From OpinionJournal : "But scholars have shown that we have greatly exaggerated the benefits of living wills. Studies by University of Michigan professor Carl Schneider and others have shown that living wills rarely make any difference. People with them are likely to get exactly the same treatment as people without them, possibly because doctors and family members ignore the wills. And ignoring them is often the right thing to do, because it is virtually impossible to write a living will that anticipates and makes decisions about all of the many, complicated, and hard to foresee illnesses you may face.
For example, suppose you say that you want the plug pulled if you have advanced Alzheimer's disease. But then it turns out that when you are in this hopeless condition your son or daughter is about to graduate from college. You want to see that event. Or suppose that you anticipate being in Terri Schiavo's condition at a time when all doctors agree that you have no chance of recovering your personhood and so you order the doctors to remove the feeding tubes. But several years later when you enter into a persistent vegetative state, some doctors have come to believe on the basis of new evidence that there is a chance you may recover at least some functions. If you knew that you might well have changed your mind, but after entering into a PVS you can make no decisions. It is not clear we would be doing you a favor by starving you to death. On the contrary, we might well be doing what you might regard as murder.
There is a document that is probably better than a living will, and that is a durable power of attorney that authorizes a person that you know and trust to make end-of-life decisions for you."
