The appeals court issued the M’Naughten rule,5 by which a mentally ill defendant may be considered insane if, at the time of the act, he:
- did not understand the nature and quality of the act
- or did not know the wrongfulness of the act.
- appreciate the criminality of his act
- or conform his conduct to the requirements of the law.
How to evaluate insanity
Prepare. Review the defendant’s medical/psychiatric records and materials pertaining to the offense, including the police report and other legal or medical documents. Indications of a prior psychiatric diagnosis may support or rebut the presence of a mental disease or defect at the time account of the of the alleged offense.
To assess the defendant’s mental state at the time of the act, note any recorded observations of his or her behavior during or around that time. You may wish to collect this information from collateral sources, as well. Look for bizarre behavior or other evidence that the defendant suffered from delusions, hallucinations, or other symptoms of a severe mental disease or defect.
Police records may contain:
- reports by witnesses, victims, and police officers about the defendant’s statements or behavior during or soon after the offense
- a defendant’s statement to the arresting officers.
Interview the defendant. Next, conduct a thorough standard psychiatric interview in person. Inform the defendant that the interview is not confidential, and that any information he or she provides may be included in a written report to the court or disclosed during trial testimony.
Consider psychiatric symptoms the defendant experienced during the offense, medication adherence, and use of alcohol or other mood-altering substances. Remember that voluntary intoxication does not provide grounds for an insanity defense, even in states that allow the irresistible impulse defense.
Obtain a detailed account of the event from the defendant. Look for:
- symptoms of a mental disease or defect when the offense occurred
- the defendant’s knowledge (or lack thereof) that the offense was wrong at that time (Table 3)
Signs that a defendant knew an act was wrong
| Efforts to avoid detection | Wearing gloves or a mask during the offense |
| Concealing a weapon | |
| Falsifying information (using an alias or creating a passport) | |
| Committing the act in the dark | |
| Disposal of evidence | Washing away blood |
| Removing fingerprints | |
| Discarding the weapon | |
| Hiding the body | |
| Efforts to avoid apprehension | Fleeing |
| Lying to authorities |
Indications that the defendant was aware at the time of the offense that his actions were wrong may include:
- behaviors during or immediately after the event, such as hiding evidence, lying to authorities, or fleeing the scene
- a rational motive such as jealousy, revenge, or personal gain.
Caveats. The defendant’s mental status during the interview may differ vastly from that at the time of the offense. Don’t be swayed if a defendant with a history of psychosis now appears symptom-free. Recent treatment may explain his or her lack of symptoms. Also be aware that the defendant may be malingering mental illness to support an insanity defense and escape criminal responsibility.7
Outcomes. Approximately 15% to 25% of criminal defendants who plead insanity are adjudicated insane. Technically, a defendant who is found legally insane has been acquitted of the offense. The insanity defense is less likely to succeed in jury than in nonjury trials.8
Although the defendant may not be punished once acquitted, he or she may be committed to a mental institution to ensure treatment compliance and protect the public.