"Are your friends making you fat?" demanded headlines displayed on a number of media networks this July in response to a study published by Nicholas Christakis, a professor at Harvard University, and James Fowler, a professor at the University of California, San Diego. Christakis spoke about the study's conclusion, that the fluctuations of a person's weight affect the weight of people in one's social network, in a lecture at the Rockefeller Center on Thursday night.
According to the study, titled "The Spread of Obesity in a Large Social Network over 32 Years," an individual's weight influences the weight of people up to three degrees of separation away from him or her.
This information helps to explain the prevalence of obesity in our society, Christakis said.
"It has become trendy to speak of the obesity 'epidemic; but is it possible to imagine that obesity is truly an epidemic, spreading from person to person?" Christakis asked the audience.
Christakis and Fowler used archival data from the National Institute of Health's Framingham Heart Study to recreate a social network that followed approximately 12,000 individuals over a 32-year period. While the Framingham Heart Study used the network to study cardiovascular disease, Christakis and Fowler compared the weight of the participants to their friends, coworkers, neighbors and other acquaintances.
"We found that there was more clustering [of obese people] than would be expected due to chance alone," he said. "This could include people you don't even know."
The study revealed that spouses, siblings and acquaintances of the same gender all affect a person's weight. Coworkers in offices with six members or fewer also have an impact, while immediate neighbors have no effect.
Geographic distances between members of a network do not change the impact, Christakis noted.
Christakis distinguished between three types of relationships. He termed the person whose weight is under analysis the "ego" and the person whose behavior may influence the ego an "alter." In a mutual friendship, the ego and the alter influence one another; in an ego-dominated relationship, an ego perceives a friendship with the alter; and in an alter-dominated friendship, the alter perceives a friendship with the ego.
The study concluded that in a mutual relationship, if an alter become obese, the ego is 171% more likely to become obese as well, and in an ego-dominated relationship, weight gain is 57% more likely. An alter-dominated relationship had no affect on the ego's size.
"This directional data is especially important because it suggests that obesity obeys a kind of sociological rule," Christakis said. "Some kind of causal thing is going on here."
Addressing a claim that the study encourages people to form friendships based on weight, Christakis said that a person's change in weight, not one's initial weight, affects the ego.
"You could be connected to a thin person who is gaining weight, and that will contribute to your gaining weight," Christakis said. "Unlike what the headlines said, you should not 'ditch your fat friends.'"
Christakis' statement clarified media claims for some attendees.
"There was definitely an emphasis in the media about making thinner friends because that will help you," Eliana Fishman '11 said. "Logically, that doesn't make sense."
