All this has given Dr. Snowdon, author of a new book on the study called "Aging With Grace" (Bantam), a rare window through which to examine why some nuns thrive and others deteriorate so much they lose speech, mobility and much of their memory. The differences show up even in nuns with virtually identical backgrounds, even those who are biologically related.
At 93, Sister Nicolette Welter still reads avidly, recently finishing a biography of Bishop James Patrick Shannon. She knits, crochets, plays rousing card games and, until a recent fall, was walking several miles a day with no cane or walker.
But a younger sibling, Sister Mary Ursula, 92, shows clear Alzheimer's symptoms, Dr. Snowdon said. Several times a day, Sister Nicolette feeds and reads prayers to Sister Mary Ursula, who uses a wheelchair and can hardly lift her head or gnarled hands.
The other day, Sister Nicolette prompted Sister Mary Ursula to remember her age and birth date, but when Sister Nicolette asked if she recalled when "Sister Julia told you to pick up the Kleenex people used after Mass and you didn't want to," Sister Mary Ursula's eyes glazed, showing no hint of recognition.
Another Welter sister, 87-year-old Sister Claverine, is still active and clearheaded. A fourth sibling, Sister Mary Stella, died in 1996 at 80.
"I wouldn't have any idea why this happened to Mary Ursula," said Sister Nicolette, "but I just feel like I'll keep my mental faculties."
Some of Dr. Snowdon's research suggests she might be right. Sister Nicolette's autobiography, written when she was 20, was full of what Dr. Snowdon calls "idea density," many thoughts woven into a small number of words, a trait correlating closely with nuns who later escaped Alzheimer's.
One sentence in Sister Nicolette's essay, for example, reads, "After I finished the eighth grade in 1921 I desired to become an aspirant at Mankato but I myself did not have the courage to ask the permission of my parents so Sister Agreda did it in my stead and they readily gave their consent."
Compare that to the essay of another Mankato nun, who is in her late 90's and has performed steadily worse on the memory tests. The nun, who sat quietly by a window the other day, wrote in her essay, "After I left school, I worked in the post- office."
The Nun Study's latest published findings offer similarly provocative ideas about how positive emotional state in early life may contribute to living longer. Experts say linking positive emotions in the autobiographies to longer life echoes other studies showing that depression increases risk of cardiovascular disease and that people rated as optimists on personality tests were more likely than pessimists to be alive 30 years later.
The findings also raise questions like, What underlies the positive emotions?
"How much of this is temperament?" Dr. Suzman said. "How much of it is affected by life events and critical relationships with parents, friends, teachers, peers?" 
Overall, Dr. Snowdon says, the nuns live significantly longer than other women. Of the 678 in the study, 295 are alive and are all 85 or older. In the Mankato convent alone, there have been seven centenarians, many free of dementia.
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