The insanity plea is likely one of the most well-known criminal defenses.  On May 7th, attorneys for James Holmes, the man accused of murdering 12 people in a movie theatre shooting in Colorado, stated that Homes wants to change his plea from not guilty to not guilty by reason of insanity.  Unsurprisingly, this has received its fair share of media attention.  In fact, given the amount of airtime devoted to cases where a defendant enters an insanity plea, it may seem that the accused are pleading insanity right and left.  However, such defenses are entered in less than 1% of felony cases and often do not succeed.

The rules governing an insanity defense vary from state to state.  Three states—Montana, Idaho, and Utah—do not even allow the insanity defense.  But the first codification of such a defense in the Western World dates back to the 1800s.  The M’Naghten rules, which guide insanity defenses in about half of all states, arose in England after the acquittal Daniel M’Naghten who was accused murdering a man he mistook for Prime Minister Robert Peel.  In that case, the courts ruled that the defendant should not be held liable for his actions if he did not know that his act was wrong or if he did not understand the nature of his crime.  Of course, since that time those standards have evolved.  A distinction has been drawn between the knowledge that something is morally wrong and the knowledge that something is legally wrong.  Some courts have held that an ignorance of both is necessary for an insanity defense, while others deem that the ignorance of only one is sufficient for a successful insanity defense.  The majority of states that do not abide by this right/wrong definition of insanity, follow guidelines set forth by the American Law Institute.  Those guidelines dictate that if the defendant lacks “substantial capacity” to understand the criminality of his conduct, then he should be found not guilty by reason of insanity.  Naturally, what constitutes “substantial capacity” is a question to be settled at trial.

In 1984 Congress passed the Insanity Defense Reform Act that changed the way insanity defenses were dealt with.  This act came after Ronald Reagan’s failed assassin was found not guilty by reason of insanity.  Prior to the Insanity Defense Reform Act, the standard of evidence for proving insanity was made more stringent and prosecutors were relieved from having to prove that a defendant was sane. Instead, it was left up to the defense to prove that an insanity defense was justifiable.

The judicial system has also developed the concept of “guilty but mentally insane.”  This allows for the court to sentence a defendant as if they were found simply guilty, but requires that a determination be made as to what type of treatment the defendant requires.  If the defendant is ever deemed cured of all mental illnesses, he is still required to serve the rest of his sentence.  A defendant who is deemed not guilty by reason of insanity may be released from the penal system once psychiatrists show that the defendant no longer poses a threat.

With all of the debate, complications, and restrictions surrounding insanity defenses, it’s no wonder that they aren’t very common.  And, while no ruling has been made on whether Holmes can change his plea to not guilty by reason of insanity, if his attorney’s request is granted, it will certainly raise a number of hurdles for the defense.