Updated October 2026 | The Bulldog Law | California Criminal Defense

The insanity defense is a plea that a defendant should not be held criminally responsible because a mental disease or defect, at the time of the offense, left them unable to understand what they were doing or to know that it was wrong. California uses the M'Naghten rule, written into Penal Code 25(b) by Proposition 8 in 1982: the defendant must prove, by a preponderance of the evidence, that they were incapable of knowing or understanding the nature and quality of the act, or of distinguishing right from wrong, when they committed it.

A successful plea produces a verdict of not guilty by reason of insanity, and that verdict does not mean release. Under Penal Code 1026 the person is committed to a state hospital for up to the maximum sentence the crime carries, extendable for as long as they remain dangerous. The defense is raised in a small fraction of cases and succeeds in a smaller one, and for most clients with a mental health condition, the tools that actually change the outcome are different ones.

The Bulldog Law's attorneys evaluate mental state in every serious case we defend, and we enter an insanity plea in the rare case that supports it. More often we use what a client's condition offers in other ways: to negate the intent the charge requires, to obtain mental health diversion, to raise competency, and to shape the sentence. What follows is the M'Naghten test as California applies it, the exclusions, who must prove what, how a not-guilty-by-reason-of-insanity trial is structured, what the commitment that follows looks like, and the alternatives that usually matter more. Call (888) 928-1609 or message our team.

The M'Naghten rule and Penal Code 25

The test comes from the 1843 English case of a man who shot the prime minister's secretary under a delusion of persecution, and the rule the House of Lords announced afterward has governed most American jurisdictions since. California followed it, abandoned it in 1978 for the broader American Law Institute test, and restored it by initiative in 1982. Penal Code 25(b) now provides that the defense of insanity shall be found only when the accused proves by a preponderance of the evidence that they were incapable of knowing or understanding the nature and quality of their act and of distinguishing right from wrong at the time of the offense. Although the statute says "and," the California Supreme Court held in 1985 that the two prongs are alternatives, as the original rule intended, so a defendant who satisfies either one is legally insane.

The first prong asks whether the person understood what they were physically doing: a defendant who believed they were squeezing a lemon rather than a throat meets it. 

The second asks whether the person knew the act was wrong, and California measures wrongfulness by both legal and moral standards, so a defendant who knew the act was illegal but, because of a delusion, believed it was morally required, can meet it. The test is cognitive. It asks what the defendant understood, not whether they could control themselves, and California does not recognize an irresistible-impulse or volitional defense. Cornell's Legal Information Institute summarizes the competing tests used elsewhere; the federal standard in 18 U.S.C. 17, which applies to federal charges, is similar to California's and requires clear and convincing proof.

What does not count: Penal Code 29.8 and the intoxication rule

The defense requires a mental disease or defect, and Penal Code 29.8 excludes three conditions from that category when they are the sole basis for the claim: a personality disorder or an adjustment disorder, a seizure disorder, and an addiction to or abuse of intoxicating substances. The intoxication exclusion is the one most often litigated. A psychosis that exists only while the person is under the influence of methamphetamine or a hallucinogen does not support the defense. A settled mental disorder, meaning one that persists after the intoxicating effects have worn off, can support it even if substance use triggered or worsened it, and the question whether a client's psychosis was transient or settled is frequently decided by the medical history before and after the offense. The defense also requires that the condition existed at the time of the act, so evidence of a disorder diagnosed later is relevant only insofar as it shows the person's state at that moment, and a client who was sane at the time of the offense but has since become ill raises a competency question, not an insanity one.

The plea, the experts, and the two trials

Insanity is pleaded, not simply argued. Under Penal Code 1026 a defendant who intends to rely on the defense enters a plea of not guilty by reason of insanity, alone or together with a plea of not guilty. When the court receives the plea it appoints two, or in some cases three, psychiatrists or psychologists under Penal Code 1027 to examine the defendant and report on sanity at the time of the offense; the defense and the prosecution may retain their own experts in addition. If the defendant pleads both not guilty and not guilty by reason of insanity, the trial is divided.

The guilt phase is tried first, with the defendant conclusively presumed sane, and the jury decides whether the defendant committed the offense with the required mental state. If the verdict is guilty, the same jury hears the sanity phase, in which the defendant bears the burden of proving insanity by a preponderance, and returns a verdict of sane or insane. A defendant who pleads only not guilty by reason of insanity admits the offense and proceeds directly to the sanity trial. The prosecution does not have to prove sanity; the defendant has to prove insanity, and that allocation, together with the expert reports, decides most cases. In practice a large share of successful insanity cases are resolved by stipulation, with the prosecution agreeing to the verdict on the strength of unanimous expert reports, rather than by a contested sanity trial.

What a not-guilty-by-reason-of-insanity verdict means

The verdict is an acquittal of criminal responsibility, not a release. Under Penal Code 1026 the court commits the person to the Department of State Hospitals, or, if the court finds it safe and the person has substantially recovered, to an outpatient program, and under Penal Code 1026.5 the commitment lasts up to the maximum state prison term the offense would have carried, calculated as if the person had been sentenced. For a felony that carries a life term, the commitment is for life. Before the maximum term expires, the district attorney may petition to extend the commitment for two years at a time on a showing, at a jury trial if the person demands one, that the person has a mental disorder that makes them a substantial danger of physical harm to others, and the extensions can be renewed indefinitely.

