“Not guilty by reason of insanity.”
These are controversial words in a court of law, when a defendant who—by their own admission—committed the crime in question is released on the grounds they lacked the wherewithal to understand their actions.
This verdict was most famously reached in the case of John Hinckley, who attempted to assassinate President Ronald Reagan in March 1981.
What does it really take to escape with an insanity defense plea?
It’s not easy.
The first thing to understand about the insanity defense is that it shifts the burden of proof.
The concept of assumed innocence given to the defendant is basic to the American criminal justice system.
It doesn’t apply with an insanity defense.
The defendant has acknowledged guilt and assumed the burden of proof beyond a reasonable doubt.
Whether or not the defendant fulfilled that burden is determined by The M’Naghten Test.
This dates back to 19th century England when Daniel M’Naghten believed the Prime Minister was leading a conspiracy against him and shot the Prime Minister’s secretary.
While M’Naghten would spend the rest of his days institutionalized, public anger over his acquittal forced a tightening of standards.
Other legal benchmarks in an insanity plea
The Irresistible Impulse Test refers to defendants who may suffer from manias or phobias that leave them temporarily incapable of grasping what they were doing.
A related test is The Modern Penal Code, which also looks for reasons a defendant may have been mentally incapable of resisting their action, even if they had the cognitive presence of mind to know it was against the law.
An important note is that if one is diagnosed as a sociopath or psychopath, they cannot use this defense.
The definition of insanity defense is ultimately difficult to pin down and leaves juries dependent on expert witness testimony.
But it’s still not an easy benchmark to reach for the defense.