The law allows for a lot of defenses in criminal cases. Some of them are used fairly often, like self defense, while others are somewhat more obscure, such as involuntary intoxication. But probably no defense has as many myths surrounding it as the insanity defense. Today I’m going to shed some light on the issue.
- The insanity defense isn’t new. It’s documented in English common law as early as the 14th century.
- The legal definition of insanity has little to do with medical definitions of mental illness. It’s not only possible, but quite common, for a person to have a documented mental illness yet not meet the legal standard of insanity.
- The legal test for insanity varies by jurisdiction. The mostly widely used is the M’Naghten Rule, developed in a British case in 1843: Daniel M’Naghten attempted to assassinate the prime minister (and instead killed the prime minister’s private secretary). In order for the M’Naghten Rule to be used successfully, the defense lawyer must prove that because of a mental disease or defect, the defendant was unable to understand the nature of his actions or was unable to distinguish right from wrong.
- Four US states have abolished the insanity defense completely (Idaho, Kansas, Montana, and Utah).
- The insanity defense is used in fewer than 1% of criminal cases.
- The insanity defense is successful only about a quarter of the time.
- If a defendant is successful with the insanity defense, he will rarely be set free right away. Instead, he’ll almost always be committed to a mental hospital until he’s deemed no longer a danger to himself or others. Unless he was accused of a serious crime, this may mean he’ll spend more time in the hospital than he would have in prison. John Hinckley, Jr., for example, has been locked up since 1982, after he shot President Reagan and three other men in an assassination attempt.
- There have been attempts to reform the insanity defense. In the 1960s and 1970s, some states adopted tests that were easier to meet than M’Naghten and more in line with scientific knowledge of mental illness. But after Hinckley was successful with the newer defense, many of these states switched back to the old rule.
Incidentally, if you’re writing a serial killer, it’s very unlikely he’ll be successful with an insanity defense. Jeffrey Dahmer wasn’t deemed insane, despite killing 16 people—and committing necrophilia and cannibalism. Although most serial killers likely have antisocial personality disorder (and/or other personality disorders), these are generally not considered to be a “mental disease or defect.” Furthermore, most serial killers are quite aware of what they’re doing and that it’s wrong.
You may have heard the term “temporary insanity.” This isn’t a separate defense. It happens when the defendant claims she was legally insane at the time of the crime but has since regained mental stability. Its use is extremely rare. Probably the most famous case was Lorena Bobbitt, who cut off her abusive husband’s penis while he slept. The jury found her not guilty by reason of insanity, but after an evaluation at a mental hospital, she was released.