In criminal law, for one to be found guilty of a crime, it must be proved that there was actus rea and mens rea. In other words the person committed the unlawful act (actus rea) and did so with malice afore thought or had reasonable knowledge that their action would result to what they are charged with (mens rea).

When one pleads insanity or NGRI they are in fact saying that although they committed the act they are not guilty because they had no malice aforethought (mens rea).

Insanity Defense in the USA

1) The M’Naghten Rule (1843)- Under this rule, one is assumed sane unless it can be proved that at the time of committing the act, they were laboring under such a defect of reason (from a disease of the mind) that they did not know the nature and quality of the act they were doing, or they did not know that what they were doing was wrong.

2) The Irresistible Impulse Rule- This view holds that an accused person might not be responsible for their acts even if they knew what they were doing was wrong if they had lost the power to choose between right and wrong. In other words, the person could not avoid doing the act in question because they were compelled beyond their will to commit the act.

3) The Durham Rule- In a 1954 US court of Appeal’s ruling, the insanity rule was broadened. Mr. Justice David Bazelon was of the view that the previous precedents did not allow for a sufficient application of scientific knowledge of mental illness and proposed a test that would be based on this knowledge. Under this rule, the accused is not criminally liable if his or her unlawful act was the product of mental disease or mental defect. This rule is often referred to as the product test.

4) The American Law Institute (ALI) Standard (1962)

This test often referred to as the “substantial capacity test” for insanity combines the cognitive aspect of McNaghten with the volitional focus of irresistible impulse holding that the perpetrator is not legally responsible if at the time of the act he or she owing to mental disease or defect, lacked “substantial capacity” either to appreciate its criminal character or to conform his or her behavior to the law’s requirements.

5) The Federal Insanity Defense Reform Act (IDRA)

This act was adopted by congress in 1984 as the standard for the insanity defense to be applied in all Federal jurisdictions. The Act abolished the volitional element of the ALI standard and modified the cognitive one to read “unable to appreciate” and in so doing made the law closer to the McNaghten rule. IDRA also shifted the burden of proof of insanity to the defense by stating that the mental disorder must be a severe one. Previously it was the prosecution responsibility to prove that an accused person was sane when the criminal act was committed.

The intent of the law in shifting of burden was to discourage the use of insanity defense and there is evidence to show that it did in fact alter litigation practices. Most states currently subscribe to the more restrictive M’Naughten standard or to the ALI standards version. In some jurisdictions when an insanity plea is filed, the case is first submitted for pretrial screening which includes a psychiatric evaluation, review of records and appraisal of criminal responsibility.

In a study done in 1996 of 190 defendants who had entered a plea of not criminally responsible, 105 were judged to be criminally responsible, charges against another 34 were dropped and 8 of the defendants were found to be insane and not responsible, (Janofsky, Dunn et al., 1996). Such defense it was noted only involved private attorneys rather than public defenders and was considered a “rich man’s defense”.

Some states like Idaho,, Montana, and Utah entirely abolished the attribution of insanity as an acceptable defense for wrongdoing. This apart from lacking compassion for those with mental illness has had a more negative effect to the intended purpose ( more effective prosecution) in that there was a corresponding rise in the use of “incompetent to stand trial” which has lead to charges actually being dismissed.

Some states have adopted the optional plea/verdict of guilty but mentally ill (GBMI). In this case a defendant may be sentenced but placed in a treatment facility rather than in a prison. In such a case a person would remain in the custody of the correctional department until the full sentence is served.