The insanity defense is rooted in the ethical assumption The insanity defense is rooted in the ethical assumption that the state cannot impose a criminal sanction unless the individual is culpable. States have taken two basic approaches to the insanity defense: a majority follow some variation of the rule enunciated in the M'Naghten case, but a substantial minority have adopted some form of the Model Penal Code definition. Six States have added the irresistible impulse test to the M'Naghten rule. States also vary in whether they place the burden of proof on the defendant to prove insanity or on the prosecution to prove sanity. The present Federal standard is basically uniform, based on the Model Penal Code's cognitive and volitional tests for insanity. The proposed legislation would limit the definition of insanity and shift the burden of persuasion to the defendant to prove insanity by clear and convincing evidence. This allows exculpation of only those defendants so afflicted by a mental defect that they are unable to tell right from wrong, thus eliminating the volitional test. These proposals would violate the fifth amendment, because the burden of persuasion they place on the mentally deficient or diseased defendant is too heavy in relation to the strict definition of the crime. The legislation also violates the eighth amendment's ban on cruel and unusual punishment, because it would allow the government to impose a qualitatively different punishment, penal imprisonment, on the mentally deficient defendant without proving a legally sufficient basis for this sanction. The present Federal insanity defense needs reform. Alternatives to the proposed legislation include making changes in the definition of insanity and burden-of-proof rules, confining the Federal definition of the cognitive prong of the Model Penal Code test, and adopting a standard whereby the defendant's burden of proof should not exceed the preponderance of evidence. The paper contains 119 footnotes.