Encyclopedia of Psychology and Law

The insanity defense is generally unpopular in pub-

lic opinion because of misperceptions about how it is

used. Survey studies reveal public beliefs that the insan-

ity defense is both frequently used and often successful.

Such beliefs are likely distortions that result from the

high degree of publicity surrounding notorious cases

such as those of Patty Hearst, David Berkowitz (“Son

of Sam”), Jeffrey Dahmer (none of whom was judged

legally insane), and John Hinckley (who was found

insane). However, neither belief is correct. Research

shows that the insanity defense is asserted in less than

half of 1% of criminal cases and more often than not it

fails. Furthermore, when the insanity defense is suc-

cessful, it is not the result of clever lawyers pulling the

wool over the eyes of naive jurors. Most successful

insanity defenses are not seriously challenged by the

prosecution; more often it is clear to all parties that the

defendant was insane at the time of the offense (accord-

ing to criteria discussed below) and should not be held

legally and morally responsible for his or her actions.

Thus, many successful insanity defenses result, in

effect, from plea agreements.

The following sections discuss (a) the criteria for

legal insanity, (b) the legal calculus for determining

when a defendant meets these criteria, and (c) the

methods used by mental health professionals to gather

evidence and formulate opinions about the mental

state of a defendant at the time of an offense.

Criteria for Legal Insanity

Historians of Western law point to the influence of

Henry de Bracton, whose writings on English law in

the 13th century introduced notions of mental capac-

ity and intent into deliberations about guilt and moral

culpability. Early language referenced notions such as

“infancy” or reasoning capacity not far removed from

that of a “wild beast” as potentially exculpating men-

tal states. The most influential English case was the

M’Naghtencase (1843), which established as the test

for insanity that the accused

was laboring under such defect of reason, from dis-

ease of the mind, as not to know the nature and qual-

ity of the act he was doing; or, if he did know it, that

he did not know he was doing what was wrong.

The emphasis in M’Naghtenon impairment in the

ability to “reason” or to “know” clearly made judg-

ments about a defendant’s cognitivecapacities central

to the legal determination of insanity. This emphasis

has survived in modern formulations that reference a

defendant’s “ability to appreciate the wrongfulness of

his conduct.” However, over the course of time modern

psychiatry has influenced the law to consider voli-

tional as well as cognitive impairments as potential

bases for legal insanity. Thus, formulations in some

jurisdictions refer to insanity resulting from criminal

behavior that was due to an “irresistible impulse” or

impaired capacity “to conform one’s conduct to the

requirements of the law” irrespective of the presence

of cognitive impairment.

Finally, some formulations specify quantitative,

albeit imprecise, levels of impairment for the insanity

test to be met. Thus, it may be that not merely

“impaired capacity” but “substantiallyimpaired capac-

ity” must be established to excuse the defendant from

being criminally responsible for his or her behavior.

The Calculus of

Legal Tests for Insanity

In jurisdictions that allow the insanity defense, judges

and juries must apply the relevant legal test. The struc-

ture of all such tests requires three findings by the trier

of fact. First, there is a predicate mental condition from

which the defendant must have been suffering at the

time of the offense. In the M’Naghten formulation

above, the requisite condition is a “disease of the

mind.” More modern formulations reference the pres-

ence of “mental disease or defect” or similar language.

It is important to note that in virtually all formulations,

the legal definition for the predicate mental condition

is neither highly precise nor tied explicitly to clinically

recognized categories of mental illness or other mental

impairments. (It is the case, however, that in many

jurisdictions certain clinical conditions are explicitly

barred by law as a basis for an insanity defense, most

commonly (a) states of intoxication due to voluntarily

consumed drugs or alcohol and (b) “disorders” defined

almost exclusively on the basis of a history of antiso-

cial behavior, such as antisocial personality disorder or

sociopathy.)

The second component in the legal test is that the

criminal act was affected by (loosely, “caused” by) the

predicate mental condition. In other words, merely

having a “disease of the mind” or a “mental disease or

defect” alone is not sufficient to excuse the defendant

from being criminally responsible. Mentally ill people

may commit crimes for all the noncrazy reasons that

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