PROBLEM: The brain is a muscle, says every teacher ever. Their point is figurative, but the brain is like a muscle in the sense that it normally loses size with age -- in some parts by as much as 25 percent.
METHODOLOGY: A long time ago, Scotland surveyed the intelligence of every Scottish child that had been born in 1936. More recently, 691 of those former children celebrated their 70th birthdays by filling out a survey about their social and intellectual pursuits and their levels of physical activity. Three years after that, they celebrated their 73rd birthdays by undergoing brain MRI scans at the University of Edinburgh in Scotland. The researchers assessed the brain images for physical signs of cognitive decline.