Psychopaths are mentally ill — insane — but as a rule have no insanity defense against criminal liability. This article explains why.
The explanation hinges upon the doctrine of mens rea, the criminal mind necessary for criminal liability. The insanity defense is an excuse, an affirmative defense for those with mens rea enough to be guilty. But the defense should take its essential purpose and shape from the doctrine of mens rea.
This relation between mens rea and the excuse of insanity is why a defendant insane as a matter of mental health may not be insane as a matter of criminal law. Only an insanity that calls into question the usual workings of the doctrine of mens rea should excuse from criminal liability. If psychopathy is not such an insanity, it should not excuse.