Mercy killing in the Netherlands for psychiatric reasons is increasingly popular. It is a supposedly neat and tidy end for untidy lives and is promoted as strictly voluntary -- a jewel in the crown of individual freedom and self-determination. For the mentally ill, however, the siren call of individual freedom dovetails too nicely with society's proven intolerance of the troubling behavior of people with mental disorders. In fact, the roots of mercy killing in modern times are lodged in the unsavory and downright savage practices of the last century.

Progressives in the early 20th century advocated voluntary euthanasia, as both a right and a practice that would benefit society. But when applied to the mentally ill and other undesirables, the euthanasia movement became tied to the eugenics movement. For those with "lives not worth living," the societal duty to die appeared quite clear in the face of pressing needs for social reformation and urban hygiene. In the Bollinger case, a handicapped baby was refused a lifesaving operation in Chicago in 1915, touching off a national debate over mercy killing the mentally disabled.

Even advocates for accepting the physically handicapped supported euthanasia for the mentally handicapped. One surprising supporter was Helen Keller. She wrote in The New Republic that the fate of the "idiot baby" whose "existence is not worthwhile" should be left to a jury of physicians. She was no friend to the idea that all human lives are inherently valuable and worthy of protection, even those of the mentally handicapped. Instead, she believed that it "is the possibilities of happiness, intelligence and power that give life its sanctity, and they are absent in the case of a poor . . . unthinking creature."

Keller thought that juries of doctors could decide the worth of human lives accurately and without prejudice. She was proven spectacularly wrong by the creation of one such jury, the Hereditary Health Courts, in Nazi Germany. More than 200,000 mentally ill and handicapped adults and children were involuntarily euthanized in what became the model for the mass extermination of Jews, gypsies, and other "undesirables."

The government and euthanasia supporters in the Netherlands are justifying the recent killing of a 29-year-old mentally ill woman in the same way that Helen Keller judged the life of the Bollinger baby: They believed future happiness was impossible. Even if the patient was firmly convinced that she would never recover, fulfilling her request in the name of individual freedom raises the question: Can a patient suffering from severe mental illness be expected to make a rational request for death?

History proves that it is altogether too easy to mix voluntary euthanasia -- promoted as the individual's right to choose death -- and involuntary euthanasia -- promoted as preventing a life of suffering or as a way to reach utilitarian goals. In the Netherlands, disabled infants who cannot consent to be killed are routinely and legally euthanized under a set of standards called the "Groningen Protocol." In Belgium, where euthanasia is also legal, adults are routinely killed without consent, accounting for over 30 percent of those euthanized, according to one study. Doctor Peter Saunders of England's Care Not Killing campaign warned, "Once you have legalized voluntary euthanasia, involuntary euthanasia will inevitably follow."

Here in the U.S., euthanasia is not yet legal, but its cousin, physician-assisted suicide, is legal in several states. Social acceptance for ending lives that are "no longer worth living" is rapidly increasing, a dangerous trend when combined with the common prejudice against the value of the lives of those who suffer from severe psychiatric diseases. Tormented and terrified, homeless or imprisoned, these can certainly seem like lives not worth living. It's easy to imagine the slide from purely voluntary suicide as a last resort to the mercy killing of the mentally ill who burden society. Looking at 20th-century history, it's easy to understand why societies should be wary of allowing that slide to ever start.