Forensic psychology is a clinical psychology specialty which intertwines the law with two types of professional services: (1) psychological evaluation and assessment to assist the defense or prosecution of criminal cases or civil commitment proceedings; and (2) "rehabilitation" of criminals once in the prison system.
When a person is charged with a misdemeanor or a felony, their mental state or mental illness may be an issue to consider before conviction or before sentencing. The attorney representing the accused person may request a psychological evaluation. Sometimes a psych evaluation is court ordered.
The state (prosecution) carries the burden of proving "beyond a reasonable doubt" that the defendant "formed the requisite mens rea to commit the crime." [A "reasonable doubt" is a doubt based on reason and common sense after a careful and impartial consideration of all the evidence in the case. It is a kind of doubt that would make a reasonable person hesitate to act in the most important of his/her own affairs. "Mens rea" is Latin for "guilty mind." It is a is a criminal law concept which focuses on the mental state of the accused. Did the person know that the act or omission was illegal or prohibited?]
When the court appoints a psychologist or psychiatrist as an expert witness, the psychologist or psychiatrist examines the defendant (patient). A determination is made as to whether there is substantial evidence that the patient suffers a mental disorder. Emotions such as fear, jealousy, anger or hatred are not considered a mental disorder.
The psychologist or psychiatrist needs to consider psychological influences at the scene of the alleged crime (for example, provocative social situations, injury, physical illness, medical neurological diseases, adverse environmental conditions, etc.). Depending on the outcome of the examination, the psychologist or psychiatrist may testify in court how the impaired mental abilities "actually caused a malformation of the mental element of the crime." An expert opinion is rendered regarding whether the "mental element probably failed to be formed."
The expert does not have to be certain that the defendant's disorder caused him or her to be unable to form the intentor "knowledge," but the expert must have some belief in the "probability or possibility" that it did. Experts need to testify with "reasonable" medical and psychological certainty.
The old "insanity defense" or "M'Naughten" test was: "At the time of the commission of the offense, as a result of a mental disease or defect, the mind of the actor was affected to such an extent that: (1.) he was unable to perceive the nature and quality of the act with which he is charged; or (2.) he was unable to tell the difference between right and wrong with reference to the act charged."