Here's a fact that shows just how quickly obesity has taken over America: In 1990, no state had an obesity prevalence of 15 percent or more. By 2010, no state was less than 20 percent obese.

Adult Obesity Prevalence in 1990 and 2010


But what's driving the epidemic? We have no answers, only theories. Some studies say it's food deserts; others say it's desk jobs. Some even claim that it's the decline in smoking, since tobacco suppresses appetite.

Along with three other researchers, Charles Courtemanche, an assistant professor of economics at Georgia State University, analyzed a number of these theories for a recent study.

Courtemanche realized that a lot of past studies came to conclusions like, "the increasing popularity of driving to work is correlated with the rise of obesity." But he wasn't sure what, specifically, was having the greatest effect on obesity's rise.

He gathered 27 things he thought might be contributing to obesity—everything from working hours to exercise trends to gas prices—and put them in what he calls a "statistical horserace." Essentially, he took all the studies that associated some factor with obesity and saw what would happen if he held everything else constant, like demographic or economic changes. (For example, he told me, "If you're just looking at cigarette prices, was it really cigarette prices, or is it just that they're related to food prices and gas prices? It's controlling for everything together.)

Only two of the factors ended up being meaningful drivers of obesity: 1) the proliferation of restaurants and 2) the rise of warehouse clubs, like Costco, and super-centers, like those Walmarts that have grocery stores in them.

As you can see from the chart, both restaurants and super-centers saw a remarkable growth since 1990, and together they explain about half the rise in class II and class III obesity—the worst varieties.

Changes in Food Store Numbers


Interestingly, regular supermarkets actually had a slight negative effect on obesity rates, so it's not just that food has become more accessible. Instead, Courtemanche said, it's that it's become much, much cheaper. At a place like Costco, you can save big bucks by buying pounds upon pounds of peanut butter all at once. Meanwhile, "Walmart super-centers undercut their competitors by 10 to 30 percent," he said. "They're really telling the truth when they talk about 'always low prices.'"

Restaurants, meanwhile, save customers a different kind of cost: time.
"You're talking about switching from seeing a restaurant as an occasion, to an environment where restaurants are everywhere, and you're always driving by one, so there's no time cost anymore," he said. "It's very easy to get restaurant food."

This doesn't mean that food should become more expensive, and Courtemanche admitted that his wife still does most of their food shopping at a Walmart. But the government could use this information—the fact that we love cheap, easy food ... maybe a little too much—and use it to regulate menu labels and calorie counts. (It's already doing this a little bit.) Or it could subsidize healthy food and tax junk food, so that peoples' overall grocery bills are still low, but they get nudged toward the carrots and away from the Oreos.

It is kind of a depressing finding, though, that obesity, which costs our healthcare system so much money, is happening because we're getting the food we love for so little.

"We'd all like something to be all good or all bad," Courtemanche said, "and the reality is that often there are things that are mostly good but have some adverse consequences."