ABC News explores some possible causes of obesity that are often overlooked.

Sure, most of the nation needs to eat less and move more. But is that the only reason America is so fat?

As more scientists and sociologists look at our bulging waistlines, some unusual explanations for the nation’s weight gain in the last 30 years are popping up.

The article discusses an intestinal bacteria that may contribute to weight gain and particular genes that may influence the success or failure of dieting.

Beyond these physical explanations, social factors may also contribute to obesity. A Harvard medical sociologist weighs in:

In 2007, an article in the New England Journal of Medicine used 30 years of data on 12,000 people to show obesity and weight loss may actually be contagious — things that spread among people who know each other.

“They key idea is that people are influenced by the behavior and actions of those around them. This applied to something that people may not have thought of, which is body size,” said Dr. Nicholas A. Christakis, author of the recent book “Connected,” which looks at how various phenomena from depression to obesity spreads through society.

Over the three decades, Christakis showed how obesity in one person in a circle of friends statistically meant more people in their circle of friends would become obese. The same was true of weight loss.

“We’re not saying we found the cause of the obesity epidemic. We’re not,” said Christakis. “Social networks have a general property that they magnify what they are seated with.”

While Christakis could show an obesity epidemic spreading through friend networks, he could only make an educated guess why.

“One possibility is that you start doing things — certain behaviors that I copy,” said Christakis.

So if one friend starts serving beer and cookies all the time, perhaps another friend will pick up the habit. Or if one friend joins a running club, perhaps another friend will join it too.

Christakis said another possibility is that “What’s spreading between people is an idea, or a norm.”

For example, if most people a person associates with are overweight, then that person’s idea of “normal weight” is likely to be bigger than what is actually healthy.

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