An insanity defense hinges on proving that you did not have the ability to understand what you were doing it the consequences of your actions when committing a crime. Insanity is a type of guilty plea by which you say you did the crime but there were extenuating circumstances that make you not liable for the crime.
According to NPR, a ruling from the U.S. Supreme Court could open the door for every state to remove the insanity option from criminal cases.
The basic opinion of the court is that the brain is too complex to understand how it works normally. Trying to understand how it works with mental illness present is even more difficult. Even experts do not understand the role that mental illness plays in culpability for a crime.
Much of the rulings on the insanity defense have a foundation in opinion and not expert knowledge of the subject.
Most states do allow for you to plead insanity, and New Jersey is one of those states. However, with the changing knowledge about mental illness and this Supreme Court ruling, there is a potential for states to start changing the rules about using it.
The ruling allows states to restrict the insanity defense. In Kansas, for example, it is only relevant during the sentencing phase of a trial. The case at the heart of the ruling is for an individual from Kansas that wanted the court to state the defense must be usable in every phase of a criminal trial. However, the ruling struck that down, giving other states the chance to alter their laws and prevent you from claiming insanity as a defense against criminal charges.