The insanity defence is well embedded in legal jurisprudence. In English and American law, for example, an individual must not only commit an illegal act (i.e. ‘actus reus’ or bad act), but also must have done so with ‘mens rea’ (i.e. a guilty mind, or mental state). The challenge has been how to develop a legal standard for determining the threshold for when an individual with a severe mental illness is so impaired as to be considered not responsible for his or her behaviour. Modern conceptualisations derive from the landmark case of Daniel M’Naghten in England in 1843. The ‘M’Naghten standard’ focused on the individual’s lack of ability to ‘know he was doing what was wrong’. This standard was adopted in many jurisdictions in the United States, until the latter part of the twentieth century. In 1962, the American Law Institute’s Model Penal Code proposed an expanded standard that included not only appreciation of wrongfulness, but also an option for defendants to be considered legally insane if they lacked substantial capacity to ‘conform conduct to the requirements of the law’. This is known as the volitional prong, focusing on ability to control impulses (in contrast to the cognitive prong of understanding and appreciating that an act is morally and legally wrong). In response to the case of John Hinckley (who shot President Reagan and was acquitted by reason of insanity), many states and the U.S. federal system modified their standards to eliminate the volitional prong. Despite the public focus on such high-profile cases, the insanity defence is infrequently used, rarely successful, and often results in acquittees being psychiatrically hospitalised for much longer periods of time than they would have been incarcerated for if found guilty.

Insanity defence evaluations in most jurisdictions are conducted only by doctoral-level psychologists or psychiatrists. Professional standards have been developed to guide these evaluations, including guidelines about the data elements to be included, the need for multiple sources of data (i.e. not relying solely on the defendant’s account, but seeking out third-party sources of information), sensitivity to the possibility of ‘impression management’ (i.e. considering whether the defendant may be malingering/exaggerating symptoms, or deliberately under-reporting symptoms because of stigma), and appropriate use of psychological testing. In addition, evaluations of mental state at the time of the alleged offence are often complicated by substance use, as use of various drugs and alcohol can mimic, enhance, or induce psychotic states.