I am using Undercover Economist with my general education class, which I love, and now I'm reading all of Tim Harford's FT columns. The one this weekend asks why are Americans and the world more generally so much heavier than a century ago? He cites evidence that even a second wage earner working 10 hours a week makes raises the chances of childhood obesity by 1.3%. But that won't explain my own waistline. He offers two explanations:

First, the cost of exercise has risen: most of us used to be paid to burn off calories in physically demanding jobs, after all. It is hard to undercut a form of exercise that pays you, and modern fitness clubs haven�t tried.

Second, food technology has tipped the balance in favour of more snacking. Think of the humble potato, once consumed in bland form, boiled or in stews - it was messy and time-consuming to make chips. But industrial processing, freezing and vacuum packing now makes crisps and chips easy to enjoy at home or in a fast-food joint. It is not just that crisps are more calorific than boiled spuds, but that they can be conveniently eaten at any time of day. Despite the attention devoted to �supersize� portions, the calories consumed at main meals have actually declined.

If the marginal cost of snacking has declined, it makes sense that there would be more snacking. And God knows how many times I have driven past the gym saying "I should go there, but I have to (fill-in-the-blank)" first. It's in fact logical that as I work harder and take on more at home, I've done less exercising -- damned near none since I became a department chair. Maybe I would eat more fruits and don the track shoes if I wasn't chair?