The Insanity Defense The insanity defense refers to that branch of the concept of insanity which defines the extent to which men accused of crimes may be relieved of criminal responsibility by virtue of mental disease. The terms of such a defense are to be found in the instructions presented by the trial judge to the jury at the close of a case. These instructions can be drawn from any of several rules used in the determination of mental illness. The final determination of mental illness rests solely on the jury who uses information drawn from the testimony of "expert" witnesses, usually professionals in the field of psychology. The