Epidemiologist and one of the world's leading experts on Alzheimer's disease, David Snowdon. In 1986 he began what he calls the "Nun Study," following a group of aging nuns to better understand why some of the sisters were able to age gracefully, retaining their mental faculties, and others were not. He studied 678 nuns who belonged to The School Sisters of Notre Dame. His study was published in The Journal of Personality and Social Psychology. It's also the subject of Snowdon's book, Aging with Grace: What the Nun Study Teaches Us About Leading Longer, Healthier, and More Meaningful Lives. Snowdon is Professor of Neurology at the University of Kentucky Medical Center.