecution from disproving mental disease in every trial. The defense is also an excuse that the defendant asserts: it concedes the act and claims that his condition removed the capacity that makes punishment fair, and the facts about that condition, such as treatment history, earlier episodes, and witnesses to his behavior, are facts he is positioned to gather. Finally, a successful defense does not end in outright release but in a special verdict of not guilty by reason of insanity, usually followed by commitment, which separates the defendant from the criminal system and sends him to a medical-custodial disposition.³ A presumption of sanity reserves that diversion for defendants who actually make the showing.

The legal question the test asks is narrow, but the evidence that bears on it is broad. Insanity turns on what the defendant understood when he acted, and a mental state cannot be observed directly. It can only be reconstructed from what surrounds the act: the defendant's history, expert testimony about his condition, and his behavior before, during, and after the offense. For that reason courts applying M'Naghten have generally admitted any evidence of aberrational behavior that is probative of the defendant's mental condition, without regard to the formal limits of the test that will later define insanity for the jury, and they have not generally confined expert witnesses to testimony about formal cognition.⁴ This explains how the preceding section could hold that a loss of self-control is no defense and still allow evidence of it: the test limits what the jury must find, and it leaves open what the jury may hear about the defendant's mind.

Timing ties all of this evidence to a single moment. The elements are measured at the time of the charged conduct, so the question is whether the disease had produced the defect of reason when the act occurred.⁵ Evidence from earlier or later periods is relevant only to the extent that it sheds light on that moment, which is why a diagnosis or an earlier finding of insanity does not satisfy the test by itself.⁶ The same separation operates across the criminal process, because insanity as a defense is defined differently from the mental condition that makes a defendant incompetent to stand trial, and a finding under one definition does not carry over to the other.⁷

Hypothetical Illustration

A defendant charged with assault raises insanity under the traditional test. He offers hospital records documenting a delusion that his coworkers had been replaced by impostors, the testimony of his sister, who heard him describe that belief in the weeks before the offense, and a psychiatrist's opinion that the delusion was active on the day of the attack. All of this reasonably tends to show his mental condition when he acted, so the jury hears it and decides whether the delusion left him unable to know the nature of the act or that it was wrong. Change one fact: the defendant offers only a diagnosis from years earlier and a record that he was found not guilty by reason of insanity on an unrelated charge, with nothing connecting his illness to what he understood during the assault. The jury learns that he is ill, but no evidence reaches the moment of the act, so he has not proved the elements and is not acquitted.

Evidence of mental disease can also serve purposes other than the insanity defense, and the jurisdiction controls those uses. In many states such evidence may be introduced only as part of an insanity defense, so a defendant who wants to show that his illness kept him from forming the intent for the crime has no separate route.⁸ Other states let the defendant use the evidence on mens rea, a route usually called diminished capacity, and prosecutors object that this gives the defense two theories, legal insanity and lack of mens rea, for the same crime.⁹ A state may also bar expert psychiatric testimony offered for the mens rea purpose without violating due process, a point from Clark v. Arizona that the next section takes up.¹⁰

Proof and Evidence

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