As Waistlines Widen, Brains Shrink
An interesting example of a headline grabbing attention but no one questioning the methods. They seem to have found a link by looking at people as they are now. What is only vaguely touched on in the article is that what may be the cause if a lifetime of inactivity, mental and/or physical.
As these people are larger, you could argue they have been too lazy to exercise, and therefore you could make the assumption that they're lazy and so haven't given their brains a workout over the years either and that is the reason their brains are smaller/older, not because of their weight.
I don't know, I'm just putting another side to this, you'd have to look at the research to see what they did, but if they just measure a person now, it's impossibly to say what caused it.