Denying the Insanity Defense: A Repudiation of Science and Justice

By Laura Karas

The Supreme Court this term faces a difficult question: Is it unconstitutional for a state to abolish the insanity defense? In the case before the court, Kahler v. Kansas, a plaintiff who suffered from multiple psychiatric disorders, including severe depression, shot and killed his wife, two daughters, and mother-in-law. He was denied the opportunity to present an insanity defense under Kansas law and was sentenced to death. Kansas is one of four states to have abolished the insanity defense, along with Idaho, Montana, and Utah.

The insanity defense in its modern form contains two prongs. First, a person is not responsible for his criminal conduct if he could not “conform his conduct” to the requirements of the law due to mental illness. This prong is sometimes referred to as the volitional prong of the insanity defense. Second, a person is not responsible for his criminal conduct if, due to mental illness, he could not appreciate the wrongfulness of his actions. Since the 1980s, many states have rejected the volitional prong of the insanity defense, and some states, such as Kansas, have gone so far as to abolish the insanity defense entirely.

The volitional prong of the insanity defense fell into disfavor because of the difficulty of assessing a defendant’s “volition,” meaning the degree of impairment of one’s will, in states of mental illness. Some have argued that impaired volition due to mental illness requires precise measurement in order to establish a threshold below which volition is lost. Only in this way, the argument goes, can we know whether a person’s deliberate will was reduced sufficiently by mental illness that he deserves a defense under the law for his criminal act. Unfortunately, psychiatry and neuroscience have not yet provided us with the means to make such measurements.

I argue here that the idea that impairment in volition must be precisely measured to be subject to jury determination is a misconception.

In many areas of criminal law, determinations are made regarding a culpable mental state without requiring any degree of precise measurement. Did the defendant engage in adequate reflection to establish willful, deliberate, premeditated murder? Did the defendant act in the heat of passion? Did the defendant have the level of awareness necessary to distinguish reckless conduct from negligent conduct? A measurable test is not necessary for a jury to reach a final determination on these matters of mental state. Instead, juries should use expert testimony—ideally testimony from neutral experts—to decide whether a person in the throes of a severe illness may have lost the ability to control his conduct and to “conform his conduct” to the law.

The argument against the volitional prong of the insanity defense persists at least in part because disorders of the mind are medical pathologies for which many assume quantifiable thresholds can or should exist. But the focus on measurement is misplaced. Instead, the focus is better shifted to a holistic appraisal of the impact of mental illness on individual behavior. Mental illnesses can be severe and disabling. Scientific findings continue to shed light on the origins of mental illness: dysfunction of the brain. A growing body of evidence suggests that mental illness has a complex genetic basis, reinforcing the fact mental illnesses are not artificial constructs or mere emotions that one can simply “will away.” Though it is not always simple to appropriately categorize and diagnose mental illness, in many cases a defendant will have a well-established history of psychiatric illness. Psychiatric history in conjunction with neutral expert testimony can help jurors assess whether an insanity defense is warranted.

Psychiatry and neuroscience have not yet advanced to the point of permitting us to gauge volition on a numeric scale as we do blood sugar or blood pressure. Yet, criminal law should not deny the very real and powerful impact of mental illness on individual behavior merely because the current state of science remains imperfect. Those who are ill deserve treatment, not criminal punishment. Rejection of the volitional prong of the insanity defense, and denial of an insanity defense entirely, amount to a repudiation both of science and of justice.

Laura Karas, M.D., M.P.H., is a J.D. candidate at Harvard Law School.  She received her medical degree from Baylor College of Medicine, her undergraduate degree from Harvard College, and a Masters of Public Health in Health Policy and Management from the Harvard T. H. Chan School of Public Health.  Her research interests include access to care for disadvantaged populations, pharmaceutical policy, and health care antitrust.


The Petrie-Flom Center Staff

The Petrie-Flom Center Staff

The Petrie-Flom Center staff often posts updates, announcements, and guests posts on behalf of others.

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