The Burden of Proof in Insanity Cases in CA

In California criminal cases, there are no defenses more confusing and complex than an insanity defense. They are also highly scrutinized.

Though it's true that mental illness can play a large role in criminal behavior, the law draws a line between mental illness and a legal sanity defense. One of the most important parts of this is who carries the burden of proof in these cases and how high that burden is.

Unlike other criminal defenses, where the prosecutor must prove guilt beyond a reasonable doubt, an insanity defense operates under very different rules. Understanding how this works is extremely important for any criminal defendant who is considering an insanity defense in California or in cases where mental health conditions may have influenced a criminal act.

Mental Illness vs. Legal Insanity in Criminal Cases Under California Law

Before getting into the burden of proof, it is really important to understand that mental illness, alone, doesn't automatically mean that mental health defenses apply. A defendant who has a mental disease, receiving treatment for mental disorders, or even have a history of hospitalization for their mental competency could still be legally sane in the eyes of the law.

California follows a modified version of the M'Naghten Rule. This defines legal insanity in a very narrow way. The criminal justice system in CA follows California Penal Code section 25(b), which says that a defendant is legally insane only if they were:

• Unable to understand the nature and quality of their act, or
• Unable to understand that the act was morally wrong

This focuses on the defendant's ability to understand the nature of their actions, not their emotional distress, impulse control, or poor judgement.

Who Has the Burden of Proof in California Criminal Cases Based on Sanity?

In California, the burden of proof is totally dependent on the defendant when using an insanity defense. This is very different from other types of cases in California courts. Typically, the prosecution must bear the burden of proof, and that must be beyond a reasonable doubt. In this case, however, the defendant's mental state must be proven for a successful insanity defense to work. Goss Law has information on juvenile defendants and the insanity defense in CA as well.

The “Preponderance of the Evidence” Standard for Insanity Defense in California

Under Penal Code 25(b), a defendant must prove that they were legally insane by a preponderance of the evidence.

This standard:

• Means "more likely than not"
• Is lower than “clear and convincing evidence”
• Is much lower than “beyond a reasonable doubt”

The jury must find it more likely than not that the defendant was legally insane at the time of the crime. If the evidence is evenly balanced, the defense fails.

How Insanity Is Litigated: The Bifurcated Trial Process

California courts use something called a "bifurcated trial system" when a defendant pleads not guilty by reason of insanity. It is a two phase process:

Phase One: Guilt Phase

The jury will first determine if the defendant is guilty of the crime in the first place. In most cases, the mental health treatment or diagnosis of the defendant is irrelevant unless it directly impacts certain elements, such as intent.

If the jury finds the defendant not guilty, nothing else happens and the defendant is free. If the defendant is found to be guilty, the case moves on to a sanity trial.

Phase Two: Sanity Phase

The sanity phase is where the defense must present evidence to show that they were insane at the time the crime occurred.

Generally, the defense attorney of the defendant will present things like psychiatric evaluations, expert testimony, and medical records from mental hospitals, outpatient treatment centers, etc. They can also advise on M'Naghten Rule vs. Model Penal Code.

The prosecution will then present a rebuttal and evidence.

Then, the jury must take all of the information they have an determine if the criminal conduct was done when the person was sane or if they were considered legally insane.

The Role of Psychiatric Experts

Psychiatrists and psychologists are central to insanity cases, but their opinions are not determinative.

Experts evaluate the defendant and testify regarding:

• Diagnosis
• Cognitive functioning
• Ability to understand right from wrong
• Mental state at the time of the offense

However, jurors are free to reject expert opinions, even when multiple experts agree. The jury may place greater weight on the defendant’s behavior before, during, and after the crime.

Evidence That Can Undermine an Insanity Defense

Because the burden is on the defendant, any evidence suggesting awareness or intent can be devastating. Common examples include:

• Planning the crime
• Attempting to conceal evidence
• Fleeing the scene
• Lying to law enforcement
• Making statements acknowledging wrongdoing

Even subtle indications that a defendant understood consequences or moral wrongfulness can defeat an insanity claim.

Voluntary Intoxication and Insanity Claims

California law is clear that voluntary intoxication cannot form the basis of an insanity defense.

Mental states caused by:

• Alcohol
• Illegal drugs
• Substance abuse disorders

do not qualify as legal insanity, even if they cause hallucinations or impaired judgment. This exclusion significantly limits the scope of viable insanity defenses.

What Happens If the Defendant Fails to Meet the Burden?

If the defendant does not prove insanity by a preponderance of the evidence, the insanity defense fails, and the guilty verdict stands.

This outcome can have serious consequences:

• The defendant is sentenced as a convicted offender
• Psychiatric testimony becomes part of the record
• Other defense strategies may be weakened

For this reason, insanity defenses must be evaluated carefully before being asserted. For help with insanity defenses, a criminal defense lawyer may be able to help.

What Happens If the Defendant Meets the Burden?

If the jury or judge finds the defendant not guilty by reason of insanity:

• The defendant is usually committed to a state hospital or other treatment facility, though the court may instead order outpatient treatment or find that sanity has been restored
• Confinement can last as long as or longer than a prison sentence
• Release requires court approval and medical clearance
• Periodic hearings determine continued confinement

An insanity verdict is not a release from custody—it is a transfer from the criminal system to the mental health system.

Why Insanity Defenses Are Rare

The standard of proof is not especially high, but insanity defenses are still uncommon and rarely successful. The legal definition of insanity is far narrower than public perception, jurors tend to be skeptical, and the defendant carries the burden.

The Importance of Experienced Criminal Defense Counsel

Raising an insanity defense without strong factual and medical support can be harmful. A qualified criminal defense attorney must:

• Evaluate whether the legal standard can realistically be met
• Coordinate expert evaluations
• Anticipate prosecutorial rebuttal
• Educate the jury on the legal definition of insanity
• Protect other defense options if insanity is not viable

Insanity cases demand strategic planning and precision.

Criminal Defense Guidance from Goss Law

In California, the burden of proof in insanity cases rests on the defendant, and meeting that burden requires solid evidence, credible experts, and a well-structured legal strategy. The decision to pursue an insanity defense should never be made lightly.

Goss Law represents clients throughout Sacramento and surrounding areas in complex criminal cases involving mental health issues. When questions of sanity, criminal responsibility, or psychiatric evidence arise, experienced legal guidance is critical.

Anyone facing criminal charges where mental illness may be a factor should consult with a criminal defense attorney who understands California insanity law and the burden of proof involved.