Why do most mental disorders not eliminate criminal responsibility?

In criminal law, the assumption of criminal responsibility is rooted in the capacity to make elementary moral distinctions and the power to adjust behavior to the requirements of the law. Most mental disorders do not eliminate this responsibility because legal tests for insanity are not intended to be scientific definitions of mental illness.12

Instead, these legal standards are designed to identify only those individuals whose incapacity is so severe that holding them responsible would violate principles of justice and social expediency. Under traditional standards, such as the M’Naghten’s Case rule, an individual is only excused if they did not understand the nature of their act or could not distinguish right from wrong.12

Because the law prioritizes the ability to conform conduct to legal requirements and understand moral culpability, many conditions that might be classified as mental disorders by medical professionals do not meet the high threshold required to negate criminal intent or responsibility. In some jurisdictions, defendants who suffer from mental illness but do not meet the strict legal definition of insanity may be found "guilty but mentally ill," meaning they remain subject to criminal sentencing despite receiving treatment.12