Is insanity a defense to murder?

by | Jul 13, 2017 | blog

As the investigation continues into the disappearance of four young men, the public might wonder about possible defenses for whoever is charged with the crime. The U.S. Constitution guarantees certain rights to every person accused of criminal activity.

One of those guarantees is the right to a defense attorney and the right not to incriminate yourself–meaning you are never required to testify at the trial even if you are the defendant. But what happens when a case might contain very damaging evidence such as DNA or eye-witness testimony? What defense can you mount when chemical analysis presumes you were definitively at the scene of the crime?

The M’Naghten Rule

Each state has a different test for determining whether a defendant is legally insane at the time a crime is committed. The Commonwealth of Pennsylvania uses the M’Naghten rule to ascertain insanity and the burden of proof is placed on the defendant.

The M’Naghten Rule is actually based in old English law and was codified by the English House of Lords in the mid-1800s. In a nutshell, the rule asks the jury to determine whether, during the commission of the crime, the accused understood both a./what he had done (i.e. murdered someone) and b./that his actions were wrong.

This is a two-pronged test. For example, in order to use the M’Naghten Rule as a defense, it would have to be proven that the accused not only knew he had shot someone, but that, in fact, shooting someone is wrong.

The M’Naghten Rule places the burden of proof on the defendant, presuming, according to Black’s Law Dictionary, that a person is sane, and therefore responsible for his actions unless: “…At the time of committing the act, he was laboring under such a defect of reason from disease of the mind as not to know the nature and quality of the act he was doing, or if he did know it that he did not know he was doing what was wrong.”

What can keep someone from understanding it is wrong?

While most of us have no problem understanding that killing someone is wrong, someone who is suffering from mental illness may not. Some diagnoses used to prove incapacity–and a concomitant lack of sanity–include paranoid schizophrenia, post traumatic stress disorder, and psychosis.