Insanity, unsurprisingly, is an excuse rather than a justification. The criminal act of an insane person is not morally acceptable or a net gain to society. But we tend not to punish the insane because being insane means that one acts without the ability to rationally control one's behavior and, therefore, also without the ability to be deterred, so long as the condition of insanity persists.

I. Common Law

The common-law doctrine of insanity is derived from a nineteenth century English case...