Mental State at the Time of the Offense, sometimes abbreviated as MSO, is a determination of a defendant’s state of mind at the time he or she committed a crime. When a defendant’s mental state is at question, a sanity evaluation may be requested by the court, the defense, or the prosecution.

An MSO evaluation usually consists of a series of three interviews performed by a forensic psychiatrist. The first focuses on getting a personal history of the defendant, the second focuses on the defendant’s mental state at the time of the crime, and the third evaluates the defendant’s present mental state. Although all three interviews are important, the first one is one of the most important. A rating system for comparing the defendant’s own recounting with police reports, attorney notes, and any other histories is a must to determine the facts. The forensic psychiatrist becomes an investigator for the courts and may spend time reviewing collateral information and interviewing other third party persons. Factors such as planning the offense, awareness of guilt, and self-control are rated along with an analysis of the collateral information.

An MSO evaluation assesses if a mental disease or defect caused a cognitive impairment at the time of the offense. There are legally two grounds for an insanity plea: that the defendant did not know the nature and quality of the criminal act, and that the defendant did not know that the act was wrong. An individual can also be found insane if he had an “irresistible impulse,” and due to mental illness, lost control over his actions at the time of the offense.

The basis of an insanity plea rests on the basic premise that the defendant was not criminally responsible for his or her actions at the time of the offense due to the influence of a mental illness. Although each case is unique and may require additional testing or evaluation, most evaluations include the following:

– A review of records related to the specific offense (e.g., police records and witness statements)
– A review of background records and history (e.g., school records and psychiatric treatment records)
– Background information: social history, psychiatric history, medical history, substance abuse history, family background
– Clinical interview(s) of the defendant
– Defendant’s self-report of the incident(s)
– A review of collateral sources (e.g., interviews with family and police) related to the defendant’s mental status around, leading up to, and at the time of the offense
– Psychological testing (e.g., personality assessment tests)
– Observation of the defendant during evaluation sessions
– Malingering assessment
– Assessment of the defendant’s mental status at the time of the offense as per the relevant legal statute and case law
– Conclusions and opinions of the forensic psychiatrist
Malingering is always considered as a possibility, as is the possibility of brain damage. Malingering is routinely investigated by the evaluator and additional objective testing may be conducted at the discretion of the evaluator to confirm or deny a diagnosis of malingering.

As in so many instances in criminal law, the forensic psychiatrist plays a key role in determining an insanity plea. His or her evaluation almost always overrides all other considerations in a case.