What Is the M’Naghten Rule For Insanity Defense?

Not many people have heard of the M’Naghten rule, but you may have heard of its other name, the insanity defense. This legal principle prevents the mentally ill from being held legally accountable for their actions if they did not know that what they were doing was wrong and illegal. The M’Naghten rule has an interesting history. Read on to find out more about the insanity defense.

When Was the Insanity Defense First Used?

The first use of the insanity defense was in 1843 in England. A person thought that the Prime Minister wanted to kill him, so he tried to kill the Prime Minister first but accidentally killed the Prime Minister’s secretary instead. The defendant’s name was Daniel M’Naghten, hence why the insanity defense is called the M’Naghten rule. At the trial, medical experts examined M’Naghten and found that he was psychotic and had no idea of his actions and the legal and moral wrong that he had done.

The first use of the M’Naghten rule was really controversial in the UK. The public had never heard of the insanity defense and did not believe that it should excuse M’Naghten from accountability for his crimes. In order to appease the public and prevent a dangerous precedent from being set, the House of Lords ordered the court to create a legal definition of insanity. Thus, the M’Naghten rule was created, and all defendants using the insanity defense must meet the criteria. The criteria for the insanity defense is slightly different in the UK than it is in USA, so we will only discuss the American M’Naghten Rule.

How to Use the M’Naghten Rule in the US

After the US adopted the M’Naghten rule, they modified the definition that must be used to determine if the insanity defense is applicable. Each jurisdiction has slight variations on the definition of criminally insane, but in general, they are as follows.

In order to use the M’Naghten rule insanity defense, the defendant must not understand the moral and legal implications of their actions. Most states do not allow those who claim volitional insanity to plead the insanity defense using the M’Naghten rule. Volitional insanity is where the person knows their actions were wrong, but their mental state made them commit their actions anyway.