Insanity Defense

A defense in which a person can be found not guilty, or not responsible, for a crime because, at the time of the crime, the accused was unable to differentiate between right and wrong, based on the fact that the accused suffers from mental illness or mental defect.


The case of the so-called Crocodile Man brings to light questions about the limbic system in the brain, and the use of the insanity defense in court.

On July 17, 1974, a man named Charles Decker picked up two teenage girls who were hitchhiking in southern Massachusetts. The three of them spent the evening drinking, smoking marijuana, and driving around back roads. Suddenly, and seemingly without provocation, Decker attacked the girls with a stonemason's hammer, striking them so hard in the skulls that they later required brain surgery. They tried to escape and he continued striking the girls with rocks, choking one of them. He then seemed to “snap out of it,” dropped them in the parking lot of a store which was open, and called his father. He told his father that he didn't know what had happened but that he “went ape.” He said he had no memory of using the hammer on the girls, and only suspected this when he noticed it was missing from his vehicle. Charles appeared calm and remorseful when police arrested him.

Following his arrest Decker was evaluated at a State Hospital in order to determine whether he was competent to stand trial. In a legal sense this only means that one must be able to understand the charges and aid in defending against them. Insanity and competence are not mutually exclusive. After psychiatric examination, he was deemed competentto stand trial, having “no significant psychiatric disorder.”

Decker's father, an endocrinologist, still feltthere may be a biochemical explanation for his son's strange violent behavior. He contacted his colleague Mark Altschule, a physician at Harvard to inquire about the possibility of an organic cause for Charles' outburst. Based on the testimony of one of the victims, Gail Sussman, as well as Charles' testimony, Altschule believed that the incident seemed to indicate a malfunction of the limbic system. The limbic system is a very primitive part of the brain, responsible for initiating reactions such as flight, fright, and aggression. It makes up the majority of the brains of reptiles such as crocodiles, hence, Decker became known as “The Crocodile Man.”

The defense called Altschule as an expert witness. He argued thatthis unprovoked attackwas not in keeping with Decker's personality and that it was the result of a dysfunction or lesion of the limbic system.

The prosecution had their own expert witness who disputed these claims, saying that they were based on conjecture and not backed by evidence. It could not be ascertained whether Decker's actions were due to an abnormality of his limbic system unless an autopsy was performed.

The case was filled with ambiguity and there was a lack of medical consensus. Furthermore, Decker had some history of outbursts which undermined the claim that this act of violence was totally unprecedented. Decker had an unusual reaction to alcohol and it was found that he did have Korsakoff's syndrome, a condition usually, but not always associated with late-stage alcoholism which causes damage to the limbic system. In Charles’ case the cause was not alcoholism, but Altschule argued that it did lead to an alcohol metabolism abnormality which could lead to violence.

The court ruled that Decker's syndrome was proof of diminished mental capacity at the time of the attack. He was charged with attempted murder, but given a suspended sentence and probation (pending no alcohol use).

The case of the Crocodile Man is notable for the way in which the defense attorney utilized the insanity defense in relation to an impairment to the limbic system, resulting in violent impulses when he consumed alcohol. The prosecution maintained that Decker should have known the effects alcohol had on him, and that biochemical abnormalities in the brain should not necessarily be a viable defense for attempted murder. The case called into question many issues of criminal responsibility and the possible biochemical basis for some violent behavior.

It is important to note that “insanity” is a legal term, not a psychological one, and experts disagree whether it has valid psychological meaning. Critics of NGRI have claimed that too many sane defendants use NGRI to escape justice; that the state of psychological knowledge encourages expensive “dueling expert” contests that juries are unlikely to understand; and that, in practice, the defense unfairly excludes some defendants. While research on NGRI fails to support most of these claims, some serious problems may still exist.

Insanity defense statistics

Successful NGRI defenses are rare. While rates vary from state to state, on average less than one defendant in 100—0.85 percent—actually raises the insanity defense nationwide. Interestingly, states with higher rates of NGRI defenses tend to have lower success rates for NGRI defenses; the percentage of all defendants found NGRI is fairly constant, at around 0.26 percent.

