ON JUNE 20, 2001, Andrea Yates, an ex-nurse from Houston with a history of severe postpartum depression, drowned all five of her children (aged six months to seven years) in a bathtub. Following a conviction in 2002 that was overturned on appeal, Yates was acquitted in 2006 as not guilty by reason of insanity. Yatess attorneys, backed by expert testimony, contended that she thought she was being persecuted by Satan and needed to protect her children from eternal damnation by killing them.

Forty-six U.S. states have some version of the insanity defense on the books, with Utah, Montana, Idaho and Kansas disallowing it. This defense is designed to protect people who are incapable of understanding or controlling their criminal actions and to help them get treatment. Nevertheless, the idea of offenders being deemed legally innocent is hard for the public to swallow. In the case of Yates, radio talk-show host Mike Gallagher captured the sentiments of many: So now, Gallagher opined, officially and formally, Andrea Yates did not drown her five children, is that it? Similarly, after the 1982 acquittal of John W. Hinckley, Jr., for the attempted assassination of President Ronald Reagan, an ABC News poll revealed that 76 percent of Americans believed that Hinckleywho was deemed delusionalshould have been convicted.

Although excusing the violence of Yates and Hinckley may seem wrong, the insanity defense is actually tailored to such situations. The concept of criminal guilt refers to more than whether a defendant committed the crime; in almost all states, it also requires that the person be deemed of sound mind when the act was performed. And although many believe the plea dumps dangerous felons back on the streets, in fact attorneys attempt the defense only rarely and typically fail in the attempt. Even when the defense succeeds, the acquitted usually end up with sentences similar to or longer than those for convictions. The main difference between an acquittal and conviction: those acquitted on the basis of insanity are usually sent to psychiatric hospitals rather than prisons.

Origins of a Plea
In 1843 Daniel McNaughton went to 10 Downing Street in London with a plan to kill the British prime minister, Robert Peel. Mistaking Peels secretary for Peel, McNaughton shot the secretary, who died five days later. McNaughton was acquitted on the grounds that he believed the government was plotting against him, but the verdict had no clear precedent and rested on fuzzy legal grounds. Reacting to public anger to the verdict, a panel of judges fashioned a guideline for insanity, now called the McNaughton rule: to be declared insane, defendants must either not have known what they were doing at the time or not have realized their actions were wrong.

The McNaughton rule, which many U.S. states adopted, hinges on cognitive factors, excusing people from legal responsibility because they lacked understanding of the crimes meaning. Some states now employ the looser guidelines set out by the American Law Institute in 1962, which broadened the insanity defense to also include cases in which a person cannot control his or her impulse to act because of a psychiatric disorder. Proponents of the defense, in either guise, regard it as a needed exception for the rare cases in which people are unable to inhibit their destructive behaviors. Most advocates believe that it is inhumane to punish individuals who did not adequately grasp what they were doing. Instead, they say, we should try to rehabilitate or least treat them.

But critics contend that excusing individuals for a crime that they unquestionably committed makes no sense. To them, the insanity defense confuses the question of whether a person should be found guilty of a crime with that of what punishment he or she should receive. Most skeptics believe that all defendants who commit a crime should be found guilty but that those with severe mental illness should sometimes receive lessened sentences.

Catering to this view, about 20 states have introduced the verdict of guilty but mentally ill, which holds a person legally accountable for a crime but permits mental illness to be considered as a mitigating factor in sentencing. This verdict is supposed to enable an ill individual to receive the treatment he or she needs. In reality, those deemed guilty but mentally ill sometimes fail to receive adequate therapy. What is more, the verdict has not led to a clear-cut reduction in the number of insanity acquittals.

Judicious Use?
Whether or not the insanity defense is justified, it is intended only for the rare instances in which a bona fide mental disorder has obliterated the psychological brakes most of us use to stop ourselves from acting immorally. Yet many Americans perceive the insanity defense to be widely invoked and commonly successful. In a 2007 study psychologist Angela Bloechl of the University of WisconsinOshkosh and her colleagues found that college students estimate that the defense is used in 30 percent of criminal cases and succeeds 30 percent of the time.

Yet data from multiple studies show that only about 1 percent of cases involve the plea, and only 15 to 25 percent of those result in acquittals. Although notorious insanity plea acquittals, such as those of Yates and Hinckley, garner outsize media attention, scores of other defendants, including Jack Ruby (who killed Lee Harvey Oswald, John F. Kennedys assassin), David Berkowitz (Son of Sam), Jeffrey Dahmer (serial killer) and Lee Boyd Malvo (one of the two Beltway snipers), have been convicted after pleading insanity.

Many people also believe that those acquitted on the basis of insanity get a quick and easy pass out of prison. A few years of treatment in a mental hospital, then presto! Shes all better now, free to be released into an unsuspecting public, Gallagher speculated about Yates. But only about 1 percent of those who use the insanity defense successfully are released immediately, and the average length of hospital stays for people let off because of insanity is about three years. Indeed, as of this writing, Yates remains institutionalized in a mental hospital in Kerrville, Tex., more than four years after her acquittal. Moreover, data collected in 1995 by sociologist Eric Silver, then at Policy Research Associates in Delmar, N.Y., suggest that those deemed not guilty by reason of insanity often remain in institutions just as long as people convicted of comparable crimes do; in some states, such as New York and California, they stay longer.

Thus, the insanity defense is far from a quick passage to freedom. Citizens and policy makers must understand the plea for what it is: an extremely rare exception that proves the rule that almost all individuals should be held legally responsible for their criminal actions.