The Art of Forensic Psychology
John Wayne Gacy was arrested in 1978 and charged with the murders of thirty-three young men. He pretended to have an alter personality, so his attorneys offered a defense of insanity. A number of psychologists and psychiatrists put him through a battery of assessment instruments and came up with a variety of diagnoses. The overriding claim was that Gacy had experienced an irresistible impulse when he killed each young man, and had either blacked out or lost his inhibitions by drinking alcohol to such an extent that he was unable to control himself. This was in keeping with a phrasing of the insanity defense at the time in Illinois that allowed that a person might realize that what he was doing was wrong was unable to stop himself. The jury rejected the defense, largely because the prosecutor was able to show that Gacy had planned several of the murders and his team's psychologist insisted that an irresistible impulse cannot be planned in advance.
Most forensic health professionals, including those on cases like this, work either in a correctional setting or in a psychiatric hospital. They usually perform the role of assessment and treatment, or therapeutic intervention, and they will have a substantial background in abnormal and social psychology, along with considerable experience in psychometric testing. For the most part, their job is to determine one of two things: competency (present functioning and understanding) or the defendant's mental state at the time of the offence. With Gacy, each professional involved ran a standard battery of assessment instruments, such as an IQ test, projective tests, and self-report inventories. Some of them also examined him for potential brain damage (and found none). Each then determined a diagnosis from the results, and lined up either for or against the claim that Gacy did not know what he was doing each time that he committed murder.
Assessments can be done for the court (civil or criminal), the government, insurance companies, or any other decision-making fact-finders involved in legal issues. A forensic behavior specialist may serve as an expert witness for the defense or prosecution, or may simply present findings as ordered by the court. They should remain impartial, but try as they might to avoid it, they may be brought into a case as a "hired gun"--a professional who says what he's paid to say. Such experts are all too readily available, although this practice is considered unethical and their reputation among their peers usually suffers.