Insanity Defense, Juries and

The insanity defense is one of the most controversial legal defenses in the U.S. legal system, as demonstrated through the constantly evolving insanity laws and the public response to insanity cases. There is extensive evidence to suggest that juror attitudes, preconceived notions, and case-relevant biases and beliefs affect their judgments in insanity defense cases. Research provides strong support for the finding 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 determining their verdicts. At the same time, a number of other factors, such as the severity of the crime, characteristics of the perpetrator, and knowledge levels, may moderate the relationship between attitude and verdict, and these factors warrant further investigation. 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 fundamentally 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 defendants 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 committed. 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 altering the balance of emphasis between the cognitive and volitional underpinnings of insanity and also changing the definitions of these concepts. Some of the standards 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 “product” rule, requiring only that the crime be the product of a mental illness; the American Legal Institute standard, which includes both cognitive and volitional reasons 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.

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