Not Guilty by Reason of Insanity: What It Takes To Prove Legal Insanity

An insanity defense can protect a criminal defendant from being held legally responsible for an offense when a qualifying mental disease or defect prevented the defendant from meeting the applicable standard for criminal responsibility. The rules vary by jurisdiction, and an insanity defense is different from using psychological evidence about a syndrome, such as Battered Person Syndrome, to support another defense such as self-defense or diminished capacity.

Having a mental illness does not automatically make someone legally insane. Courts apply specific legal tests to determine whether a defendant’s mental condition affected their understanding, judgment, or ability to control their conduct in a way the law recognizes. A successful insanity defense may result in a verdict of not guilty by reason of insanity, but that does not necessarily mean the defendant walks out of the courthouse. Commitment for psychiatric treatment may follow, sometimes for an extended period.

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What Is an Insanity Defense?

Rather than disputing whether the conduct occurred, the defense argues that the defendant should not be held criminally liable because their mental state met the jurisdiction’s legal standard for insanity when the crime occurred.

Legal insanity is not necessarily the same as having a mental illness or psychiatric diagnosis. A defendant may have a diagnosed mental health condition and still face criminal liability. The question is whether the defendant satisfies the specific legal test for insanity that applies to the criminal charges.

Proving Legal Insanity: Four Legal Tests

A psychiatric diagnosis may be relevant, but the diagnosis alone does not determine whether a defendant can avoid criminal responsibility. The legal question focuses on the defendant’s mental state at the time of the crime. No single definition of legal insanity applies nationwide. States and federal courts use different legal standards to evaluate a defendant’s mental state at the time of the offense.

Legal insanity is also different from competency to stand trial. Competency concerns the defendant’s current mental competence and the defendant’s ability to understand the criminal trial and work with their attorney. An insanity defense, by comparison, looks at the defendant’s mental condition when the alleged offense occurred.

It is also separate from a diminished capacity defense. States differ significantly in whether they recognize diminished capacity. They also vary on how mental health evidence can be used for that purpose. Voluntary intoxication may also affect a defendant’s mental state in some cases, but it does not automatically establish legal insanity.

1. M’Naghten Rule

The M’Naghten rule evaluates what was going on in the defendant’s mind at the time of the crime. Did a mental disease or defect prevent the defendant from understanding what they were doing? Could they recognize that the conduct was wrong?

Under the traditional version of the rule, a defendant may be considered legally insane if, because of a mental disease or defect, they did not understand the nature and quality of their actions or did not understand that their actions were wrong. Versions of this rule remain widely used and influential. However, the exact language and requirements can vary by jurisdiction.

California uses the M’Naghten test for legal insanity. Under this test, the defense applies if the defendant did not understand the nature of the criminal act or did not understand that they were doing something morally wrong.

2. Model Penal Code Test

The Model Penal Code test takes a broader approach than the traditional M’Naghten rule. It considers two things: the defendant’s ability to understand and control their conduct at the time of the offense.

Under this approach, the defendant lacks substantial capacity to appreciate the criminality or wrongfulness of their actions. Alternatively, the defendant lacks substantial capacity to conform their conduct to the requirements of law. Some jurisdictions have adopted versions of this test. Others have modified or rejected parts of it.

3. Irresistible Impulse Test

The irresistible impulse test focuses on a defendant’s ability to control their conduct. A person may understand that an action is wrong but still claim that a mental disease or defect made them unable to resist the impulse to commit it.

This approach is often discussed as a supplement to cognitive tests for legal insanity. Its use and application vary among jurisdictions.

4. Durham Rule

The Durham rule took a broader approach to determining legal insanity. Under this test, a defendant could be excused from criminal responsibility if the unlawful act resulted from a mental disease or defect.

The rule focused on whether the defendant’s mental condition caused the criminal conduct. Although courts have largely moved away from the Durham rule, it remains an important part of the history and development of insanity law.

How Do Defendants Raise an Insanity Defense?

Raising an insanity defense usually involves more than telling the court that the defendant has a mental illness. Criminal procedure rules often require the defense to take specific steps before presenting the issue at trial.

Depending on the jurisdiction, the defendant may need to formally notify the prosecution and court, enter a special plea, or file documents asserting the defense.

Who Has the Burden of Proving Insanity?

Raising an insanity defense does not always shift the burden of proof in the same way. The rules depend on where the case is being tried. In some jurisdictions, the defendant must prove legal insanity by a preponderance of the evidence or another standard. In others, the prosecution may have to address the issue once the defense has properly raised it.

Mental health professionals and psychiatric testimony can play an important role in meeting that burden. However, even compelling testimony from a qualified expert does not automatically establish the legal defense. The evidence must satisfy the legal standard used by the court.

