The Official Newsletter of
updated May 31, 2010
Is Obesity 

by Ron Brown, author of The Body Fat Guide 

"Ron Brown is a certified fitness trainer who doesn't have an inch of flab on his body. He'll tell you what you can do to become fit and trim too." 
Washington DC

IS OBESITY CONTAGIOUS? Is the obesity epidemic that is plaguing developing countries caused in part by people catching obesity from each other, much as they catch a contagious disease during an infectious epidemic? Some researchers believe the answer to this question is yes—people can pass along an unhealthy amount of excess bodyweight to others, although not through microorganisms. Rather, these researchers believe obesity can be transferred from one person to another through social factors. 

In approaching the problem of obesity, Blanchflower, Oswald, and Landeghem (2008) hypothesized that, for most people, satisfaction with one’s own bodyweight is relative to other people’s weight, and  would be dependent on how much other people weighed. In other words, if you are overweight, and if people around you tend to be overweight, you will be more satisfied with your own weight when comparing your weight to other people, rather than if people around you tended not to be overweight. In addition, the researchers hypothesized that people subconsciously adjust their lifestyle behaviors to allow their weight to keep up with their overweight neighbors, which the researchers described as “keeping up with the weight of the Joneses.”

In the words of these researchers, “…it may be easier to be fat in a society that is fat.” Viewed in this manner, efforts to break away from the norm, and remain as slim or slimmer while the population grows increasingly overweight, may be considered deviant behavior. Nevertheless, even slim people may be affected by the population’s rise in bodyweight. For example, if a slim person allows his or her weight to rise by the same percentage as the rest of the population, he or she would still derive satisfaction from remaining relatively slim in comparison to other people’s weight. Nevertheless, the researchers noted that some slim people, rather than allowing their weight to rise with the general tide, will attempt to directly oppose the trend within the population, and endeavor to become even thinner than before. The researchers found that these people tend to be more educated and wealthier, and they tend to set their own standards—without following the herd. The researchers suggested that socially contagious forms of obesity are possible, and that "imitative obesity" warrants further study.

American researchers Christakis and Fowler (2007) made a more in-depth examination of the person-to-person mechanisms by which obesity spreads in a social network. These researchers analyzed data from the Framingham Heart Study in Massachusetts , which consisted of a densely interconnected social network of 12,067 subjects who were periodically assessed for 32 years between 1971 and 2003. Weight gain was monitored during the Framingham study,  and Christakis and Fowler analyzed this data to determine if a subject’s weight gain was associated with weight gain in the subject’s friends, spouse, neighbors, and siblings. 

Christakis and Fowler hypothesized that if a person has a significant social relationship with someone who becomes obese, it might change a person’s point of view toward obesity, increase that person’s acceptance of obesity, and even cause that person to imitate similar eating and sedentary behaviors of their obese friend or family member.

In their analysis, Christakis and Fowler examined clusters of obese people who were closely connected to others within the social network. The researchers hypothesized that cluster formation among obese subjects was due to the social influence of people in mutual friendships. In comparison to random clusters generated in their social network model, the researchers found strong evidence to support this hypothesis. The researchers also discovered that the influence of obesity in clusters extended to friends of friends, and even to the friends of friends of friends! In other words, the influence of obesity in a mutual friendship extended to a depth of three degrees of separation.

The results of the study by Christakis and Fowler showed that having a mutual friend who became obese increased the chances of a person becoming obese by 171%. Friends of the same gender also had a stronger influence than friends of opposite gender. However, there was no increase in the social influence of opposite-sex friendships. Interestingly, among same-sex friendships, males had a stronger probability, 100%, of becoming obese if a mutual male friend became obese, while females had an insignificant probability of becoming obese due to same-sex friendships.

Christakis and Fowler found that opposite-sex siblings did not influence each other to become obese. However, this was not the case among same-sex siblings. The social influence between two brothers increased the chances of one brother becoming obese in response to the obesity of the other brother by 44%. Among sisters, the chances of an obese sister influencing the other sister to become obese were increased by 67%. Among married opposite-sex couples, an obese husband increased a wife’s probability of obesity by 44%, and an obese wife increased the husband’s probability of becoming obese by 37%. 

Another significant finding by Christakis and Fowler was that the physical distance between mutual friends did not change the strength of the social influence. This suggested to the researchers that the social influence of obesity was based on beliefs and attitudes rather than on observing behaviors. To illustrate, if a male travels to visit his best male friend, whom he hasn’t seen in years, he may be shocked to see that his best friend has recently put on 100 pounds or more, and has become obese. This may influence the male to be less concerned about his own weight in comparison to his friend, whom he still holds in high esteem. The result might be that the male relaxes restrictions on his own eating habits, and allows himself to put on 100 pounds also, but he rationalizes his behavior by saying, “At least I’m no heavier than my best friend!”

After publicizing their study, Christakis and Fowler were falsely accused by the media of advocating that people should drop their obese friends, which prompted very bad reactions from the public. In response, the researchers went back to the data to look at the outcome of people who dropped their friends who had become obese. Rather than improving their outcome, the people who dropped their friendship actually became more obese themselves. The researchers emphasized that getting rid of obese friends, and reducing one’s social network, is not the answer to prevent the social effect of the spread of obesity. 

When Christakis and Fowler examined other data that showed social clusters of smokers, they noticed that whole

clusters of smokers disappeared over time. This reduction in smokers coincided with the decrease of smoking within the population. However, the researcher’s analysis showed that when people quit smoking, they usually influenced others within their social network to do the same, and the entire cluster quit together. This type of social influence provided an example of how social networks can improve health behavior. These supportive social networks can play an important role in obesity prevention, and in other health interventions where groups of people gather, such as in schools, the workplace, and community organizations. 

An earlier research study by Wing and Jeffery (1999) examined the benefits of recruiting friends to increase social support for weight loss and weight maintenance. This study included 166 participants who were divided into two groups. In one group, participants followed the weight loss program alone, while participants in the other group followed the program with three friends or family members. All participants received a standard behavioral treatment, but the participants in groups of friends or family members also received social support strategies. After the four-month treatment, and six months later, participants who recruited the support of social groups had greater weight losses. In addition, among the single participants, 76% completed their treatment, and 24% maintained their weight loss six months after treatment, while 95% of participants in groups with friends or family members completed their treatment, and 66% of these participants maintained their weight loss after six months. 

In conclusion, research evidence demonstrated that social networks can influence the epidemic-like spread of obesity through an effect that has been called imitative obesity. However, this social network influence also has great potential to be harnessed in a positive way to help spread preventive interventions that stop and reverse the obesity epidemic. While two thirds of the U.S. struggles to find ways to avoid the increased chronic disease risks of obesity and excess bodyweight, it is important to educate people about the advantages available to them through supportive social networks.


Blanchflower, D. G., Oswald, A. J., & Landeghem, B. V. (2008). Imitative obesity and relative utility. National Bureau of Economic Research, Working Paper No. 14337.

Christakis, N. A., & Fowler, J. H. (2007). The spread of obesity in a large social network over 32 years. The New England Journal of Medicine, 357, 350–379.

Wing, R. R., & Jeffery, R. W. (1999). Benefits of recruiting participants with friends and increasing social support for weight loss and maintenance. Journal of Consulting and Clinical Psychology, 67, 132–138.



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