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Your income and social status are written all over your body, Adam Drewnowski says.

Drewnowski is an epidemiologist. He directs the Center for Obesity Research at the University of Washington in Seattle, and he lectures frequently. "I can pretty much guess the income of an audience by the number of obese women in the room," he says.

"If one-third of my audience is obese, I don't think, 'Oh, my God, these are people with weak willpower or who made bad choices.' I say, 'These are women who do not make more than $40,000 a year.'"

Poor women pack on pounds

Several studies, including Drewnowski's, show that poorer Americans are more likely to be obese than those who are wealthier. But when you look deeper into these studies, University of California, Davis, nutritionist Marilyn Townsend says, the difference is mostly among women: Poor women are much, much more prone to obesity than their wealthier counterparts.

The discrepancy between men and women isn't well understood. The drive for status may be what keeps wealthier women thinner, suggests Townsend, who specializes in behavioral change and works with other researchers, including Drewnowski.

Often, the obese are blamed for being morally weak, says a 2010 study in the International Journal of Obesity.

"People say things like, 'Why don't they just make a cheap pot of lentil soup and live on it for a week?'" Drewnowski scoffs. "Obesity is not the result of excessive indulgence; it's a symptom of lower socioeconomic standing in this country."

Lower-cost diets that are high in fat and sugar and lower in nutrients are more often consumed by people with lower education and incomes, Drewnowski says in research published last year in the American Journal of Clinical Nutrition.

In smaller studies in Seattle, Drewnowski has found obesity as low as 6% in wealthy neighborhoods and as high as 30% in poor areas. (Obesity is a body mass index over 30; you can calculate your own body mass here.)

Drewnowski has even linked obesity to grocery stores: Customers of lower-cost Seattle supermarkets were up to 10 times more likely to be obese. As much as 40% of shoppers at cheaper grocery stores, such as Safeway, were overweight, compared with as little as 4% at stores such as Whole Foods.

"The minute you move to an area served by Wal-Mart -- because they place themselves in lower-income areas -- you will be surrounded by obesity," Drewnowski says.

The point is not that a particular store makes you fat or thin but that people who eat cheaply are prone to getting fat. Those who spend more generally are thinner.

There's "almost a straight-line correlation between wealth of the neighborhood and obesity among women," Drewnowski says. The 2012 federal poverty line for a family of four in the Lower 48 states is $22,050; that takes in nearly one in six Americans.

A spreading epidemic

Five years ago, only one state -- Mississippi -- had an obesity rate of more than 30%. But by 2010, 12 states had joined that club, according to a report published last year by the Robert Wood Johnson Foundation and Trust for America's Health, two nonprofit health advocacy organizations.

America's obesity problem is worst in the South. It's least bad in the Northeast and West. But here's how fast it's growing: The leanest state in 2011, Colorado, with an adult obesity rate of 19.8%, would have been the fattest in 1995. Two decades ago, not one state had a rate above 15%.

The two health advocacy organizations, among others, also link the problem with poverty. Their report says 33% of American adults earning less than $15,000 per year are obese, compared with 24.6% making more than $50,000.

Mississippi, with an obesity rate of 34.4%, is the poorest state in the union. Its poverty rate was 22.4% in 2010, the Census Bureau reported recently.