Julian travels to the Bellows’ home state and finds evidence that Americans are lardy. And indeed they are. But not evenly so. He links to this convenient map to show the uneven distribution of American girth. This prompts Julian to note:

So maybe, instead of talking about the American obesity “epidemic,” we should be talking about the narrower problem of obesity in specific regions where, for instance, lower population density means people are apt to drive rather than walk when they go out day-to-day.

Except evidence seems to show that while sprawl, or low-density living, is correlated with obesity, it doesn’t necessarily cause it. Rather, hefty persons choose to live in sprawling or low-density areas so they don’t have to walk so damn much. Now there are a number of ways to look at such a finding, but my personal interpretation is this: for an obese person, moving about involves a certain amount of disutility. The more obese you are, the less you want to do it. This cost of obesity is significantly reduced by growth in auto-friendly developments, and this encourages people to consume more obesity, so to speak. If automobile travel is improperly underpriced, due to a failure to account for pollution and congestion externalities, then far too much low-density development will take place, leading to overconsumption of obesity.

In other words, if we passed a carbon tax and that led to a general increase in urban density and alternate forms of travel, then obesity would fall. Not necessarily because people would be walking more, but because the added cost of driving would force people to drive less and walk more–and more walking would make obesity a less attractive option. So to speak.