Elizabeth Kolbert, writing in the New Yorker, asks a simple question: Why are we so fat? The short answer -- We eat too damn much -- is unbefitting a proper New Yorker essay, and so we're served a long, fascinating look at the literature of "weight-gain" books that aim to explain how Americans gained more than a billion pounds in the last ten years. The most convincing answer I found is: Price and elasticity of appetite.

Kolbert, reviewing The Fattening of America by Eric Finkelstein and Laurie Zuckerman, notes that food has gotten cheaper relative to the other goods and services. And fatty foods have gotten a lot cheaper. The price of soft drinks, for example, have dropped by more than 20 percent, and they now account for seven percent all calories in the US. The New York Times offered a striking graph that told just that story:

But, Kolbert follows up with a good question: Sure, food became cheaper -- fatty food especially. But that doesn't mean our bellies had to grow at an equal and opposite pace. Why didn't we just spend less money easting less? How did our appetites grow so suddenly in the 1980s and 90s?

The most convincing idea she floats is the "elasticity of the human appetite." Simply stated: We have no idea how hungry we are, so we respond to growing portion sizes with growing appetites. In one depressing experiment, participants ate bowls of pasta soup that were secretly being refilled through a hidden tube. On average, people eating out of these "trick bowls" consumed 73 percent more than the regular bowl eaters. Somewhere between our brain and our stomach, the word "stop" is swallowed -- along with a lot of calories.

The elasticity of our appetites is perhaps most visible in our relationship with fast food and chain restaurants. McDonald's used to offer one size of french fries, with 200 calories. Now it also offers a large, with 500 calories. This jives with a graph, also produced by the New York Times, that compares time spent eating and national obesity.

The conclusion I would draw is not that you should spend more quality time with your Burger King Quad Stacker. I'm more interested in the relationship between the fatter countries and fast-food proliferation. If you draw a horizontal line just under Canada to isolate the countries with the highest obesity rates, you get a group of six countries. Four of those countries -- the US, New Zealand, Australia and Canada -- happen to be the four countries with the most McDonald's per capita in the world (the UK is number 9). Our evolutionary instinct to maximize caloric intake befriends our economic instinct to buy cheap, and the result is, well, something like this.

*Thanks to Suzanna Pacaut for soup/pasta mixup. Pasta, in retrospect, could be prohibitively difficult to refill through a tube.