As the Wolff case illustrates, insanity is not an easy defense to prove, particularly in those states that apply the M'Naghten test. While Wolff was decided over 50 years ago, the M'Naghten test is still used in approximately half of the states.

It also remains difficult for defendants to use the defense successfully, as demonstrated in the highly publicized Texas case of Andrea Yates. Yates confessed to having drowned her five young children in the bathtub because she heard voices telling her to kill them in order to "save them from Satan."6 The case was tried in Texas, a state that still follows the M'Naghten "right from wrong" test. Therefore, to prove insanity, the defense had to show not just that Yates was mentally ill, about which there was no dispute, but also that she was not aware what she had done was wrong. In a three-week-long trial, the defense called as witnesses psychiatrists, relatives, and friends to testify that Yates suffered from severe post-partum depression. The prosecution countered by arguing that Yates's prompt action in reporting the drowning to the police established that she did know what she had done was wrong. The jury apparently agreed with the prosecution that Yates was not legally insane and convicted her of murder. She was sentenced to life in prison and sent to a prison psychiatric ward to receive treatment for her mental illness.

It is also possible that the jury found her guilty not because they thought she was sane when she drowned her children, but because they were afraid that a not guilty by reason of insanity verdict would have resulted in her being released from state custody. What the jury did not know, because by Texas statute7 they could not be told, is that even a not guilty by reason of insanity verdict would most likely have resulted in her immediate involuntary commitment to a mental institution.8 When a trial ends in a verdict of "not guilty" by reason of insanity, the defendant is absolved of any criminal responsibility but is often civilly committed to a mental health facility for treatment. And unlike a finding of guilt that results in a fixed sentence, a finding of not guilty by reason of insanity can result in defendants being committed for an indefinite term, potentially for life, until they are deemed to no longer be a threat to themselves or society.

As a result both of the difficulty of proving the defendant was insane at the time the crime was committed, and the uncertain result, the insanity defense is rarely used. Studies have shown that it is alleged in less than one percent of all criminal cases.

However, even if a defendant was sane at the time the crime was committed, he or she can be considered legally incompetent at the time of trial. If a defendant cannot understand the legal process or assist in his or her own defense by, for example, talking about the case with the attorney or testifying meaningfully at trial, the defendant may not be tried until the court determines the defendant is mentally competent.

Which of the three major insanity defenses do you think is the most appropriate? Do you support the concept of "guilty but insane" verdict? Why or why not?