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NEW YORK - Carrying a spare tire or two around the waist has become socially acceptable in the United States as the population's waistlines have expanded, according to a study released yesterday.

Economic researchers from Florida State University and the Federal Reserve Bank of Boston found the weight of the average woman rose by 20 pounds (9.2 kilograms) or 13.5 per cent between 1976 and 2000 - but their ideal weight also edged up.

In 1994 the average woman tipped the scales at 147 pounds (66.8 kilograms) but she wanted to weigh only 132 pounds - but less than a decade later the average woman weighed 153 pounds but said her desired weight was 135 pounds.

"This is a social force that we are trying to document because the rise in obesity has occurred so rapidly over the past 30 years," Frank Heiland, an assistant professor of economics at Florida State University, said in a statement.

"Medically speaking, most agree this trend is a dangerous one because of its connection to diabetes, cancer and other diseases. But psychologically, it may provide relief to know that you are not the only one packing on the pounds."

Heiland and Federal Reserve Bank of Boston economist Mary Burke, who reported their findings in the journal Economic Inquiry, said people had adjusted their perceptions of what is normal body weight as the population's weight ballooned.

They said this was one of the first studies to suggest that weight norms may change and are not set standards based on beauty or medical ideals.

Heiland said according to 2001-2004 data, 33.2 per cent of American women over the age of 20 are classified as obese.

But he said the fact that even women's ideal weight had increased suggested there was less social pressure to lose weight.

He cited a previous study in which 87 per cent of Americans, including 48 per cent of obese Americans, believed their body weight fell in the "socially acceptable" range.

The researchers believe a combination of social, economic and biological factors have contributed to the expanding size of Americans.

Their findings are based on information from the National Health and Nutrition Examination Survey and the Centers for Disease Control and Prevention.