A headline at MSNBC.com stated ‘‘One way to ward off Alzheimer’s: Take a hike. Study: Walking at least one mile a day reduces risk of cognitive impairment by half.” The article reports on a study showing that elderly people who walked a lot tended to have more brain mass after nine years and were less likely to develop dementia that can lead to Alzheimer’s disease than subjects who walked less. At the start of the study the researchers measured the walking habits of the elderly subjects and then followed up with measures of brain volume nine years later. Assuming that active walkers really did have more brain mass and fewer dementia symptoms, is the headline justified?