The Insanity Defense in Forensic Practice

By David Shapiro, Ph.D.
January 1, 2006 - Last updated: May 31, 2011

Early in November, the Texas Supreme Court announced its decision to reverse and remand for a new trial, the case of Andrea Yates. It will be recalled that Yates, despite the abundant evidence of severe mental illness was convicted of killing her five children by drowning them in a bath tub. While the reversal was not totally unexpected because the expert for the prosecution gave testimony that was inaccurate, the decision of the Texas Supreme Court focused attention on an issue that has been hotly debated for decades now, what the appropriate standard for legal insanity should be.

Until the early 1980s, most states defined legal insanity as some variation of the standard proposed by the American Law Institute in the late 1960s. This standard mandated the presence of a mental disease or defect which substantially impaired the defendant’s appreciation of the criminality of her/his conduct or substantially impaired her/his ability to conform behavior to the requirements of the law. Thus, the test addressed both cognition and impulse control.

Following John Hinckley’s being found not guilty by reason of insanity in 1982, there emerged a cry for reform of the insanity defense. Despite empirical evidence to the contrary, the public believed that the insanity defense was grossly overused, and that defendants were getting away with murder. Several studies during this time period placed public perception of the frequency of insanity acquittals between 45 percent and 70 percent of criminal offenses. The actual percentage is more like one-tenth of 1 percent.

Nevertheless, public misperception leads to poor public policy. In 1984 the United States Congress enacted the Insanity Defense Reform Act. This act eliminated the so called “volitional” prong of the insanity defense, and restricted the defense only to those defendants whose mental illness was so impaired that she/he could not appreciate the wrongfulness of her/his behavior. The above noted dimension, of acting on an impulse because of the influence of a delusion or hallucination was therefore eliminated, unless the delusion or hallucination also interfered with the understanding that the act was wrong. If a defendant, for example, has a delusional belief that the Devil wears a blue suit, and kills someone wearing a blue suit, then flees the scene because he fears apprehension, he would be regarded as legally sane because his fear of apprehension was evidence of his appreciation of wrongfulness. In short, the delusional basis of the act is deemed, under the law, to be irrelevant.

Approximately 60 percent of states now utilize this very restrictive insanity defense and another five have totally abolished the insanity defense. This represents a step backward to the very primitive thinking about mental illness which existed in the 1840s when the right/wrong test for insanity became widely accepted. While there is admittedly much we still do not know about mental health and mental illness, we certainly do know more than the purely cognitive. Advances in neurosciences, for instance, have demonstrated to us the multitude of subtle ways that the brain’s functioning can influence behavior; it is certainly not as simple as knowledge of right and wrong. By eliminating the volitional prong from these defenses, we have effectively eliminated any testimony about these ways in which the brain influences behavior. We are asked such questions as whether the defendant, because of his brain damage, could not appreciate the wrongfulness of his conduct.

Critics of this narrowing approach therefore point out that the law is not responsive to changes in our understanding of mental and emotional processes; they suggest that the law needs to adapt itself to these new insights. On the other hand, there is a school of thought that believes that the law should not change and that psychologists must learn ways to incorporate their findings into the law as it exists now.

It is clear that as long as these two schools of thought maintain their rigid positions, little can be accomplished. We need to open up a dialogue where recent findings from psychology and neuroscience can be evaluated and determine to what extent these findings can fit into the existing legal structure and to what extent laws may need to become more flexible to accommodate these findings. Certainly, an insanity defense so rigid that it allows the conviction of someone as seriously mentally ill as Yates is sorely in need of reform.


David Shapiro, Ph.D., is an associate professor of psychology at Nova Southeastern University in Fort Lauderdale, Fla., and maintains a forensic psychology consulting practice in Fort Lauderdale. He has been involved in forensic and correctional work for 30 years and has published four books on forensic assessment. He can be reached by e-mail at:

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