Is Obesity Contagious?

A social contagion is a process where the network in which people are embedded, influence their behaviors. 

The idea of obesity being socially contagious is not new.^1,2,4 The idea is this: If you live around people who have healthy behaviors, you might be influenced to have healthy behaviors. However, if you live around people with unhealthy behaviors, you might follow suit - thus increasing your risk of obesity.

It has been shown, for example, that between mutual friends, one's risk of obesity increased by 171% if the friend became obese.¹ So, obesity not contagious, in the communicable sense.

Rather, “obesity may spread in social networks in a quantifiable and discernable pattern that depends on the nature of social ties.”¹ This is how the research explains it. 

So, it is our close, social network that can influence our behaviors that subsequently increase risk of obesity.

However, the recent study behind the recent headlines found that “military families assigned to installations in counties with higher obesity rates were more likely to be overweight and/or obese than their counterparts assigned to installations in counties with lower obesity rates.”³ 

Interestingly, this effect of social influence on our behavior occurred, no matter what type of built environment one lived in. 

Even if I live in a town where there is health foods stores and gyms on every corner, if all my friends and family eat unhealthfully, then I might follow suit - because social distance appears to be more important than geographic distance.

Okay, but how much more likely are they to gain weight when they move to a county with higher obesity rates?

[WARNING!] For those interested, I will try and make this next part as painless as possible.

The likelihood of an effect of the county’s obesity rate on a person’s body weight, is indicated by  this number right here, 0.08. It is called a beta weight, and tells us that independent of the environmental characteristics of the county, an increase in the county obesity rate predicts an increase in an adults body weight. 

Based on this information:

For every 1 standard deviation increase in the independent variable (IV), there is a (beta) standard deviation change in the dependent variable (DV).

IV = County Obesity Rate; 29.77 ± 4.20

DV = Adult BMI; 27.83 ± 4.52

Beta = .08

They found that for every 1 SD increase in County Obesity Rate (4.2 kg/m²), there was a .08 SD (.36 kg/m²) change in parent body mass index (BMI).

Wait, what?

Think of it this way – if a 5’5”, 150 lbs person (a BMI = 25; overweight) moved from a county that the average BMI was 25 (overweight) to 29.2 (more overweight), their weight would increase around 2 lbs to 152 lbs. - maintaining a BMI of 25 (25.36 to be exact).

Conclusion

Thus, the effect is very small - only slightly contagious. Yet, I am calling this one clarify, because obesity is technically not contagious. However, those closest to us, such as friends and family, can influence our unhealthy behaviors, increasing our risk of overweight or obesity OR increasing the opportunity for a healthy lifestyle.

References

1. Christakis, N. A., & Fowler, J. H. (2007). The spread of obesity in a large social network over 32 years. New England Journal of Medicine, 357(4), 370-379.
2. Cunningham, S. A., Vaquera, E., Maturo, C. C., & Narayan, K. V. (2012). Is there evidence that friends influence body weight? A systematic review of empirical research. Social Science & Medicine, 75(7), 1175-1183.
3. Datar, A., & Nicosia, N. Association of exposure to communities with higher ratios of obesity with increased body mass index and risk of overweight and obesity among parents and children. JAMA Pediatrics.
4. Powell, K., Wilcox, J., Clonan, A., Bissell, P., Preston, L., Peacock, M., & Holdsworth, M. (2015). The role of social networks in the development of overweight and obesity among adults: a scoping review. BMC Public Health, 15(1), 996.