Encyclopedia of Psychology and Law

explanations during trial admonitions, and reinforcing

charges at the end of the trial. Lessons from broader

memory research suggest that any means to intercept

errant information before or at the time it is encoded

into memory is likely to be more successful than an

attempt to remove the inadmissible evidence after it is

merged into memory. Jury deliberation also may be

expected to limit the influence of inadmissible evi-

dence; however, few studies have addressed this spe-

cific hypothesis. The research and legal communities

will benefit from future research that attends to creative

solutions for the problem of inadmissible evidence.

Nancy K. Steblay

See alsoJuries and Judges’ Instructions; Pretrial Publicity,

Impact on Juries; Story Model for Juror Decision Making

Further Readings

Steblay, N., Hosch, H., Culhane, S., & McWethy, A. (2006).

The impact on juror verdicts of judicial instruction to

disregard inadmissible evidence: A meta-analysis. Law

and Human Behavior, 30,469–492.

Wistrich, A,. Guthrie, C., & Rachlinski, J. (2005). Can judges

ignore inadmissible information? The difficulty of

deliberately disregarding. University of Pennsylvania Law

Review, 153,1251–1345.

INSANITYDEFENSE, JURIES AND

The insanity defense is one of the most controversial

legal defenses in the U.S. legal system, as demon-

strated through the constantly evolving insanity laws

and the public response to insanity cases. There is

extensive evidence to suggest that juror attitudes, pre-

conceived notions, and case-relevant biases and

beliefs affect their judgments in insanity defense

cases. Research provides strong support for the find-

ing that negative attitudes toward the insanity defense

have a robust effect on mock jurors’ verdict decisions.

Additionally, there is evidence that jurors, rather than

relying on instructions and legal definitions, tend to

rely on their own commonsense notions of what is

considered sane and insane and to use these in deter-

mining their verdicts. At the same time, a number of

other factors, such as the severity of the crime, char-

acteristics of the perpetrator, and knowledge levels,

may moderate the relationship between attitude and

verdict, and these factors warrant further investiga-

tion. This entry briefly examines the evolution of

insanity law, jurors’ attitudes to and knowledge of the

insanity defense, the influences on jurors’ insanity

verdicts, and the role of experts in insanity trials.

The insanity defense can be raised in criminal cases

when a defendant has a mental illness that interferes

with his or her capacity for criminal responsibility. The

concept underlying the insanity defense is that it is fun-

damentally unfair to hold a person responsible for a

crime when he or she lacks the capacity to form intent

because of a mental illness. The idea that certain defen-

dants should not be held responsible for their actions

due to their mental state has been well established for

centuries, starting with the “wild beast” test of the

1700s. Since then, the law has struggled to establish

guidelines as to what constitutes insanity. This has led

to a constantly evolving standard in these cases.

The changing standards for insanity reflect the

difficult nature of the defense. The M’Naghten test,

established in 1843, held that defendants were not

responsible for their actions if they could not tell that

their actions were wrong at the time they were com-

mitted. This test was subsequently criticized because it

put heavy emphasis on the cognitive aspects of right

and wrong but failed to take into consideration

the issue of the defendant’s volitional control. The

M’Naghten test underwent many changes, each alter-

ing the balance of emphasis between the cognitive and

volitional underpinnings of insanity and also changing

the definitions of these concepts. Some of the stan-

dards currently in use include the M’Naghten test; the

M’Naghten test with an allowance for the defendant

having an “irresistible impulse”; the Durham or “prod-

uct” rule, requiring only that the crime be the product

of a mental illness; the American Legal Institute stan-

dard, which includes both cognitive and volitional rea-

sons for insanity, and the Insanity Defense Reform Act

of 1984, which includes only the cognitive element

and requires the mental illness to be severe. Many of

these changes in standards were in response to highly

publicized insanity defense cases in which the verdicts

were viewed unfavorably by the public. The most

influential of these cases was the trial of John Hinkley

for the attempted assassination of President Reagan.

Juror Decision Making

in Insanity Cases

The changes in the law described above have resulted in

multiple insanity standards, which raises the question of

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