If your friends are obese, ...

If your friends are obese, your risk of obesity is 45 percent higher. ... If your friend's friends are obese, your risk of obesity is 25 percent higher. ... If your friend's friend's friend, someone you probably don't even know, is obese, your risk of obesity is 10 percent higher. It's only when you get to your friend's friend's friend's friends that there's no longer a relationship between that person's body size and your own body size.
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 Nicholas A. Christakis

Quotes from the same author

Social media and the Internet haven't changed our capacity for social interaction any more than the Internet has changed our ability to be in love or our basic propensity to violence, because those are such fundamental human attributes.
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 Nicholas A. Christakis
Social networks are these intricate things of beauty, and they're so elaborate and so complex and so ubiquitous that one has to ask what purpose they serve.
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 Nicholas A. Christakis
People have just assumed that... if we call our Facebook acquaintances our friends, we must be influenced by them, too. But we're not.
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 Nicholas A. Christakis
One reason citizens, politicians and university donors sometimes lack confidence in the social sciences is that social scientists too often miss the chance to declare victory and move on to new frontiers.
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 Nicholas A. Christakis