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  • Michael Mullan

What is the Insanity Defense and why do so many prisoners have a mental illness?

To begin, what do we mean by the insanity defense? The Legal Information Institute defines the insanity defense as follows: “The insanity defense refers to an affirmative defense that a defendant can plead in a criminal trial. In an insanity defense, the defendant admits the action, but asserts a lack of culpability based on a mental illness.” The types of mental illnesses that are most likely to be successful in a criminal trial are what are known as SPMI (serious and persistent mental illness), which includes schizophrenia, bipolar disorder and severe depression. In order to successfully plead an insanity defense, a defendant must pass a given legal test for insanity. In the U.S. there are 3 predominant tests used. The M’Naghten Test stems from an old English common law case in the 1800s. Under this test, a defendant will be deemed legally insane if he cannot understand the nature and quality of the act, or cannot understand the wrongfulness of the act. The Irresistible Impulse Test focuses on volition, rather than cognition. Under the Irresistible Impulse Test a defendant will be deemed legally insane if he cannot conform his conduct to the law. The Model Penal Code test is essentially a combination of the first two tests. It also worth noting at this point that the burden of proof to assert an insanity defense rests on the defendant.

We now move to consider the background of how U.S. society has treated the mentally ill. Historically it was a case of ‘out-of-sight-out-of-mind’ whereby the responsibility for caring for the mentally ill was firmly within the family setting. Then with the onset of the welfare state, this population experienced a period of institutionalization whereby they were taking care of in state mental health hospitals. However, in the 1970s a number of factors led to a period of deinstitutionalization from these state hospitals. First of all, the media’s reporting of poor conditions and treatment of the mentally ill brought political pressure to bring about deinstitutionalization, in the form of the 1963 Community Mental Health Act. Other factors that led to this included developments in World War II whereby one of the leading findings of unfitness to serve was ill mental health. Also, the severe mental health crises that occurred post-WWII including widespread PTSD also contributed to a heightened consciousness of mental health concerns. The rise of psychotic drugs, including the patenting of Thorazine, also had a role to play, as did the civil rights movement more generally.

However, despite the god intentions of deinstitutionalization to treat those with mental illnesses with community mental health care, a lack of funding followed this intention. Soon this population experienced trans-institutionalization whereby those with a mental illness were released from psychiatric institutions and ended up in criminal justice institutions. Although this began in the 1970s, there are still currently a disproportionate number of prisoners and jail inmates with mental illnesses. It has been estimated that 50-70% of U.S. prisoners have a mental illness. Despite this alarmingly high number, the insanity defense is raised in less than 1% of felony cases, and is successful in less than 25% of these cases. Here we can see the disconnect between legal and medical insanity.

One of the reasons for such disproportionate numbers of prisoners having a mental illness is that mental illness has become criminalized. In this regard, certain lifestyle choices of the mentally ill mean that they are targeted by police in some instances, including public intoxication, trespassing and loitering. Another driving force is that this population is disproportionately affected by the general criminogenic factors that lead to increased police contact and increased likelihood of prison entry. This includes homelessness, poverty, unemployment and discrimination.


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