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Original Investigation
March 2018

Assessing Social Contagion in Body Mass Index, Overweight, and Obesity Using a Natural Experiment

Author Affiliations
  • 1Center for Economic and Social Research, University of Southern California, Los Angeles, California
  • 2RAND Corporation, Boston, Massachusetts
JAMA Pediatr. 2018;172(3):239-246. doi:10.1001/jamapediatrics.2017.4882
Key Points

Question  Does exposure to communities with higher rates of obesity increase the body mass index (BMI) and risk of overweight/obesity of individual residents?

Findings  Using data from military service members assigned to installations around the country, this study found that exposure to counties with higher rates of obesity (relative to counties with lower obesity rates) was associated with higher mean BMI and greater odds of obesity in parents and higher BMI z scores and greater odds of overweight/obesity in children. Associations were stronger among families who had resided longer in a given location and with off-installation residence; no evidence supported self-selection or shared built environment as explanations for these results.

Meaning  Exposure to communities with higher rates of obesity is associated with higher BMI and greater risk of overweight and/or obesity in parents and children, and this may suggest the presence of social contagion.

Abstract

Importance  Little is known about whether the substantial clustering of obesity and overweight within social and geographic networks results from causal pathways, such as social contagion and shared environments, or from self-selection.

Objectives  This study aimed to determine whether exposure to communities with higher rates of obesity increases the body mass index (BMI) of individuals, calculated as weight in kilograms divided by height in meters squared, and their risk of being overweight or obese, and whether social contagion, shared environments, or self-selection can account for identified differences.

Design, Setting, and Participants  This natural experiment study used the routine assignment of military service members to installations as a source of exogenous variation in exposure to communities with higher vs lower rates of obesity. The study, which used data collected by the Military Teenagers' Environments, Exercise, and Nutrition Study, examined families from 38 military installations around the United States to determine if individuals had higher BMI and greater odds of overweight and obesity when assigned to installations in counties with higher rates of obesity. The study also examined if the relationship persisted after controlling for shared built environments. The participants included 1 parent and 1 child aged 12 or 13 years from 1519 families of Army-enlisted personnel. Data analysis was completed from November 2016 to October 2017.

Exposures  Adult obesity rate in the county where the assigned installation of the service member was located. Time at installation and location of residence (on-installation vs off-installation) were used to measure the degree of exposure.

Main Outcomes and Measures  For parents, outcomes were BMI, overweight/obesity (BMI, ≥25) and obesity (BMI, ≥30). For children, outcomes were BMI z score, overweight/obesity (BMI percentile for age and sex, ≥85), and obesity (BMI percentile for age and sex, ≥95). These outcomes were based on self-reports for parents, self-reports and parent reports for all children, and anthropometric measurements for a subsample of children.

Results  Members of 1519 families participated, including 1314 adults (of whom 740, or 56%, were fathers) and 1111 children (of whom 576, or 52%, were boys); anthropometric measurements were performed on 458 children. The sample was 40% white, 22% black, 24% Hispanic, and 14% other races/ethnicities. A 1–percentage point higher county obesity rate was associated with a higher BMI (a difference of 0.08; 95% CI, 0.02-0.13) and greater odds of obesity (adjusted odds ratio [aOR], 1.05; 95% CI, 1.02-1.08) in parents, and a higher BMI z score (0.01; 95% CI, 0.003-0.02) and greater odds of overweight/obesity (aOR, 1.04; 95% CI, 1.01-1.06) in children. The evidence supported stronger associations among families with more time at installation and off-installation residence. Associations persisted even after controlling for shared built environments.

Conclusions and Relevance  Exposure to counties with higher rates of obesity was associated with higher BMI and higher odds of overweight and/or obesity in parents and children. There was no evidence to support self-selection or shared built environments as possible explanations, which suggests the presence of social contagion in obesity.

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