What are the main differences between the legal definition of insanity vs. the medical definition?

It varies from jurisdiction to jurisdiction, but in most U.S. states, the legal definition of insanity requires the presence of a diagnosable major mental illness (as determined by a mental health professional) and the mental illness must have interfered with the individual’s ability to differentiate between right and wrong. It is the legal system which determines whether these two conditions have been met and if they have, an insanity defence can be successful.

Insanity is no longer a medical term. It used to be used to describe what would now be considered psychotic features – that is, a detachment from reality. So a medical practitioner would not diagnose someone as “insane.” Even for legal purposes, a medical practitioner would provide a diagnosis (e.g., schizophrenia or delusional disorder) which might meet the requirements of the legal definition of insanity, but the diagnosis would be conveyed in the standard medical terminology.

As far as “responsibility,” the notion of the insanity defence is that if you are incapable from distinguishing right from wrong, you really can’t be held responsible for your actions. In Canada, the term in Not Criminally Responsible – not sure if that is used in the States.