NEW YORK (Reuters Health) – Obesity may be contagious because most people feel good about themselves if they are about as heavy as the people around them, according to new research from an international team of economists.

This could explain the rapid rise in the prevalence of overweight around the world, the researchers say. That is, the norm that most people compare themselves to has become fatter and fatter, feeding a cycle of “imitative obesity.”

“What we’re finding is that human beings are probably driven tremendously by comparison. Unless you understand those comparisons, you’re not going to understand the rate of obesity,” Dr. Andrew J. Oswald of the University of Warwick in the UK told Reuters Health. “Understanding the sociology of obesity is much more important than understanding the biology.”

Last year, Oswald and his team note, Drs. Nicholas Christakis of Harvard and James Fowler of the University of California, San Diego published a study showing that people were more likely to become obese if their friends and family members were heavy.

In the current analysis, which they presented at the National Bureau of Economic Research conference July 25 in Cambridge, Massachusetts, Oswald and his colleagues attempt to use an economic model to show why this happens. They analyzed data from several sources on body mass index (BMI) and people’s perception of their weight for 29 European countries.

More than one-third of Europeans think they are too fat, the researchers found, and people who are more educated were more likely to think they are overweight.

The researchers also found that for women, satisfaction with their weight depended on their own BMI in relation to the average BMI for a woman of their own age living in the same country. For their part, men who were overweight tended to be happier if the people around them were overweight too.

The link between people’s relative BMI and their general life satisfaction is likely unconscious, according to Oswald. “They may not be aware of it. Our computers can trace out these patterns without the individual necessarily knowing them.”

So the average person doesn’t mind being overweight if people around him are too; hence he is “keeping up with the fat Joneses,” Oswald explained.

However, for “high-status” individuals, being thin is becoming more and more important, he added; this may explain the rise of super-skinny models and actresses, as well as the prevalence of anorexia among upper-middle class girls and boys.

It might be possible to change people’s weight-related norms by having them look at images and movies from decades ago — when people were, on average, 20 pounds lighter, Oswald suggested.

“They don’t have to be 220 pounds,” he said. “Their parents got on fine in their life at similar ages weighing many pounds less.”

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