§ 26.02 Diminished Capacity: Mens Rea Defense

§ 26.02 Diminished Capacity: Mens Rea Defense

[A] Nature of the Defense

Consider three hypothetical cases. First, D1 is charged with first-degree murder based on the claim that he intentionally killed V. D1 wishes to introduce expert testimony that at the time he killed V he was suffering from a mental illness. Although the testimony will not show that D1 was insane at the time of the crime, it is intended to prove that, as a result of his abnormal mental condition, he did not form the intent to kill V.

Second, suppose that D2 is prosecuted for rape and wishes to introduce evidence that as a result of mental disability, he genuinely believed that the female with whom he was having intercourse consented.

Finally, suppose that D3 is prosecuted for assault with intent to commit rape, but wishes to prove that due to a delusion he believed that the woman he was forcibly attacking was his wife.

In each of these cases, the proffered expert testimony forms the basis of the mens rea version of the diminished capacity defense. The testimony that each defendant seeks to introduce speaks to the "question of whether the defendant in fact possessed a particular mental state [intent to kill, by D1; intent to have nonconsensual intercourse, by D2; intent to rape, by D3] which is an element of the charged offense."⁷

The word "defense," as used here, is a "legal colloquialism," because the doctrine "is not designed to defeat a case the State has otherwise established."⁸ Indeed, it is potentially "confusing to refer to [the mens rea version of] diminished capacity as a 'defense'."⁹ This is because "the 'diminished capacity defense' . . . does not provide any grounds for acquittal not provided in the definition of the offense. Properly understood, [diminished capacity] is . . . not a defense at all but merely a rule of evidence."¹⁰ As one court put it, "[f]or the purpose of determining criminal guilt, diminished capacity either negates the state of mind required for a particular offense, if successful, or it does not."¹¹

[B] Law

[1] Overview

Because of judicial confusion regarding the concept of diminished capacity, and legislative imprecision in the enactment of relevant statutes, the law of "diminished capacity" is unclear in many states. However, certain observations and a general summary are possible.

American law is sharply divided regarding the extent to which evidence of an abnormal mental condition not amounting to legal insanity may be introduced for the purpose of negating the mens rea of an offense. As examined below, some states, primarily those that follow the Model Penal Code, permit introduction of such evidence, when relevant,¹² to negate the mens rea of any crime. Other states limit the admissibility of such evidence to some or all specific-intent offenses. A third group bars "diminished capacity" mental health evidence in prosecutions of all offenses.

[2] Defense-to-All-Crimes (Model Penal Code) Approach

Currently, 22 states, consistent with the Model Penal Code,¹³ provide that evidence that the defendant suffered from a mental disease or defect at the time of his conduct is admissible if it is relevant to prove that he lacked a mental state that is an element of the charged offense.¹⁴ The Colorado Supreme Court in Hendershott v. People¹⁵ has succinctly explained the reasoning behind this rule:

Once we accept the basic principles that an accused is presumed innocent and that he cannot be adjudicated guilty unless the prosecution proves beyond a reasonable doubt the existence of the mental state required for the crime charged, it defies both logic and fundamental fairness to prohibit a defendant from presenting reliable and relevant evidence that, due to a mental impairment beyond his conscious control, he lacked the capacity to entertain the very culpability which is indispensable to his criminal responsibility in the first instance.

Applying the Model Penal Code rule to the hypotheticals posited in subsection [A], the defendants in each case would be entitled to introduce evidence of their respective mental abnormalities. The effect of the evidence—if believed or, at least, if it created a reasonable doubt as to the defendants' mens rea—would be to acquit them of the crimes charged, although they might be guilty of a lesser offense.¹⁶ In contrast, as an example, a murder defendant who seeks to show that, due to mental disability, he is easily persuaded to follow the wishes of others, would not be permitted to introduce expert evidence in this regard in an intent-to-kill prosecution, because such testimony, even if believed, would not negate the required element of intent.¹⁷

[3] Limited-Use Approach

A few states arbitrarily limit the introduction of mental-condition evidence to murder prosecutions.¹⁸ Thus, in the hypotheticals in subsection [A], D1 would be permitted to introduce evidence of his mental illness in order to avoid a first-degree intent-to-kill conviction of murder, but D2 and D3 would not be allowed to introduce evidence of their abnormal mental conditions.

A far more common distinction is one drawn between specific-intent and general-intent offenses: Evidence of an abnormal mental condition may be introduced if it tends to show that the defendant could not, or did not, form a specific intent specified in the offense, but such evidence is inadmissible in the prosecution of a general-intent (or strict liability) crime.¹⁹ Thus, D1 could raise his diminished capacity claim to prove that he lacked the capacity to form, or did not in fact form,²⁰ the specific intent to kill. D3, as well, could introduce evidence...