Legal Definitions - MPC test

The MPC test refers to the insanity defense standard adopted by the Model Penal Code (MPC), a set of proposed criminal laws developed by the American Law Institute. This test is also known as the Substantial-Capacity Test.

Under the MPC test, a person is not criminally responsible for their conduct if, at the time of the conduct, as a result of a mental disease or defect, they lacked substantial capacity either to appreciate the criminality (wrongfulness) of their conduct or to conform their conduct to the requirements of the law.

This standard is more flexible than some older insanity tests because it does not require a complete lack of capacity, only a "substantial" lack. It also considers both a person's ability to understand right from wrong (cognitive capacity) and their ability to control their behavior (volitional capacity).

Here are some examples illustrating the MPC test:

• Example 1: Cognitive Incapacity

  Imagine a person, Sarah, who suffers from severe paranoid schizophrenia. During a psychotic episode, she genuinely believes that her neighbor is a demon sent to harm her family and that killing the neighbor is the only way to protect them. She acts on this delusion, causing harm to her neighbor. Under the MPC test, a court might consider whether, due to her mental disease, Sarah lacked the substantial capacity to appreciate the criminality or wrongfulness of her actions. If her delusion was so profound that she truly believed she was acting righteously to save her family from a demonic threat, she might meet the criteria for insanity under the MPC test, as she could not appreciate that her actions were criminal or wrong in the conventional sense.

• Example 2: Volitional Incapacity

  Consider Mark, who has a severe, diagnosed impulse control disorder that manifests in uncontrollable kleptomania, especially when under extreme stress. Despite knowing that shoplifting is wrong and illegal, he experiences an overwhelming, irresistible urge to steal items when he is in a store, particularly during periods of intense personal crisis. He has tried therapy and medication, but the urges persist. If Mark were to shoplift during such a period, his defense might argue that, due to his mental defect, he lacked the substantial capacity to conform his conduct to the requirements of the law, even though he understood that shoplifting was wrong. The MPC test allows for consideration of this inability to control one's actions.

• Example 3: Combined Cognitive and Volitional Impairment

  A veteran, David, suffers from severe Post-Traumatic Stress Disorder (PTSD) and dissociative episodes. During a particularly stressful event, he experiences a flashback so intense that he genuinely believes he is back in a combat zone, under attack. In this dissociated state, he reacts violently to a perceived threat that, in reality, was a harmless civilian interaction. A defense using the MPC test would argue that, due to his mental disease (PTSD leading to dissociation), David lacked the substantial capacity to both appreciate the wrongfulness of his actions (because he perceived himself to be in a life-or-death combat situation) and to conform his conduct to the law (as his actions were an uncontrollable, automatic response to a perceived threat within his dissociated state).