Not Guilty by Reason of Insanity: What the Law Says, What Psychiatry Knows, and Why They Do Not Agree

On the morning of March 30, 1981, John Hinckley Jr. shot President Ronald Reagan outside a Washington hotel. Reagan survived. Hinckley was arrested at the scene. Two years later, a jury found him not guilty by reason of insanity.

The public was outraged. Congress changed the law within months. Several states abolished the insanity defense entirely.

What almost no one stopped to ask was whether the jury got it right.

That question sits at the center of one of the oldest and most misunderstood arguments in criminal law. What does it mean to be legally insane? Who decides? And does the law's answer have anything to do with what science actually knows about the human mind?

The Most Misunderstood Defense in Law

Before going further, it helps to know how rare the insanity defense actually is. It is raised in less than one percent of felony cases in the United States. Of those, it succeeds roughly twenty percent of the time. When it does succeed, the defendant almost never goes free. They are almost always committed to a secure psychiatric facility, often for longer than a prison sentence would have been. [1]

The defense is not a loophole. It is not a trick. It is a narrow legal argument that says a person's mental state at the time of the act was so severely impaired that holding them criminally responsible would be unjust.

Whether the law draws that line in the right place is a very different question.

Lens One: The Scientific Evidence

The insanity defense in American law is built primarily on a standard that dates to 1843. That year, a Scottish man named Daniel M'Naghten killed the secretary to the British Prime Minister while experiencing paranoid delusions. The House of Lords established what became known as the M'Naghten standard: a defendant is not criminally responsible if, at the time of the act, they did not know the nature of what they were doing, or did not know that what they were doing was wrong. [2]

That standard, with some variations, is still the law in most of the United States today.

The scientific problem with it is significant. Psychiatry has learned an enormous amount since 1843 about the nature of severe mental illness. What the research shows is that the experience of psychosis is far more complicated than a simple question of knowing right from wrong.

Consider Andrea Yates, who drowned her five children in a bathtub in 2001. Yates had a long history of severe postpartum psychosis and had been hospitalized multiple times. At the time of the killings, she believed she was saving her children from Satan. She knew, in a technical sense, that drowning a child was something society considered wrong. But her understanding of reality had been so completely restructured by her illness that the act existed in an entirely different moral framework inside her mind. [3]

Her first jury convicted her. An appeals court later overturned the conviction on other grounds, and a second jury found her not guilty by reason of insanity. The science had not changed between the two trials. The question of what counted as legally insane had been interpreted differently.

Lens Two: The Neuroscience

To understand why the law's standard is so difficult to apply, you need to understand what psychosis actually does to the brain.

Psychosis is not confusion. It is not being upset or out of control. It is a fundamental disruption in how the brain constructs reality. Research using neuroimaging has shown that during active psychotic episodes, the brain exhibits dysregulation in the dopamine system, disrupted communication between the prefrontal cortex and the regions responsible for processing sensory information, and altered activity in areas involved in distinguishing between internal thoughts and external events. [4]

What this means in plain language is that a person in a psychotic episode is not experiencing the same world that everyone else is. They may hear voices that are neurologically indistinguishable from real voices. They may hold beliefs that are neurologically processed the same way that you process factual knowledge. Their brain is not generating these experiences as metaphors or exaggerations. It is generating them as reality.

The M'Naghten standard asks whether a person knew what they were doing was wrong. But if the brain has constructed a reality in which the act is not wrong, the question becomes nearly unanswerable. Knowing requires a functioning reality. Psychosis, at its most severe, dismantles the foundation that knowing rests on.

There is also a category of mental illness that M'Naghten does not address at all. Some defendants are not delusional but have impulse control disorders, severe dissociative conditions, or neurological damage that fundamentally disrupts their ability to regulate behavior even when they understand, abstractly, that something is wrong. The prefrontal cortex, which governs planning, judgment, and impulse control, can be damaged by trauma, disease, or developmental disruption in ways that are invisible on the surface but profound in their effects on behavior. [5] The law, as currently written, has very little room for these cases.

Lens Three: The Legal Interpretation

After the Hinckley verdict in 1982, Congress passed the Insanity Defense Reform Act, which narrowed the federal standard significantly. The burden of proof shifted from the prosecution to the defendant, and the threshold for what qualified as legally insane was raised. [6]

Several states went further. Kansas, Montana, Idaho, and Utah abolished the insanity defense entirely, replacing it with a verdict of guilty but mentally ill, which allows a conviction while theoretically mandating mental health treatment during incarceration. [7]

Legal scholars and psychiatrists have criticized this approach for a fundamental reason. Guilt requires not just an act but a mental state. The law has always held that punishing someone who truly could not have understood or controlled what they did serves no legitimate purpose of justice. It does not deter future behavior, because the conditions that produced the act are medical, not rational. It does not rehabilitate, because prison is not a psychiatric hospital. And it does not satisfy a coherent theory of moral responsibility, because responsibility requires the capacity for it.

The American Bar Association and the American Psychiatric Association have both published statements calling for a broader insanity standard that reflects current scientific understanding of severe mental illness. [8] Neither recommendation has become federal law.

The result is a system in which the legal standard for insanity was written before germ theory, before neuroimaging, before the field of psychiatry existed in any modern form, and has survived largely intact while the science around it has been transformed.

Putting All Three Lenses Together

The scientific evidence shows that severe mental illness can completely restructure a person's experience of reality in ways that are neurologically measurable and clinically documented. The neuroscience explains the mechanism: psychosis disrupts the brain systems responsible for constructing reality, not just interpreting it. The law, working from an 1843 standard, asks a question that the science has shown is far too simple to capture what is actually happening.

This does not mean everyone who pleads insanity should be acquitted. It means the standard for what counts as insanity should reflect what we actually know about the brain.

There is a genuine moral question underneath all of this, and it is one that courts, lawmakers, and psychiatrists have not fully resolved. If a person's brain, through no choice of their own, generates a reality in which a harmful act is not only acceptable but necessary, what does justice require of us? Punishment assumes the person could have chosen otherwise. But choice is a function of the brain. And the brain, in severe mental illness, is not functioning the way the law assumes.

John Hinckley was released from supervised outpatient status in 2022, after four decades of psychiatric treatment and monitoring. Andrea Yates remains in a state psychiatric facility. The law reached different conclusions in their cases. Whether those conclusions reflect justice, or simply the limits of a standard that has not kept pace with science, is a question Mind on Trial believes belongs in the public conversation.

The next post will look at solitary confinement, and what neuroscience has to say about what isolation does to the human brain.

Sources

[1] Callahan et al., "The Volume and Characteristics of Insanity Defense Pleas," Bulletin of the American Academy of Psychiatry and the Law, 1991: pubmed.ncbi.nlm.nih.gov

[2] M'Naghten's Case, 8 Eng. Rep. 718 (H.L. 1843): accessible via law.cornell.edu/wex/mcnaghten_rule

[3] PBS Frontline, "The Yates Case": pbs.org/wgbh/pages/frontline/yates

[4] NIH, dopamine dysregulation and psychosis: pmc.ncbi.nlm.nih.gov/articles/PMC4219066

[5] NIH, prefrontal cortex and impulse regulation: pmc.ncbi.nlm.nih.gov/articles/PMC3621648

[6] Insanity Defense Reform Act of 1984, 18 U.S.C. 17: law.cornell.edu/uscode/text/18/17

[7] Melton et al., Psychological Evaluations for the Courts, Guilford Press: guilford.com

[8] American Psychiatric Association, Statement on the Insanity Defense, 1982: psychiatry.org