In some studies, as many as 70 percent of NGRI defendants withdrew their plea when a state-appointed expert found them to be legally sane. In most of the rest, the state did not contest the NGRI claim, the defendant was declared incompetent to stand trial, or charges were dropped. High-profile NGRI cases involving rich defendants with teams of experts may grab headlines and inflame the debate, but they are very rare.

Problems with NGRI

Some problems, however, have emerged with NGRI. Regulation concerning who can testify as to the sanity of a defendant is very inconsistent from state to state. According to one national survey, only about 60 percent of states required an expert witness in NGRI determinations be a psychiatrist or psychologist; less than 20 percent required additional certification of some sort; and only 12 percent required a test. So the quality of expert witnesses may vary from state to state.

The quality of post-NGRI psychiatric treatment may be another problem. Treatment varies from state to state in both duration and, some say, quality; some defendants spend more time in mental institutions than they would have spent in jail had they been convicted, some less. NGRI defendants tend to spend more time in institutions than patients with similar diagnoses who were not accused of a crime, which undercuts somewhat the argument that treatment, not punishment, is the goal.

In terms of preventing repeat offenses, psychiatric treatment seems to help. Some studies suggest high post-treatment arrest rates, but these arrests tended to be for less serious crimes. At least one study indicated that average time to arrest of these patients after release is no higher than for the general population.

Mock jury studies indicate that jurors do carefully consider and discuss many factors in an insanity defense, but may be ignoring the local legal definitions of insanity. Mock juries tended to render the most NGRI verdicts when the defendant showed a lack of both ability to understand and ability to resist committing the crime, even though no state requires both and some consider ability to resist to be irrelevant. In addition, personal feelings about the legitimacy of the insanity defense may influence jurors’ decisions.

One of the most devastating arguments against NGRI is that it may unfairly exclude many defendants. Studies suggest high rates of psychiatric illness in the general prison population. Many mentally ill defendants never get a chance to plead NGRI; some obviously psychotic defendants fight to prevent their attorneys from mounting an insanity defense for them.

The unwillingness of many states to accept an actus rea defense bothers some experts. Biochemical studies indicate that some people have biochemical abnormalities that may make them unable to control their impulses. If this is true, and the research is not altogether clear on this point, these people cannot voluntarily conform to the law, and therefore they have grounds for NGRI. On the other hand, a huge proportion of the prison population may suffer from varying degrees of such a mental defect—and finding them all NGRI would probably be dangerous to society as well as not viable.

Guilty but mentally ill

As an alternative to NGRI, some states have added a third possible verdict to the usual trio of guilty, not guilty, and NGRI—the verdict of “guilty but mentally ill” (GBMI). In theory, this recognizes when a defendant's mental illness played an important role in a crime without entirely causing it. The state incarcerates the defendant for the crime, but also treats him or her for the mental illness.

Unfortunately, states with GBMI verdicts have sometimes neglected to provide for treatment; therefore many of these defendants are jailed without treatment, exactly as if they had been found guilty. Another dilemma with the GBMI verdict may be an “easy out” for jurors. If a jury finds the defendant guilty, they may not spend time worrying about whether he or she may be sane; because they find the defendant mentally ill, they may not address the fact that the defendant should actually be found NGRI. Hence, the insanity defense “problem” will not yield to easy solutions.



Bartol, Curt, and Anne Bartol. Introduction to Forensic Psychology, 4th edition. Los Angeles: Sage, 2015.

Bartol, Curt, and Anne Bartol. Psychology and Law: Research and Practice. Los Angeles: Sage, 2015.

Costanzo, Mark, and Daniel Krauss. Forensic and Legal Psychology: Psychological Science Applied to Law, 2nd edition. New York: Worth, 2015.

Prins, Herschel. Offenders, Deviants, or Patients?: An Introduction to Clinical Criminology, 5th edition. New York: Routledge, 2016.


American Board of Forensic Psychology, .

International Association for Correctional and Forensic Psychology, 897 Oak Park Boulevard, #124, Pismo Beach, CA, 93449, (910) 799-9107,, .