Release before the maximum term runs through Penal Code 1026.2: the person, or the hospital director, petitions for restoration of sanity, and the court first decides whether the person would be a danger if treated and supervised in the community, which ordinarily requires a year in the community outpatient program, CONREP, before the court will hold the final restoration hearing.

A person committed after an insanity verdict is also subject to a lifetime firearm prohibition. For a client facing a determinate sentence on a mid-level felony, the practical calculus can be that the hospital commitment, with extensions, is longer and less certain than the prison term, and our attorneys lay that out before any plea is entered.

What usually helps instead

Because the defense is narrow, the burden is on the defendant, and the commitment can outlast a sentence, our attorneys use three other tools far more often. The first is diminished actuality under Penal Code 28. California abolished the diminished capacity defense in 1982, but Penal Code 28(a) still allows evidence of a mental disease, defect, or disorder on the question of whether the defendant actually formed the specific mental state the charge requires, such as premeditation, deliberation, malice, or a specific intent. In a murder case, that evidence can reduce the offense to second degree or to manslaughter without any finding of insanity; our page on Penal Code 25 discusses the distinction.

The second is competency under Penal Code 1368, which asks a different question, whether the defendant can presently understand the proceedings and assist counsel, and which suspends the case for treatment rather than deciding guilt. The third, and the most important for the majority of clients, is mental health diversion under Penal Code 1001.36, which allows a court to divert a defendant with a qualifying disorder that was a significant factor in the offense into up to two years of treatment, with dismissal and sealing at the end, for most misdemeanors and felonies other than murder, voluntary manslaughter, and a short list of serious offenses. Our 2023 discussion of defense options for clients with mental illness covers how those tools fit together, and a client who was placed on a psychiatric hold around the time of the offense has, in the hold records, the beginning of the evidence for all three.

Frequently asked questions

What is the insanity defense?

A plea that the defendant should not be held criminally responsible because, at the time of the crime, a mental disease or defect left them unable to understand what they were doing or to know that it was wrong. In California the defense is set by Penal Code 25(b), which adopts the M'Naghten test: the defendant must prove, by a preponderance of the evidence, that they were incapable of knowing or understanding the nature and quality of the act, or incapable of distinguishing right from wrong, at the time of the offense. A successful plea results in a verdict of not guilty by reason of insanity and commitment to a state hospital rather than prison.

What is the M'Naghten rule?

The legal test for insanity that most American states, including California, use. It comes from an 1843 English case and asks whether, because of a mental disease, the defendant did not know the nature and quality of the act they were doing, or, if they did know it, did not know that it was wrong. California wrote the test into Penal Code 25(b) by ballot initiative in 1982 and requires the defendant to meet either prong. The test looks at cognition, what the person understood, not at whether the person could control their conduct; California does not recognize an irresistible-impulse defense.

What happens if you are found not guilty by reason of insanity?

The defendant is not released. Under Penal Code 1026 the court commits the person to a state hospital, or in some cases to outpatient treatment, for up to the maximum prison term the offense carries. The commitment can be extended in two-year increments for as long as the person remains dangerous because of a mental disorder, which for a serious violent offense can mean a longer confinement than the prison sentence would have been. Release comes through a restoration-of-sanity petition under Penal Code 1026.2, which ordinarily requires a year of successful outpatient treatment first.

Who has to prove insanity in California?

The defendant, by a preponderance of the evidence, meaning more likely than not. Insanity is an affirmative defense in California, and the presumption is that every defendant was sane. The prosecution does not have to prove sanity. That allocation is why the plea is rarely entered without strong expert support: the defense must affirmatively persuade the jury, with psychiatric testimony and records, that the defendant met the M'Naghten test at the moment of the offense.

Can you use the insanity defense for drug-induced psychosis?

Not when the psychosis was caused solely by the intoxication. Penal Code 29.8 bars the insanity defense when the condition is based solely on a personality or adjustment disorder, a seizure disorder, or an addiction to or abuse of intoxicating substances. A settled mental disorder that was triggered or worsened by substance use, and that persists independent of the intoxication, can still support the defense, and the line between a transient drug-induced state and a settled psychosis is one of the most contested issues in these cases.

How often does the insanity defense work?

Rarely. National studies have found that the defense is raised in roughly one percent of felony cases and succeeds in about a quarter of those, and most successful pleas are resolved by agreement between the prosecution and the defense on the basis of the expert reports rather than by a contested trial. In California the defense is used mainly in homicide and serious violent cases where the evidence of a severe psychotic disorder is strong, and for most clients with a mental health condition the more useful tools are mental health diversion, a competency evaluation, and evidence of the condition offered to negate the mental state the charge requires.

Related reading from our attorneys

• Penal Code 25 and mental health defenses
• Pretrial and mental health diversion under Penal Code 1001
• Murder under Penal Code 187: malice and the degrees

Mental state is the case. Let us evaluate it early

The Bulldog Law's criminal defense attorneys handle insanity pleas, diminished actuality defenses, competency proceedings, and mental health diversion across California, with the psychiatric experts retained at the start of the case rather than before trial. Call (888) 928-1609 or reach us online.