Insanity is neither a medical term nor a psychiatric diagnosis; it has a speciﬁc meaning only in the legal arena. In U.S. courtrooms where a defendant may plead “not guilty by reason of insanity,” or NGRI, insanity is deﬁned as a state of mind in which the defendants were either not aware of the nature of what they were doing, or they did not know that the action was wrong. Technically this determination is referred to as Mental State at the time of the Offense, or MSO.
The deﬁnition of insanity given above is based on what is known as the M’Naughten rule, which derives its name from a case heard in England in 1843. M’Naughten, the defendant, suffered paranoid delusions, including a conspiracy against himself involving the prime minister. M’Naughten, believing himself to be acting in self-defense, went out with a gun to preemptively strike against the conspirators, but he accidentally shot the prime minister’s secretary instead. This was the ﬁrst case in history in which a plea of not guilty by reason of insanity was successfully used.
In the United States, only half of the states still follow the M’Naughten rule, and some have attached to it the idea of “irresistible impulse” as an additional condition of insanity. This clause is not often invoked, since even the best attorneys experience some difﬁculty explaining the subtle distinction between an impulse that was “irresistible” and one that was simply “unresisted.”
The M’Naughten rule was supplanted in many jurisdictions in 1954 by the Durham rule, which was established by the District of Columbia Federal Court of Appeals. According to the Durham rule, the insanity defense can be used if the criminal act was a product of a mental disease or defect. Proving this turned out to be just as challenging as it sounds, and both the M’Naughten rule and the Durham rule were reﬁned further by the 1972 Model Penal Code rule, now used by twenty U.S. states, which declares that “a person is not responsible for his conduct if at the time of such conduct, as a result of mental disease or defect, he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law.” This differs from the M’Naughten rule in that the words “substantial capacity” and “appreciate” add an emotional, volitional side to the whole thing. A person can understand cognitively that an act is wrong and still be emotionally unable to appreciate its criminality.
The newest wrinkle in insanity defenses is the verdict of “guilty but mentally ill” (GBMI). About twelve states allow this verdict when an insanity plea has been entered. This means the defendant receives the same sentence that would accompany a guilty verdict, but time is served initially in a psychiatric hospital or similar setting. Post-treatment time is served in prison. It was a popular idea at ﬁrst, but hasn’t caught on very widely because juries ﬁnd it very confusing to be asked to distinguish between mental illness that led to insanity and mental illness that didn’t. The scarcity of GBMI verdicts may actually be a good thing, as research shows that offenders who succeed in getting a GBMI verdict only rarely receive adequate treatment and are often actually given harsher sentences than those who are simply found guilty. Some defendants have even received GBMI death sentences. Following treatment in a hospital setting, they have death row to look forward to.
Another legal issue that often involves psychologists is the determination of competency to stand trial. This is deﬁned as the defendant’s capacity to understand the criminal process and to consult with an attorney in preparation of a defense. Unlike an insanity defense, this involves present functioning rather than the mental state at the time of the offense. IQ tests play an important role in this process, because most courts will rule that a person with mental retardation at the moderate or more severe levels is incompetent to stand trial. For other types of incompetence, trials can be delayed while defendants undergo treatment. When they have recovered sufﬁciently to be found competent, the trial proceeds.
In recent years the insanity plea has become a controversial topic among politicians, who sometimes argue against it to appear tough on crime. Typically an emotional appeal is made to the voters based on the idea that the plea is being used too often, allowing murderers to walk the streets who should be in prison. Such appeals to the voters are often successful, and several states have now abolished the insanity defense. All too often missing in the political debate are statistics on the actual incidence of NGRI verdicts. Insanity is actually invoked as a complicating factor in fewer than 1 in 1,000 of all trials for violent crimes, and the NGRI plea is actually successful in less than one quarter of those. Also contrary to popular belief, recidivism rates among those found not guilty by reason of insanity are lower than among those actually convicted of similar crimes, suggesting that those defendants did receive the correct verdict.
- Walker, L. E. A., and Shapiro, D. L. Introduction to Forensic Psychology: Clinical and Social Psychological Perspectives. New York: Plenum, 2003.