A defendant must be acquitted of a crime if, at the time he committed the crime, he was legally insane. An acquittal in these situations means that the defendant will have been found “not guilty by reason of insanity.” When analyzing the possibility of an acquittal based on an insanity defense, the two issues that factor into the equation are 1) whether, at the time he committed the crime, the defendant had an impairment that is severe enough to be considered legal insanity and 2) if so, whether that impairment affected the defendant to the point that an acquittal based on insanity grounds is legally warranted. Both elements must be present at the time the defendant committed the crime in order for him to be able to successfully use an insanity defense.
Although there are a number of different variations of insanity, which we will discuss later in this section, all of them require that the defendant have some sort of mental impairment. However, it should be noted that not all mental impairments can form the basis for an insanity defense. Traditional mental diseases like psychosis and schizophrenia can theoretically be the basis for an insanity defense. In addition, mental retardation can also be the basis for an insanity defense. However, psychopathic behavior, which the Model Penal Code refers to as an abnormality manifested only by repeated criminal or otherwise anti-social conduct, cannot be the basis for an insanity defense.
Finally, involuntary intoxication can also form the basis of an insanity defense if the intoxication produces the necessary effect on the defendant’s mind. See
The general rule is that a mental impairment, by itself, is not enough to sustain an acquittal based on an insanity defense. A defendant is not guaranteed an acquittal simply because he has a mental impairment. Rather, the impairment must also have created a certain effect on the defendant’s mental condition at the time he committed the crime. The various insanity tests take into account two different types of mental impairment: the cognitive impairment and the volitional impairment. The cognitive impairment centers on the defendant’s ability to perceive reality and to behave accordingly. In other words, the cognitive impairment centers on the defendant’s basic understanding of right and wrong. The volitional impairment, on the other hand, centers on the person’s ability to control his behavior and to avoid committing acts that he knows are wrong.
The difference between cognitive impairments and volitional impairments is at the center of the issue that divides jurisdictions today. Some jurisdictions hold that the legal standard for insanity should be based solely on the cognitive impairments that a defendant might have while other jurisdictions believe that the insanity defense based on the defendant’s volitional impairments should also be allowed. In other words, some jurisdictions believe that an insanity defense should only be permitted where the defendant’s mental impairment rendered him unable to distinguish right from wrong, while other jurisdictions feel that the insanity defense should also be allowed where the defendant’s illness prevented him from being able to avoid committing acts that he knew were wrong.
The Insanity Tests
The original insanity test is known as the M’Naghten Rule, and it arose from the British courts in the mid 1800s. Under the M’Naghten Rule, the defendant must be found not guilty by reason of insanity if, due to his mental impairment, he either did not know the nature and quality of his criminal act, or, he did not know that the act was wrong at the time he committed it. Please note that the M’Naghten rule, which centers on a defendant’s understanding of right and wrong, is a cognitive test.
Most courts hold that a defendant does not know the nature and quality of his act if he does not understand the consequences of his actions. For example:
Fred shoots Barney with a gun. However, because of a mental illness that Fred has, he does not understand that shooting someone with a gun can hurt or kill the person. Because Fred does not understand the consequences of his actions, he is considered to not know the nature and quality of his act.
Most jurisdictions define the term “wrong” as legally wrong. In these jurisdictions, if Fred shoots Barney with a gun, Fred will be found not guilty by reason of insanity if his mental illness caused him to believe that shooting Barney was legally permissible.
The second insanity test is called the "Irresistible Impulse test." The Irresistible Impulse test dictates that a defendant should be found not guilty by reason of insanity if, because of a mental impairment, he was unable to control himself and avoid committing the illegal act. See
The third test is the one codified by the Model Penal Code and it says that a defendant should be found not guilty by reason of insanity if, because of a mental impairment, he was unable to either 1) “appreciate the criminality of his conduct” or to 2) “conform his conduct to the requirements of law.” If you look at the model penal code test carefully you will see that it has taken the cognitive elements from the M’Naghten Rule and the volitional elements from the Irresistible Impulse test and combined them into one larger test.
The fourth test is called the "Durham" Rule. It says that a defendant must be found not guilty by reason of insanity if the crime was the product of a mental impairment or illness that the defendant had at the time. See
When a defendant is tried for committing a crime, there is a general presumption of sanity. Under the old rules, the burden was on the defendant to, at the very least, present some evidence indicating that he was insane at the time he committed the crime. Once the defendant did that, the burden then shifted to the prosecution to prove, beyond a reasonable doubt, that the defendant was, in fact, sane at the time he committed the crime. However, in 1981, after John Hinckley was found not guilty by reason of insanity in the attempted assassination of President Ronald Reagan, a trend developed requiring the defendant to prove, by clear and convincing evidence, that he was, in fact, insane when he committed the crime.
Typically, if a defendant is tried for a crime and found guilty, he is sent to prison. If he is found not guilty, he goes home. However, since a defendant who is acquitted on insanity grounds may still pose a risk to society, any defendant that is found not guilty by reason of insanity is automatically committed to a hospital for a certain period of time, which ends only when the treating doctors opine that the defendant is ready to rejoin society. Please note that if the defendant’s metal impairment is severe enough or is not cured, he can be forced to remain in the hospital for a period of time longer than his jail sentence would have been had be been convicted. For example: