In the state of California, the McNaghten Rule is used in cases where the defendant wants to plead insanity. If the defendant is found to be legally insane at the time of the commission of the crime, he or she cannot be found guilty of that crime. There are two times when in the state of California a defendant is considered to be legally insane. These two times are when a defendant is unable to understand the nature of the criminal act or when the defendant is incapable of understanding that what he or she was doing was morally wrong. In 1978, the McNaghten Rule was modified by the American Law Institute Rule which allows the defendant to prove insanity based on either disease or mental defect. To plead an insanity defense successfully in California, a defendant must prove by the preponderance of the evidence that he or she was legally insane. Therefore, although the burden of proof is on the defendant to prove he or she was insane as insanity is an affirmative defense in the state of California, the standard for this burden of proof is sufficiently lower than the beyond a reasonable doubt standard that is required for the state to prove for the defendant to be convicted.
The proof of whether a defendant is ‘not guilty by reason of insanity’ is given at the defendant’s sanity trial, a separate phase of the trial to determine whether the defendant was sane at the time the crime was committed. Although drug and alcohol use can cause a defendant to meet one of the two requirements set forth in the McNaghten Rule, in the state of California a defendant cannot assert a defense of legal insanity simply because he or she was addicted to drug or alcohol at the time that the crime was committed. In addition, if you are successful in your assertion of a guilty plea in California, this does not mean that you go free. “If you convince the jury at your California criminal jury trial that you are not guilty by reason of insanity, then you will be committed to a state mental hospital instead of being sent to prison” (California’s Insanity Defense: The McNaghten Rule).
For a defendant to prove that he or she is legally insane at the time of the offense, this requires the testimony of a forensic psyachrist or psychologist. These professions have corut acceptable standard methods or proving whether or not a defendant was legally insane by administering certain tests. “There are two clinical tests used to evaluate a defendant’s mental competency and appropriate labeling of legally-mentally insane, they are: the Slobogin Mental Screening Evaluation (Slobogin et al, 1984), and the Rogers Criminal Responsibility Assessment Scales (Rogers, 1984). Ultimately, the assessment of insanity is a complex and time-consuming process. Forensic psychiatrists and psychologists must evaluate information from a number of sources and conduct clinical diagnostic interviews and tests” (Fact Sheet: The Insanity Defense). In the state of California, the defenses must proffer such documentation to prove that forensic psychology services were rendered and tests were administered to meet the legally acceptable standard which has been set forth by the court system.
There is no set requirement for documentation other than that the person testifying as a forensic psychologist must met the standard for being considered an expert by that court and the documentation regarding tests performed to evaluate the defendant must conform to the acceptable tests of the forensic psychologist’s profession. Therefore, a forensic psychologist cannot present documentation of his own new novel theory of why the defendant is insane; this may not be true concerning the clinical work of a psychiatrist who, as a medical doctor, may formulate new theories concerning a new mental disease that qualifies the defendant as insane.