The Art of Forensic Psychology
During the 1840s, Daniel M'Naghten believed that Prime Minister Robert Peele was out to get him. In a delusional frame of mind, he went gunning for the man but mistakenly killed his private secretary instead. He was arrested and sent to trial. His defense counsel argued that while he knew what he was doing, given his paranoid ideas, he was unable to control himself. The court acquitted M'Naghten, but the public reacted badly to this decision, so a royal commission was appointed to study the issue, and they devised the M'Naghten Rule for future such cases. The House of Lords then required that to establish a defense on the grounds of insanity, it had to be proven that "at the time of the committing of the act, the party accused was laboring 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."
Insanity is a legal definition, although it's often used incorrectly to refer to someone who is psychotic, or even just eccentric. It means a lack of responsibility for one's actions due to a mental disease or defect, which reduces criminal intent. The law recognizes that responsibility for committing a crime depends on two things: actus reus, which is evidence that the accused engaged in the act, and mens rea, the mental state required to have intended to commit the act or foreseen its consequences. The legal system assumes that people are generally rational and that they can freely make decisions for which they are morally responsible. Mental health professionals may undermine this notion in certain cases by describing psychological factors that mitigate culpability. Thus, triers of fact must consider excusing the behavior of those without mens rea, because the acts of deranged people cannot be judged in the same way as the acts of rational people.
Insanity standards have a long history in English common law, which has been adopted by the U.S. Yet from one state to another, there is no uniform standard for determining insanity, and a person found sane in one jurisdiction might be considered insane in another. Federal and state courts use a variety of standards, generally grounded in knowledge of right and wrong but sometimes also including the ability to conform one's actions to the requirements of the law.
The key issue in a mens rea determination is the defendant's state of mind specifically at the time of the offense. In other words, even if they are proven to have a several mental illness, such as schizophrenia, the illness must be related in a meaningful way to the crime. A legal defense of "not guilty by reason of insanity" presents the idea that defendants have committed crimes but by reason of "disease or defect" are not responsible for their actions. Thus, they ought not to be held criminally accountable for something they could not help. The job of a forensic mental health professional is to make an evaluation about the relationship of their illness to their crime, and if there is good reason to believe that the defendant did not appreciate what he or she was doing, or that it was wrong, the jury must acquit.
This defense is not used nearly as often as the public believes, and it succeeds even less often, but certain sensational cases have put media focus on it -- especially when someone who has been evaluated, treated and released goes out to offend again. The job of making this determination is difficult, since it's specific to a time and experience to which the professional was not a witness, but even harder than this task for psychologists is the request for a risk assessment.