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Applying these measures to the sisters' autobiographies, Snowdon and Kemper found to their astonishment that the elderly sisters who showed signs of Alzheimer's had consistently authored essays low in both idea density and grammatical complexity a half-century or more earlier. One of the lowest-scoring samples begins: "My father, Mr. L.M. Hallacher, was born in the city of Ross, County Cork, Ireland, and is now a sheet-metal worker in Eau Claire." By contrast, one of the highest-scoring essays conveys the same type of information but in a dramatically different way: "My father is an all-around man of trades, but his principal occupation is carpentry, which trade he had already begun before his marriage with my mother."
Idea density turns out to be an astonishingly powerful predictor of Alzheimer's disease--at least among the School Sisters of Notre Dame. Snowdon found by reading nuns' early writings, he could predict, with 85% to 90% accuracy, which ones would show the brain damage typical of Alzheimer's disease about 60 years later. "When we first looked at the findings," says Snowdon, "we thought, 'Oh my God, it's in the bag by the time you're in your 20s.'"
But Alzheimer's is not that simple. One especially telling case: Sister Bernadette (not her real name), who had shown no outward signs of Alzheimer's and whose youthful autobiography was rich with ideas and grammatical complexity, turned out at death to be riddled with the plaques and tangles of Alzheimer's (see diagram). Says Snowdon: "Lesson No. 1 in my epidemiology training is that there are hardly any diseases where one factor alone, even in infectious disease, will always cause illness."
These results posed a chicken-and-egg problem: Did higher brain capacity protect the sisters from developing the symptoms of dementia, or were those with lackluster biographies already suffering very early signs of some brain abnormality that predisposed them to mental decline later? That question remains unanswered--but follow-up studies, to be published next month in the journal Psychology and Aging, suggest that exercising what brain capacity you have offers some protection. While all the sisters show age-related decline in mental function, those who had taught for most of their lives showed more moderate declines than those who had spent most of their lives in service-based tasks. And that, says Kemper, supports the commonsense idea that stimulating the brain with continuous intellectual activity keeps neurons healthy and alive. (Of course, notes Snowdon, these activities are not absolute protectors. For some, a genetic predisposition may override even a lifetime of learning and teaching.)