The M’Naghten rule established a modern standard for determining insanity that is still used in many states in the U.S. This rule established the precedent for the “not guilty by reasons of insanity” (NGRI) verdict, which usually leads to the offender receiving medical treatment for his crime instead of imprisonment. In the 1843 trial which established this rule, the accused, Daniel M’Naghten, was attempting to assassinate the British Prime Minister but killed the Minister’s assistant instead, under the delusional belief that the assistant was actually the Prime Minster. During the trial, M’Naghten was found not guilty by reason of insanity.

According to the criteria established by the M’Naghten trial, the defense must prove that at the time of the criminal act the offender was suffering from a mental illness. Specifically, the defense must prove that as a result of the mental illness the offender did not completely understand the nature of the criminal act, or that he was not able to tell the difference between right and wrong. Thus the M’Naghten rule became known as the “right from wrong” test.

There have been many criticisms made against using this as a standard of insanity. First, the legal definition of mental illness as established by this case is not made clear. Second, it is also not clear how it can be determined whether the offender really didn’t know the difference between right and wrong. Finally, as many critics have pointed out, some mentally insane people may still know the difference between right and wrong but nonetheless cannot control their behavior.