Georgia is the home state of cornbread, barbecue, peach pie, and a whole lot of really fat people—21.1 percent of the state’s population, to be exact, is obese. Ten years ago, the cornbread, barbecue, and peach pie were all just as good as they are now, but only 9.5 percent of Georgians were obese. What changed?

Whatever changed, it changed everywhere. Obesity is skyrocketing in every age group, in every race, in both genders, and in every state of the union. The most obese region in the country remains the South—four of the five most obese states are below the Mason-Dixon line—but the spectacular recent growth in obesity is nationwide, led by Georgia but followed by New Mexico, Virginia, California, and Vermont. In 1991, a little over 12 percent of the country was obese; by 1999, it was almost 20 percent. (Click here to see whether you’re part of the trend.)

OK, so what’s changed in the past decade or so? Well, one thing that’s changed is the portion sizes at fast food joints like McDonald’s. In 1970, McDonald’s offered one size of French fries; today that size is called “small.” Eventually, it introduced a new size and called it “large”; today that size is called “medium.” There’s a new larger large size, but you don’t have to settle for that—you can always “supersize it” and go a step beyond large.

So, are we fatter because we’re being fed more? Not so fast: Bigger portions don’t necessarily translate into bigger meals. When portions were small, you could buy two orders of fries and eat them both; now that portions are enormous, you can buy one supersized order and share it with your family. How much of that goes on? We simply have no idea.

And even if people are eating more fries these days, there’s still a which-came-first-the-chicken-tenders-or-the-Egg-McMuffin question: Do big meals cause obesity or does obesity cause big meals? Did McDonald’s decide on a whim to fatten us up or did its market research reveal that bigger customers were demanding bigger portions? My money’s on the latter. After all, McDonald’s was presumably just as greedy in 1970 as it is today, so if we had wanted supersizes back then, we’d presumably have gotten them. That means we still have to figure out why people want to be fatter now than in the recent past.

Well, what else has changed? Here’s a thought: Ten years ago, you couldn’t read a magazine without walking to the newsstand or at least to the mailbox; today you’re reading Slate from the same chair where you work, chat with friends, and do half your shopping. Did Bill Gates make us fat?

The facts suggest otherwise: Obesity tends to be highest in states where computer ownership is lowest, and that’s true even after you control for income. Furthermore, increases in obesity tend to be highest in states where increases in computer ownership are lowest. So, the evidence goes against the computers-as-instruments-of-the-devil theory. Instead, computers seem to keep us trim—maybe because they’re so fascinating that we forget to eat, or maybe because we burn calories in silent rage every time the system crashes.

What about smoking habits? A lot of people have quit smoking lately, and maybe that’s what’s making them fat. After all, those who quit often eat more to satisfy their oral fixation. But the numbers tell a different story. It’s true that in the 25 states where smoking fell during the ‘90s, obesity rose 55 percent—but in the 25 states where smoking increased, obesity rose by an even greater 59 percent. Minnesota, where smoking plummeted faster than anywhere else, ranked only 38th in obesity growth. New Mexico, which led the nation in new smokers, ranked second in obesity growth.

So, what else has changed? Incomes have risen, but that cuts both ways: As we get richer, we can afford more food, but we can also afford better quality food and better quality health clubs. On net, there is no statistically significant correlation (in either direction) between changes in income and changes in obesity.

Which brings us back to the same question: What else changed? What happened in the early ‘90s that could have triggered an obesity epidemic? Did the advent of Rush Limbaugh make obesity stylish? Did the Americans With Disabilities Act make obesity less of a handicap in the job market?

Here’s one plausible story: The ‘90s saw the advent of drugs like Pravachol and Lipitor that can dramatically cut your cholesterol and increase your life expectancy. With medical advances like that, who needs to be thin? Of course obesity is still bad for you—but it’s not as bad for you as it used to be. The price of obesity (measured in health risks) is down, so rational consumers will choose more of it.

With the success of the human genome project, even greater advances are just over the horizon, making obesity an even greater bargain. Today’s expanding waistlines might reflect nothing more than a rational expectation of future progress against heart disease.

If you don’t like that story, here’s another: The ‘90s were the era of low-fat foods. At fewer calories per serving, it makes sense to eat more servings. The net effect can be either an increase or a decrease in weight. For example: Suppose a scoop of ice cream a night would add 10 pounds to your weight, and you’ve decided that’s not worth it, so you don’t eat ice cream. Now along comes a low-fat ice cream that allows you to eat two scoops a night and add 10 pounds to your weight. That’s a better deal, and a perfectly rational being might well opt for it. So when low-fat foods come along, some people sensibly decide to become fatter. (Other people, equally sensibly, use low-fat foods to become thinner. Therefore, the overall impact of low-fat foods on obesity could in principle go in either direction.)