Landmark study links body weight to the people around you

Whether you are a size 4 or a 24 may depend on the friends you keep.


U.S. researchers have found evidence obesity is socially contagious and weight gain can spread from person to person through a social network in a domino effect.


The study, the first of its kind to look at how complex social environments affect obesity, revealed a person’s risk of gaining weight increases by 57 per cent if one of their friends becomes obese.


A cascade effect is seen in up to three degrees of separation, to a person’s friend of a friend of a friend. Surprisingly, researchers found friends have more influence on weight than siblings or spouses.

“It’s not that birds of a feather flock together, that obese people hang out with obese people and normal-weight people hang out with normal-weight people,” said Nicholas Christakis, a physician and professor at Harvard Medical School and a study co-author. “We found evidence that one person’s weight gain causes another person’s weight gain, that they actually influence each other.”

People may pass health behaviours — good or bad — from person to person to person, causing the social spread of obesity, Christakis added. “So, if I eat badly, then you eat badly, and if I gain weight, you gain weight.”

But, he added, it’s more likely social norms are shifting over time, so weight gain among a group of friends becomes more acceptable, which then changes attitudes about acceptable body size.

“We were surprised that people to whom you are directly connected have such a strong effect on your obesity outcome — and how far that effect seems to spread,” said study co-author James Fowler, assistant political science professor at San Diego’s University of California.

Published yesterday in the New England Journal of Medicine, the study is getting rave reviews from obesity researchers, who say the findings highlight new ways of combating the obesity epidemic.

Social ties

  • Researchers analyzed 32 years of data from the long-running Framingham Heart Study. After two years of work, they were able to recreate the social ties of more than 12,000 people in a network map and immediately found clusters of thin people and obese people.