The n e w e ng l a n d j o u r na l of m e dic i n e special article The Spread of Obesity in a Large Social Network over 32 Years Nicholas A Christakis M D Ph D M P H and James H Fowler Ph D A BS T R AC T Background From the Department of Health Care Policy Harvard Medical School Boston N A C the Department of Medicine Mt Auburn Hospital Cambridge MA N A C the Department of Sociology Harvard University Cambridge MA N A C and the Department of Political Science University of California San Diego San Diego J H F Address reprint requests to Dr Christakis at the Department of Health Care Policy Harvard Medical School 180 Longwood Ave Boston MA 02115 or at christakis hcp med harvard edu The prevalence of obesity has increased substantially over the past 30 years We performed a quantitative analysis of the nature and extent of the person to person spread of obesity as a possible factor contributing to the obesity epidemic N Engl J Med 2007 357 370 9 Results Copyright 2007 Massachusetts Medical Society Methods We evaluated a densely interconnected social network of 12 067 people assessed repeatedly from 1971 to 2003 as part of the Framingham Heart Study The bodymass index was available for all subjects We used longitudinal statistical models to examine whether weight gain in one person was associated with weight gain in his or her friends siblings spouse and neighbors Discernible clusters of obese persons body mass index the weight in kilograms divided by the square of the height in meters 30 were present in the network at all time points and the clusters extended to three degrees of separation These clusters did not appear to be solely attributable to the selective formation of social ties among obese persons A person s chances of becoming obese increased by 57 95 confidence interval CI 6 to 123 if he or she had a friend who became obese in a given interval Among pairs of adult siblings if one sibling became obese the chance that the other would become obese increased by 40 95 CI 21 to 60 If one spouse became obese the likelihood that the other spouse would become obese increased by 37 95 CI 7 to 73 These effects were not seen among neighbors in the immediate geographic location Persons of the same sex had relatively greater influence on each other than those of the opposite sex The spread of smoking cessation did not account for the spread of obesity in the network Conclusions Network phenomena appear to be relevant to the biologic and behavioral trait of obesity and obesity appears to spread through social ties These findings have implications for clinical and public health interventions 370 n engl j med 357 4 www nejm org july 26 2007 Downloaded from www nejm org at LIBRARIES OF THE UNIV OF COLORADO on August 2 2007 Copyright 2007 Massachusetts Medical Society All rights reserved The Spread of Obesity in a Large Social Network Over 32 Years T he prevalence of obesity has increased from 23 to 31 over the recent past in the United States and 66 of adults are overweight 1 2 Proposed explanations for the obesity epidemic include societal changes that promote both inactivity and food consumption 3 The fact that the increase in obesity during this period cannot be explained by genetics4 5 and has occurred among all socioeconomic groups1 provides support for a broad set of social and environmental explanations Since diverse phenomena can spread within social networks 6 10 we conducted a study to determine whether obesity might also spread from person to person possibly contributing to the epidemic and if so how the spread might occur Whereas obesity has been stigmatized in the past attitudes may be changing 11 12 To the extent that obesity is a product of voluntary choices or behaviors the fact that people are embedded in social networks and are influenced by the evident appearance and behaviors of those around them suggests that weight gain in one person might influence weight gain in others Having obese social contacts might change a person s tolerance for being obese or might influence his or her adoption of specific behaviors e g smoking eating and exercising In addition to such strictly social mechanisms it is plausible that physiological imitation might occur areas of the brain that correspond to actions such as eating food may be stimulated if these actions are observed in others 13 Even infectious causes of obesity are conceivable 14 15 We evaluated a network of 12 067 people who underwent repeated measurements over a period of 32 years We examined several aspects of the spread of obesity including the existence of clusters of obese persons within the network the association between one person s weight gain and weight gain among his or her social contacts the dependence of this association on the nature of the social ties e g ties between friends of different kinds siblings spouses and neighbors and the influence of sex smoking behavior and geographic distance between the domiciles of persons in the social network Network Ascertainment For our study we used the offspring cohort as the source of 5124 key subjects or egos as they are called in social network analysis Any persons to whom the egos are linked in any of the Framingham Heart Study cohorts can however serve as alters Overall 12 067 living egos and alters were connected at some point during the study period 1971 to 2003 To create the network data set we entered information about the offspring cohort into a computer This information was derived from archived handwritten administrative tracking sheets that had been used since 1971 to identify people close Glossary Ego The person whose behavior is being analyzed Alter A person connected to the ego who may influence the behavior of the ego Node An object that may or may not be connected to other objects in a network In this study nodes represent people in the Framingham Heart Study cohorts Tie A connection between two nodes that can be either one way directed or two way bilateral In this study all family ties e g between siblings and parents as well as marital ties are bilateral but friendship ties are directional since a subject may identify someone as a friend who does not necessarily identify that person as a friend in return Degree of separation The social distance between two people as measured by the smallest number of intermediaries between an ego and other members of the network For a given ego alters are degree 1 since they are directly connected to the ego Nodes