The issue of criminal responsibility, also known as insanity, has to do with a defendant’s mental state at the time of the alleged offense. The basic philosophy is that to convict a person charged with a crime he or she must be considered responsible for his or her behavior. For most crimes there are two elements that must be proved by the prosecution in order for a defendant to be found guilty: the actus reus or physical act of the crime and the mens rea or mental capacity and intention to commit the crime. Most often the issue of criminal responsibility or insanity has to do with the second element that must be proved—the mens rea. As a general statement, defendants for whom mental disease or defect has resulted in an inability to form the intention to commit a crime or has resulted in an inability to know right from wrong may be considered not criminally responsible for their actions and thus found not guilty of the crime for which they were charged. This article describes the various standards for criminal responsibility in the United States and provides information on the public’s perception of the insanity defense.

Legal Standards for Criminal Responsibility

In the United States, several varieties of insanity defense standards have been used across various jurisdictions and at various points in time. Currently, the two most common insanity defense standards used are the American Law Institute’s (ALI) formulation and a slightly restricted version of the traditional M’Naghten test. Each of these, as well as various other standards, is described below.


The oldest test of legal insanity stems from the case of Daniel M’Naghten, which took place in England in the early 1840s. In this case the court established what has come to be known as the M’Naghten standard; also called the knowledge/right-wrong test of insanity. This standard requires that, in order to establish a defense on the grounds of insanity,

it must be clearly proved that, at the time of the committing of the act, the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing; or, if he did know it, that he did not know he was doing what was wrong. (M’Naghten’s Case, 1843, p. 722)

Thus, the M’Naghten test requires that: (1) the defendant be suffering from a disease of the mind (interpreted as being a mental disorder), which causes impairment in terms of either, (2) not understanding the nature and quality of his or her actions, or (3) not knowing that those actions were wrong. The careful reader will note that there is a link between the mental disorder of the accused and the impairment in either knowledge of his or her actions (he or she did not know what s/he was doing) or in knowing right from wrong (he or she did not know that what s/he was doing was wrong) such that the mental disorder must be directly related to one of these impairments.

This test has often been criticized as being either too narrow or too rigid and, as a result, other tests have been proposed including, the irresistible impulse standard, the Durham rule, the American Law Institute standard, and the Insanity Defense Reform Act standard.

Irresistible impulse standard

In response to criticisms that the M’Naghten test was too cognitive (recall that this test focused on whether the defendant knew what he or she was doing or knew that what he or she was doing was wrong), an irresistible impulse clause was added in order to include a volitional component to the insanity test. That is, an accused might be considered insane if he or she knew what he or she was doing and knew that it was wrong but was unable to control his or her behavior and avoid performing the action.

Durham rule

The Durham rule, which has also been called the product rule, was used in the federal system for a period of time in the 1950s and 1960s and is currently only used in New Hampshire. The Durham rule was adopted in response to criticism of the M’Naghten test as being too narrow and too cognitive in focus, even with the additional of an irresistible impulse clause. Thus, the Durham or product rule established that an accused could be found not criminally responsible if his or her actions were the product of a mental disease or defect. Obviously, this significantly broadened the standard for insanity, which brought with it a host of additional problems and the Durham rule was replaced in federal court by the American Law Institute standard.

American Law Institute (ALI) standard

The ALI standard, also called the Brawner rule, was developed in an attempt to resolve the issues of the Durham rule being too broad in nature and the M’Naughten test being too cognitive in nature. The ALI standard established that an accused is not criminally responsible if,

as a result of mental disease or defect, he [or she] lacks substantial capacity either to appreciate the criminality [wrongfulness] of his [or her] conduct or to conform his or her conduct to the requirements of the law. (American Law Institute, 1962, p. 401)

Thus, the ALI standard includes both a cognitive and a volitional component and appears to be more comprehensive than either the M’Naghten test or the irresistible impulse test while being less broad than the Durham or product rule. The ALI standard is currently used in about half of the United States.

