M'Naghten Rule vs. Model Penal Code: Understanding Criminal Law

One of the most complex and misunderstood parts of criminal law is what is meant by legal insanity during a criminal act. Mental disease and illness can play a role in criminal behavior. However, not every mental illness meets the legal definition of insanity. In the US, including in California, courts use specific legal standards to determine legal insanity. Two of the most common standards are the M'Naghten Rule and the Model Penal Code test, or MPC test.

Both of these standards address criminal responsibility and mental illness, but they are very different in how they define insanity, prove it, and apply it to the criminal justice system.

Understanding the difference between the M'Naghten Rule and the MPC test is important for anyone facing criminal charges due to being mentally ill, either permanently or temporarily. Goss Law can advise on who decides the sanity of a defendant in CA.

What Is the M'Naghten Rule in Determining Legal Insanity?

The M'Naghten Rule is one of the oldest tests for legal insanity and is one of the most widely used in the US. It actually originated in England in 1843 during the case of Daniel M'Naghten. He was accused of murdering a government official, but he also suffered from severe mental illness, including delusions.

Under the M'Naghten Rule, a criminal defendant is considered legally insane only if, during the time of the offense, they had a relevant mental defect that caused them to A) not understand the nature of the unlawful act, or B) did not understand that their criminal conduct was legally or morally wrong.

The M'Naghten Rule is almost entirely focused on the person's cognitive awareness and if they knew what they were doing and knew it was wrong.

In California, there is a modified version of the M'Naghten Rule, which makes it difficult to prove insanity, but not impossible.

Key Characteristics of the M'Naghten Rule

The M'Naghten Rule has several parts, which makes it particularly restrictive:

• It does not consider a defendant’s ability to control their behavior

• It excludes emotional disturbance, impulse control issues, or poor judgment

• It requires proof of a qualifying mental disease or defect

• It places the burden of proof on the defendant

When applying this, a defendant who understood that they were doing something wrong, but couldn't stop such conduct due to a mental illness, is still considered legally sane. This is one of the biggest criticisms of the M'Naghten Rule.

What Is the Model Penal Code Test?

The Model Penal Code is a test that was developed by the American Law Institute in the mid-20th century. It is meant to modernize and standardize criminal law. This test is much broader and more flexible than the M'Naghten test.

Under MPC, a defendant is not criminally responsible if, at the time of the crime, they lacked the mental capacity to:

• Understand that their actions was criminal or wrong, OR

• Conform their conduct to the requirements of the law

This means that both cognitive and volitional elements are at play. It recognizes that may impair both understanding and behavior. Some great criminal defense attorneys in Sacramento may be able to help advise more on these tests.

Key Characteristics of the Model Penal Code Standard

The MPC approach differs from M'Naghten in several important ways:

• It allows partial impairment rather than requiring total incapacity

• It recognizes lack of impulse control as a factor

• It uses “substantial capacity” rather than absolute inability

• It gives courts greater flexibility in evaluating psychiatric evidence

Because of this broader scope, the MPC test is often viewed as more consistent with modern psychiatric understanding.

Major Differences Between the Two Standards

The most significant difference between the M'Naghten Rule and the Model Penal Code lies in how they define responsibility.

The M'Naghten Rule asks a narrow question: Did the defendant know what they were doing and know it was wrong? If the answer is yes, the defense fails.

The Model Penal Code asks a broader question: Did mental illness substantially impair the defendant’s ability to understand or control their actions?

Another key difference is how each standard treats impulse control. Under M'Naghten, a defendant who knew an act was wrong but felt compelled to do it is still criminally responsible. Under the MPC, that same defendant may qualify for an insanity defense if they could not conform their behavior to the law.

Which Standard Does California Use?

California uses the M'Naghten Rule, not the Model Penal Code. This means that defendants in California must meet a very high bar to succeed with an insanity defense.

Mental illness alone is not enough. Mental health conditions such as depression, anxiety, PTSD, or personality disorders typically do not qualify unless they completely impair the defendant’s ability to understand the nature or wrongfulness of their actions at the time of the offense.

This also means that time spent in a mental health facility or in a mental hospital don't always help to prove a defendant's mental state at the time of the criminal act.

California also places the burden of proof on the defendant, who must establish insanity by clear and convincing evidence, a demanding legal standard for criminal proceedings.

Why the Model Penal Code Is Not Used in California

California lawmakers have historically favored a strict approach to criminal responsibility. Critics of the MPC argue that it is too lenient and gives juries excessive discretion. Supporters of M'Naghten believe it provides clearer boundaries and prevents abuse of the insanity defense.

High-profile cases in the 20th century led many states, including California, to reaffirm narrow insanity standards. Because of this, the Model Penal Code and other tests, like the Irresistible Impulse Test and the Durham Rule, have been rejected in California criminal courts.

Mental Competence and the Practical Impact on Criminal Defense Cases

The difference between these two standards has major consequences for those facing charges. In MPC jurisdictions, a wider range of mental health evidence may lead to an insanity defense. In M'Naghten jurisdictions like California, a defense like this must be carefully considered. If it doesn't work, it could backfire. Though the criminal prosecution must prove beyond a reasonable doubt, using this plea could damage the case.

Criminal Insanity Defense Guidance from Goss Law

An established insanity defense must adhere to complex legal standards, psychiatric evaluations, and procedural requirements. California’s reliance on the M'Naghten Rule makes these cases especially challenging during a criminal trial, but an insanity verdict is not impossible when someone has mental disorders and wasn't aware of his or her conduct.

Goss Law represents clients throughout Sacramento and surrounding areas in serious criminal matters involving mental health issues. They have insight on the burden of proof in insanity cases in CA. When questions of insanity, mental defect, or criminal responsibility arise, careful legal analysis and strategic planning are essential.

Anyone facing criminal charges where mental health may be an issue should seek experienced criminal defense representation to evaluate all available options under California law. Goss Law is available 24/7. Reach out now for a free case evaluation.