摘要：Objectives . We aimed to test the hypothesized role of shared body size norms in the social contagion of body size and obesity. Methods . Using data collected in 2009 from 101 women and 812 of their social ties in Phoenix, Arizona, we assessed the indirect effect of social norms on shared body mass index (BMI) measured in 3 different ways. Results . We confirmed Christakis and Fowler's basic finding that BMI and obesity do indeed cluster socially, but we found that body size norms accounted for only a small portion of this effect (at most 20%) and only via 1 of the 3 pathways. Conclusions . If shared social norms play only a minor role in the social contagion of obesity, interventions targeted at changing ideas about appropriate BMIs or body sizes may be less useful than those working more directly with behaviors, for example, by changing eating habits or transforming opportunities for and constraints on dietary intake. In recent years, health researchers have begun to explore how chronic, noninfectious disease might proliferate through social contagion, as people learn from and react to those around them. 1 The idea that social ties serve as conduits for the spread of ill health (and conversely, health) has considerable implications for public health policy and practice; yet, the sociocultural mechanisms by which such types of social contagion might occur remain poorly specified. 2 One of the most highly cited examples of this type of social contagion model appears in a study by Christakis and Fowler. 3 Using longitudinal data from the 32-year Framingham Heart Study, they demonstrated that obesity spreads through social ties, especially mutual friendships, over time. As with other studies that have observed social clustering of obesity, Christakis and Fowler could not determine from the data the specific social, cultural, and psychological mechanisms by which such contagion occurred. 3 , 4 However, they speculated about underlying pathways, including both socially shared norms about the acceptability of being overweight and socially shared behaviors. According to the first social norm hypothesis, social affiliation fosters shared norms or ideals (e.g., about the acceptability of being overweight), which then lead to similarity in body mass index (BMI) through the actions of these ideals on diet and physical activity. Studies have found clustering of both body attitudes and eating behaviors. 5 – 7 However, to our knowledge, Christakis and Fowler's 3 proposition that shared norms account for the observed social contagion of obesity has not been empirically tested. We specifically tested the proposed role played by norms of acceptable body size in observed patterns of clustering in obesity. Specifically, we examined 3 potential pathways for the influence of norms within social relationships ( Figure 1 ). People may come to share a close friend's or other network member's norm for body size, which in turn influences their BMI-related behaviors (pathway 1). For example, through conversations about fatness and thinness people may come to share friends’ ideals of appropriate body size, ideals that may in turn influence how they eat and exercise. Second, people may not internalize their friends’ norms for body size, but they may still feel pressure to fit their friends’ norms and thus modify their BMI-related behaviors (pathway 2). Specifically, people may have a heavier ideal body size than do their friends, but because of subtle comments and shaming, they may feel compelled to lose weight. Finally, people may not necessarily share their friends’ ideals for body size. However, friends’ body size may shape people's own ideals, which could lead them to eat and exercise differently in an attempt to approximate that ideal (pathway 3). Open in a separate window FIGURE 1 Three potential pathways by which norms might account for the observed relationship between ego and alter BMI. Note . alter = respondent in Phase 2 sampled from an ego's social network; BMI = body mass index (defined as weight in kilograms divided by height in meters squared); ego = study respondents in Phase 1. Although each of these pathways is theoretically plausible, several field studies have indicated that norms, beliefs, and ideals are rarely related to behaviors in such a direct manner. 8 – 10 To assess these proposed pathways, we measured key variables—body size and 3 different assessments of body size–related norms—of women and their closest network members in Phoenix, Arizona (101 women and 812 of their social ties). After confirming clustering of obesity and BMI within social ties, we then assessed whether the observed social clustering of BMI could be accounted for by the indirect effect of these different norms about acceptable body size.