Insanity Defense Reform Act (IDRA)

In 1984 the Insanity Defense Reform Act was passed. The insanity standard included in this act indicated that for a defendant to be found not criminally responsible he or she needed to prove that “as a result of a severe mental disease or defect, he [or she] was unable to appreciate the nature and quality or wrongfulness of his [or her] act” (p. 201). In effect, the IDRA standard represents a slightly restricted version of the original M’Naghten standard, which includes only a cognitive prong, and eliminates the volitional prong of the ALI standard. The IDRA standard is used in federal courts and slightly restricted versions of the traditional M’Naghten test, such as the IDRA standard, are used in about half of the United States.

Guilty But Mentally Ill (GBMI)

Since the much-publicized trial of John W. Hinckley, Jr., who was found Not Guilty by Reason of Insanity (NGRI) after his attempt to kill President Regan in 1981, a great deal of court reform and legislative revision with regard to the insanity defense has occurred. Five states (Montana, Idaho, Utah, Nevada and Kansas) have abolished the insanity defense altogether, and others have instituted alternatives such as diminished responsibility and Guilty But Mentally Ill (GBMI) provisions. These alternatives allow for a finding of guilt but a reduction in either the seriousness of the charge or the severity of the punishment if mental disorder was determined to have influenced the criminal act. The GBMI verdict is available in approximately thirteen states and has been widely criticized on many grounds including the fact that it has not served to reduce the number of NGRI acquittals nor does it ensure that offenders receiving this verdict will receive effective treatment.

Public Perceptions of the Insanity Defense

Public perceptions of the insanity defense are that it is frequently used, frequently successful, and serves as a “loophole” guilty people use to go free; however, empirical research on whether these public perceptions are born out reveals that the public overestimates both the use and success of the insanity defense and underestimates the length of confinement of insanity acquittees.

Although the exact rates of use and success vary by jurisdiction, as a general statement, the insanity defense is rarely used and even more rarely successful. Silver, Cirincione, and Steadman (1994) compared public perceptions of the insanity defense with empirical data on its actual use and found that the public estimates the use of the insanity defense to be 37% (or 37 per 100 felony indictments) whereas the actual use is 0.9% (less than 1 per 100 felony indictments), representing a public estimate that is 41 times greater than its actual use. Similarly, with respect to estimates regarding the success of the insanity defense, these authors report that the public estimates the success rate to be 44% (or 44 acquittals per 100 insanity pleas) whereas the actual rate of success is 26%, representing a public estimate of success that is 81 times greater than the actual success rate. That is, there are nine insanity pleas for every 1,000 felony cases, about only two of which are successful.

Silver and colleagues (1994) also examined the distribution of charges for persons pleading insanity to determine whether the public’s perception—that the majority of defendants pleading insanity are murderers—was accurate. These researchers reported that approximately 14% of defendants pleading insanity were charged with murder, whereas about 54% were charged with other violent offenses and 32% were charged with nonviolent offenses. Again, public perceptions of defendants pleading insanity greatly overestimate the proportion of defendants charged with murder.

With respect to public perceptions regarding what happens to “successful” insanity acquittees, the public estimates that approximately half of these defendants are hospitalized whereas Silver and colleagues (1994) report that, in reality, about 85% are hospitalized. Similarly, these authors report that the public overestimates the proportion of insanity acquittees that “go free” upon acquittal, with a public estimate of 26% going free compared to the actual rate of 15%. These authors also indicate that if conditional release and outpatient treatment are excluded from the definition of “going free”, then the actual rate drops to a mere 1%.

When Silver and colleagues (1994) compared public estimates of the length of confinement of insanity acquittees to actual lengths of confinement they found, again, that public perceptions represent an underestimate of the actual length of confinement with the public estimating an average length of confinement of just under two years (21.8 months) versus an actual average length of confinement of over two and a half years (32.5 months) for all persons (regardless of charge) and of almost six and a half years (76.4 months) for those charged with murder.


Silver, E., Cirincione, C., & Steadman, H. J. (1994). Demythologizing inaccurate perceptions of the insanity defense. Law and Human Behavior, 18, 63-70.

Author’s Note: This information was excerpted from Roesch, R., Zapf, P. A., & Hart, S. D. (2010). Forensic psychology and law. Hoboken, NJ: Wiley.