Harvard researchers published in the  British Medical Journal  a study of which occupations lowered the rate of Alzheimer's deaths. They looked at American Alzheimer's death records of more than 400 occupations, including when they died. The two occupations with the lowest rate of Alzheimer's death were taxi and ambulance drivers. More specifically, these two jobs had death rates 56% lower than the general population at any given age. An earlier British study provided some clues. Since 1865, London taxi drivers had too pass what's known as "the Knowledge," a grueling test of a person's memory of thousands of streets, landmarks and routes within the city. They had to study for years to become qualified. In 2000, London neuroscientists compared MRI brain scans of cabbies and other occupations, and  found that the former had changes in the hippocampus, the brain region dedicated to memory and navigation.   The longer cabbies worked, the larger their hippocamp...