The rich are getting fat.

Further complicating attempts to compare income and obesity are cultural factors. Certain racial and ethnic groups positively equate a man's girth with wealth -- it's a sign of success, Drewnowski said.

"I would caution against any attempts to interpret these data to say social differences have disappeared," he said. "It just shows that obesity is a general problem and it's now affecting pretty much everybody. ... But it would be very shortsighted to stop paying attention to the people who are most vulnerable."

Yet today, the obesity remedies most often recommended for Americans in general -- eat fresh salads, go ride a bike -- are impossible for many low-income families, Drewnowski said

Exercise can be hard in inner cities, where the streets may be too dangerous after working hours. Many grocery stores in low-income neighborhoods don't stock expensive fresh produce. And people who work two or three jobs have little time to make home-cooked meals.

Robinson agreed: "I don't want to take focus away from the serious racial and ethnic disparities in health."

But, she said, it's likely that different factors play a role in spurring obesity among the middle class than the poor. "We need to have a lot more research ... to tailor our interventions to specific populations."

1) I'd be interested in how the genetic makeup of the rich has changed in the past 30 years. Not just ethnic changes, but more general changes as well. For instance, are they as smart as they once were?

2) Who's doing the cooking? Sure, an Italian, heaps-of-pasta-on-the-table mother might make you fat. But I'd guess that a career-minded, busy, let-the-kids-scrounge, junk-food-in-the-fridge mother is worse.

3) How many people, as a percentage of the population, are fit because of conscious effort? I'd guess that the numbers must be pretty low. It's hard work. Hell, I ran 80 miles in April and gained 14 pounds because I wasn't watching my diet. It takes a great deal of conscious effort to overcome your environment. The vast majority of people just go with the flow.