Insanity Defense

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.

Juror Decision Making in Insanity Cases

The changes in the law described above have resulted in multiple insanity standards, which raises the question of how jurors will respond to these variations and if they are able to distinguish among them. The standards that jurors are supposed to use in any specific insanity defense case are delivered to the jurors via jury instructions. Research has investigated jurors’ responses to these various standards as presented in jury instructions. This research has shown that jury decision making is not substantially affected by the standard that is used or by variations in jury instructions. Additionally, jurors who are given instructions and those who are given no instructions do not seem to significantly differ in their decisions. Whether a standard is present and, if so, which type of standard is used appears to have little effect on jurors’ ultimate decisions, even though the standards are based on very different legal notions. This should not be interpreted to mean that jurors do not take the instructions into consideration when they are deliberating, nor does it indicate that they do not take their duties seriously. Some scholars suggest that jurors interpret insanity cases based on their commonsense understanding of mental illness and of the defense itself. It is argued by Norman Finkel (1988) and others that jurors may not distinguish among the varying standards because they rely on their own interpretation of insanity when judging the appropriateness of the defense.

If jurors are basing their decisions in insanity cases in part on their commonsense understanding of the defense, it is important to determine what this commonsense understanding might be. Michael Perlin has written extensively about the common misunderstandings that might be relied on in decision making in insanity cases. He identified eight “myths” that drive public perceptions of the insanity defense. These myths include the belief that the insanity defense is overused, defendants who plead insanity are usually faking, the insanity defense is used almost exclusively in cases that involve violent crimes, pleading not guilty by reason of insanity (NGRI) is a strategy used by criminal defense attorneys to get their clients acquitted, there is no risk to the defendant who pleads insanity, trials involving an NGRI defense almost always feature “battles of the experts,” NGRI acquit-tees spend much less time in custody than do defendants convicted of the same offense, and NGRI acquittees are quickly released from custody. Perlin’s myths are examples of the flawed knowledge about insanity that exists in the public domain. Each of these myths has been refuted by empirical findings from multiple sources. However, this misinformation has the potential to negatively influence jurors’ consideration of the insanity defense in specific cases.

In addition to the faulty knowledge that prospective jurors might have, jurors may also have preexisting attitudes about the insanity defense that could affect their decision making. Surveys as well as experimental studies have revealed that people hold strong negative attitudes toward this defense. Many prospective jurors report viewing the insanity defense as a loophole in the legal system through which dangerous mentally ill people could reenter society or by which truly guilty criminals who were not mentally ill could be acquitted. In addition, people perceive the insanity defense as one that is too frequently used as well as abused. Research also indicates that negative attitudes about mental illness are largely fueled by this misinformation about mental illness. For example, people have a tendency to overestimate the number of defendants who plead insanity and who are acquitted by reason of insanity, while they tend to underestimate the period of confinement for insanity acquittees. The relationship between insanity knowledge and attitudes is such that more accurate knowledge is related to more favorable attitudes.

Negative attitudes have been shown to decrease jurors’ willingness to consider and to render NGRI verdicts. Research indicates that attitudes toward mental illness and the insanity defense exert significant influence on mock jurors’ verdicts in insanity cases, even more so than the case facts. Jurors with negative attitudes are far less likely to render NGRI verdicts. Attitudes toward the death penalty are also related to decision making in insanity cases. Jurors with positive attitudes toward the death penalty are crime-control oriented, tend to hold negative attitudes toward the insanity defense, and are significantly less willing to render NGRI verdicts.

Another focus of research on the origins of potential jurors’ beliefs about the insanity defense has been in the study of insanity prototypes, or the concept of the typical insanity defendant. In several prototype studies, researchers have found that jurors’ notions of insanity included extreme impairments at the time of the offense as well as extreme psychosis. They tended to inflate symptoms of psychosis, as well as portray the offender as extremely violent. These prototypes could produce expectations about defendants in insanity trials that could in turn affect decision making, although there has been little research on this phenomenon.

Once a trial in which insanity is claimed begins, it is the responsibility of the jurors to assess the evidence presented to them and the viability of the insanity defense in that case. Research has investigated the impact of varying case facts on decision making in insanity cases. Mock jurors seem to construe case information differently depending on their prior beliefs and attitudes. As noted above, there is a tendency for jurors in insanity defense cases to rely more on their own notions of insanity than on the facts of the case. The type of mental illness can be influential, and much research has focused on schizophrenic defendants. Some case facts are also influential in insanity verdicts. For example, some research has found that defendants who had been more reckless, committed more gruesome crimes, and behaved with premeditation were found guilty more often.

Research has also investigated the impact of the personal characteristics of the jurors themselves. The importance of personal contact with mental illness (either through a personal experience or that of a relative or a friend) has been examined, with mixed results. For example, contact with people with mental illness has a somewhat positive effect on attitudes toward mental illness. Studies have shown that college students who had direct interactions with people suffering from depression or psychosis made more positive attributions about the causes of these illnesses. On the other hand, some research has found that those suffering from mental illnesses were less accepting of others who had mental illnesses. Juror gender has also been found to exert an influence on verdict in a limited number of studies; there is some indication that females may be more accepting than males of the insanity defense and of mental illness as a factor in determining criminal responsibility.

Expert Witnesses and the Insanity Defense

Expert testimony is typically proffered in trials where the defendant is raising an insanity defense. The typical successful insanity defense requires a showing of significant mental illness or impairment through expert testimony. The role of experts in insanity defense trials is somewhat unique. There has been substantial controversy about the role of expert psychological testimony in insanity defense trials, and some have advocated doing away with experts in these cases. The law places a number of constraints on experts in insanity trials. Psychologists in most instances testify about a diagnosis for the defendant and the symptoms associated with that diagnosis, and they give their opinions regarding the defendant’s ability to understand the difference between right and wrong. However, after the Hinkley case, the Federal Rules of Evidence were amended to disallow expert mental health testimony on the ultimate issue—whether or not the defendant was sane or insane at the time of the alleged offense. This decision was left to the trier of the fact. Limited research has examined the effect of expert testimony in general or of ultimate opinion testimony on juror decision making. A consistent finding is that the ultimate opinions proffered by experts do not have a significant effect on decision making, contrary to the concerns underlying their prohibition.


  1. Finkel, N. J. (1988). Insanity on trial. New York: Plenum Press.
  2. Hans, V. P., & Slater, D. (1984). “Plain crazy”: Lay definitions of legal insanity. International Journal of Law and Psychiatry, 7, 105-114.
  3. Perlin, M. L. (1996). The insanity defense: Deconstructing the myths and reconstructing jurisprudence. In B. D. Sales & D. Shuman (Eds.), Law, mental health, and mental disorder (pp. 341-359). Belmont, CA: Thomson Brooks/Cole.
  4. Poulson, R. L., Braithwaite, R. L., & Brondino, M. J. (1997). Mock jurors’ insanity defense verdict selections: The role of evidence, attitudes, and verdict options. Journal of SocialBehavior & Personality, 12, 743-758.
  5. Zapf, P. A., Golding, S. L., & Roesch, R. (2006). Criminal responsibility and the insanity defense. In I. B. Weiner & A. K. Hess (Eds.), The handbook of forensic psychology (3rd ed., pp. 332-363). Hoboken, NJ: Wiley.

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