In federal criminal cases, the Insanity Defense Reform Act places the burden on the defendant to prove insanity by clear and convincing evidence. Even then, the prosecution must still prove the elements of the charged offense beyond a reasonable doubt.

What Happens After a Successful Defense of Insanity?

A successful insanity defense generally results in a verdict of not guilty by reason of insanity, but it does not necessarily mean the defendant is immediately released. The official court ruling is a verdict that finds the defendant not guilty, with phrasing such as “by reason of insanity.”

After a finding of insanity, the defendant may be placed in a mental health facility, face civil commitments, undergo psychiatric treatment, or participate in periodic review of their mental condition. Release may depend on whether the person continues to present a danger and on the laws of the applicable jurisdiction.

Depending on the circumstances, commitment to a treatment facility could last longer than the criminal sentence the defendant would have received if they were convicted.

What Is a Syndrome Defense?

The term “syndrome defense” is sometimes used to describe a defendant’s reliance on psychological evidence involving the effects of trauma or abuse. Battered Person Syndrome, also historically called battered women’s syndrome, is one example.

However, a syndrome generally does not operate as an independent criminal defense. Instead, expert testimony about a syndrome or psychological condition may help a jury understand how a defendant’s experiences affected their perceptions, beliefs, or behavior.

How Can Battered Person Syndrome Support a Criminal Defense?

Battered Person Syndrome can arise in criminal cases involving a history of domestic violence or repeated abuse. Expert testimony may explain the psychological effects of that abuse and provide context for the defendant’s perceptions and behavior. This can be important when a defendant’s actions are difficult to understand without considering the circumstances they experienced over time.

The evidence does not automatically establish a legal defense. Instead, it may help the jury evaluate a particular issue raised by the defense, including whether the defendant reasonably perceived a threat or had the required mental state when the alleged offense occurred.

Syndrome Evidence and Self-Defense

Evidence about Battered Person Syndrome may be relevant when a defendant claims self-defense. Self-defense generally requires an analysis of what the defendant believed about the threat they faced and whether the defendant’s use of force was legally justified under the circumstances.

Expert testimony about the effects of repeated abuse may help a jury understand the defendant’s perception of danger. For example, the evidence may provide context for why the defendant believed they faced an imminent threat or believed that using force was necessary. Courts have recognized that this type of testimony may be relevant when determining whether a defendant’s belief in the need for self-defense was reasonable.

Specific self-defense requirements vary by jurisdiction, so syndrome evidence does not automatically establish the defense.

Syndrome Evidence and Diminished Capacity or Mental-State Defenses

Psychological evidence may also be relevant when the issue is whether a defendant formed the mental state required for a particular offense. In some jurisdictions, evidence concerning trauma or a psychological condition may support a diminished capacity defense or another defense involving the defendant’s state of mind.

This is different from an insanity defense. An insanity defense focuses on whether the defendant met the jurisdiction’s legal definition of insanity at the time of the offense. Diminished capacity and other mental-state defenses may instead address whether the prosecution proved that the defendant had the specific mental state required for the charged offense.

The availability of diminished capacity varies from state to state. Some jurisdictions recognize the defense, while others limit or reject it. The same is true of the rules governing psychological evidence, so an attorney must determine whether and how this evidence can be used under the law that applies to the case.

What Other Types of Syndrome or Psychological Evidence May Arise in Criminal Cases?

Depending on the facts, attorneys may present evidence about trauma, mental disorders, or other psychological conditions to address a specific issue in the case.

The purpose of the evidence matters. It may be offered to support a defense, challenge whether the prosecution proved a required mental state, or help a jury understand the defendant’s perceptions or behavior.

Why Are Mental Health Defenses So Complex?

Presenting a mental health defense can be complicated because it requires in-depth analysis. The defendant’s mental state is at the core of the analysis. The analysis is more complicated because it concerns the defendant’s mental state at the time of the offense, not their current mental state. That can be difficult to establish long after the alleged crime occurred.

A diagnosis by itself does not establish legal insanity. The defendant must meet the specific legal standard that applies to the case. Mental health professionals may need to review the defendant’s medical and personal history, past behavior, statements, and other evidence to form an opinion about their condition at the relevant time. The prosecution may have its own experts who reach a different conclusion.

Talk to a Criminal Defense Attorney

Legal insanity depends on the standard used in the applicable jurisdiction and focuses on the defendant’s mental state when the offense occurred. An experienced criminal defense lawyer can evaluate the facts, determine whether a mental health defense may apply, and explain what to expect if you pursue it.

Use the Super Lawyers directory to locate a criminal defense attorney in your area.