What Is an Insanity Plea?

Crime television shows and movies sometimes portray a legal defense often referred to as the ‘insanity plea.’ What is an insanity plea, and how does it work? Keep reading to find out.

The Insanity Defense Defined

The insanity defense is a criminal defense strategy where the defendant may plead innocent by reason of insanity. When the defendant makes an insanity plea, they still acknowledge that they were involved but were not of sound mind at the time of the crime.

The insanity defense is sometimes confused with diminished capacity – a defense where the defendant is pleading to a lesser crime instead of removing themselves from the criminal act entirely. Unlike diminished capacity, the insanity plea is a full defense against the crime, meaning the accused is pleading not guilty.

This defense also falls under the excuse defense category instead of a justification defense. Generally, an excuse defense utilizes an excuse to remove the defendant from liability for the crime but still acknowledges that the result of their actions was negative. In other words, the defendant was not responsible for their actions, which resulted in harm to others.

How To Prove “Insanity”

Insanity Under the Law

In order to make an insanity defense work, the defense has to prove that their client, the defendant, was “insane” at the time of the crime. In the legal realm, “insane” does not necessarily mean the same thing as what we might think of when hearing the term insanity.

Under the law, if someone is not guilty by reason of insanity, they may not have been in their right mind when the crime was committed. In some cases, this defense may refer to a severe episode of mental illness, but not always. A defendant may be considered insane if they were pushed into criminal activity by political ideology or acting out after a traumatic event.


The most crucial factor in the insanity defense is competency. In a criminal case, the defense must prove that their client is legally incompetent. This means that the accused is incapable of rational communication with their attorney about the crime and unable to comprehend the nature of the crime they are accused of committing.

If the judge allows an insanity defense to stand, there may be a competency hearing to determine whether the defendant is legally incompetent. During the hearing, the defense has to provide evidence that their client is legally incompetent in addition to a psychological evaluation.

Historically, the threshold for proving competency is low, meaning as long as the defendant is considered incompetent, they cannot stand trial. The threshold for competency is low, but the likelihood of the court accepting an insanity plea is even lower.

In most cases, the insanity defense is not the best course of action because it is difficult to prove, and the defense strategy may not convince the jury. It’s also important to note that because the court system is overburdened, scheduling additional hearings is not preferable.

Types of Insanity

In the legal system, case results can become precedent or the standard for specific situations. There are several precedents for the insanity defense that affect how the court handles these cases and the outcome if the defendant is found legally incompetent.

The M’Naghten Rule

The M’Naghten Rule applies to the jury when hearing medical testimony provided by the defense. Under this rule, the jury must presume the defendant is sane unless proven to be otherwise.

This rule also directly applies to the defendant’s cognition. Cognition is different from mental capacity in that it is the human ability for reason and problem-solving. Mental capacity is the ability or inability to recall events or communicate decisions.

The M’Naghten Rule establishes a framework for proving cognition. There are two components: the defendant’s ability to know that what they were doing was a crime and whether the defendant understands that what they did was wrong.

So, even if the defendant is aware of the criminal nature of their actions, they can still be found incompetent if they do not understand why their actions were wrong objectively. The M’Naghten Rule does not apply to more complicated mental illnesses or volitional impairment.

The Irresistible Impulse Test

Another measure for insanity is the Irresistible Impulse test. As the name would suggest, this test exists to prove that the defendant committed the crime because of an irresistible urge or uncontrollable compulsion.

This form of the insanity defense is unique because it makes regular convictions unethical under the rules of criminal justice. If a person commits a crime because they are incapable of controlling themselves, they lack criminal intent and are simply constricted by an impulse.

Proving an irresistible impulse is incredibly difficult because there is little to no scientific measure of impulse control that applies to these cases. It can also be too broad of a stroke for the court to accept.

The Durham Rule, or Product Test

The now primarily defunct Durham Rule focuses on the idea that the accused is not criminally responsible if the crime directly resulted from mental illness or defect. This rule relies on scientific psychological evaluations and medical evidence rather than situational evidence.

The Durham Rule became unacceptable when the courts realized that at the time, the science of psychology was unrestricted and more or less up to the opinion of the practitioner. In other words, there were no standards or significant research/evidence to back psychology at the time.

Since the Durham Rule fell out of favor in the early 1970s, most states have rejected its use in court. Now, only New Hampshire maintains the rule as a feasible test in court.

The Model Penal Code

To establish a more rigid definition of insanity, legal specialists introduced the Model Penal Code in 1972. Under this rule, a person cannot be guilty of a crime if they cannot understand criminality or are unable to conform to the law. This rule’s main focus is the moral failing of a person suffering from severe mental illness or other impairments.

Comprehensive Crime Control Act

By 1984, the insanity defense had become overcomplicated, so lawmakers introduced the Comprehensive Crime Control Act. Since then, the law requires the defense to prove that the defendant was incapable of understanding their actions as a direct result of mental illness.

Unlike previous precedents, the Comprehensive Crime Control Act eliminated many grey areas that would allow the defense to use an insanity plea as a strategy to avoid conviction. Instead, defense attorneys are held to a higher standard of proof. They must prove beyond a reasonable doubt that their client has a severe mental illness that makes them incapable of acting reasonably.


The insanity defense is very difficult to prove, so many courts do not accept it during a criminal trial. It can still apply to some cases, but the burden of proof is more than a regular trial. There have been many precedents and legal changes to the law regarding insanity that have made it more inclusive and more restrictive.

Want to know more about how the law applies to your case? Contact the Hester Law Group today.