Temporary insanity is a defense that can be used when the defendant believes they shouldn't be held criminally liable for their actions. First, the defendant must have a mental disease or defect.In an insanity defense, the defendant admits the action but asserts a lack of culpability based on mental illness. John Wayne Gacy was arrested in December 1978, suspected of being involved with the disappearance of a 15-year-old boy. In order to make an insanity defense work, the defense has to prove that their client, the defendant, was "insane" at the time of the crime. Lawson and Berry have significant criminal defense experience and know how to expertly demonstrate that you were insane at the time of the offense. There are no statewide standards for evaluating which defendants should plead the insanity defense. What Happens If I Plead Insanity? Date Published. 1981.