though it is logically relevant to whether a requisite
mens rea was in fact present.
The logic of the mens rea variant is impeccable.
Crimes are defined by their elements, and the prosecu-
tion must prove all these elements beyond a reasonable
doubt. If the prosecution is unable to prove an element,
then the defendant should be acquitted of a crime
requiring that element. The defendant using the mens
rea variant of diminished capacity seeks simply to use
evidence of mental abnormality to cast reasonable
doubt on the presence of a mental state element that is
part of the definition of the crime charged. Such use of
mental abnormality evidence is not a full or a partial
affirmative defense. It is functionally and doctrinally
indistinguishable from the use of any other kind of evi-
dence for the same purpose, and it thus does not war-
rant a special name as if it were a unique doctrine.
Justice or fairness seems to require permitting a
criminal defendant to use relevant evidence to cast rea-
sonable doubt on the prosecution’s case when criminal
punishment and stigma are at stake. Nonetheless, a
criminal defendant’s right to introduce relevant evi-
dence may be denied for good reason, and the U.S.
Supreme Court recently held that the Constitution does
not require the admission of most kinds of mental
abnormality evidence offered to negate mens rea, even
if such evidence is logically relevant and probative.
About half the American jurisdictions exclude mental
abnormality evidence altogether when it is offered to
negate mens rea, and the other half permit its introduc-
tion but typically place substantial restrictions on the
use of the evidence.TToottaall EExxcclluussiioonn ooff MMeennttaall
AAbbnnoorrmmaalliittyy EEvviiddeennccee
The most common justifications for exclusion of
mental abnormality evidence to negate mens rea are
that courts and legislatures confuse the mens rea claim
with a partial or complete affirmative defense, that
mental abnormality evidence is considered particu-
larly unreliable in general or for this purpose, and that
permitting the use of such evidence would compro-
mise public safety. If mens rea negation is wrongly
thought to be an affirmative defense, it may appear
redundant with the defense of legal insanity or a court
might believe that creating a new affirmative defense
is the legislature’s prerogative. If mens rea negation
were an affirmative defense, these might be good rea-
sons to reject the admission of mental abnormalityevidence, but these reasons are unpersuasive because
they rest on a confused doctrinal foundation.
The unreliability rationale for exclusion is stronger
in principle because courts are always free to reject
unreliable evidence. The difficulty with this rationale
is that mental abnormality evidence is routinely con-
sidered sufficiently reliable and probative to be admit-
ted in a wide array of criminal and civil law contexts,
including competence to stand trial, legal insanity,
competence to contract, and others. Criminal defen-
dants are afforded special protections in our adversary
system because the defendant’s liberty and reputation
are threatened by the power of the State. For the same
reason, there is also a powerful motivation to provide
defendants special latitude to admit potentially excul-
patory evidence, especially when evidence of the
same type is admitted in other contexts where much
less is at stake. It seems especially unfair to exclude
evidence of mental abnormality, which is rarely, if
ever, the defendant’s fault, when most jurisdictions in
some circumstances routinely admit evidence of vol-
untary intoxication to negate mens rea.
The public safety rationale is also sound in principle.
If a mentally abnormal and dangerous defendant uses
abnormality evidence successfully to negate all mens
rea, outright acquittal and release of a dangerous agent
will result. Virtually automatic involuntary civil com-
mitment follows a successful affirmative defense of
legal insanity, but the State has less effective means to
preventively confine dangerous defendants acquitted
outright.
The problem with the public safety rationale is
practical rather than theoretical. Mental disorders may
cause agents to have profoundly irrational reasons for
action, but they seldom prevent people from forming
intentions to act, from having the narrow types of
knowledge required by legal mens rea, and the like.
Moreover, the mens rea termed negligence—unreasonable
failure to be aware of an unjustifiable risk that one has
created—cannot be negated by mental abnormality
because such failure is per se objectively unreasonable.
Consequently, very few defendants with mental disor-
der will be able to gain outright acquittal by negating
all mens rea or will even be able to reduce their con-
viction by negating some mens rea. Public safety
would not be compromised by the mens rea variant.
The only possible exception to the observation that
mental abnormality seldom negates mens rea is the
mental state of premeditation required by many juris-
dictions for conviction for intentional murder in the218 ———Diminished CapacityD-Cutler (Encyc)-45463.qxd 11/18/2007 12:42 PM Page 218