摘要：Objectives. We used social network analysis to examine how weight status affects friend selection, with an emphasis on homophily and the social marginalization of overweight youths. Methods. We used an exponential random graph model to assess the effects of body mass index (BMI) on friend selection while controlling for several alternative selection processes. Data were derived from 58 987 students in 88 US middle and high schools who took part in the 1994 to 1996 National Longitudinal Study of Adolescent Health. Results. On average, overweight youths were less likely than nonoverweight youths to be selected as a friend; however, this effect differed according to the BMI of the person initiating the friendship. Nonoverweight youths were 30% more likely to select a nonoverweight friend than an overweight friend, whereas overweight youths were largely indifferent to the weight status of their friends. Friendship ties from overweight youths to nonoverweight peers were more likely than ties in the reverse direction. Conclusions. We found evidence consistent with homophily and social marginalization but only for the selection behavior of nonoverweight youths. We conclude that avoidance of overweight friends is the primary determinant of friendship patterns related to BMI. Social network analysis offers a tool to understand the complex social and biological relationships that contribute to health. 1–3 A tenet of the social network paradigm is that individual behaviors are interdependent owing to processes such as normative influences, social learning, and contagion. 4–6 At the same time, network structure is not independent of behavior. Networks are dynamic, complex systems in which ties (e.g., friendships) are constantly evolving in conjunction with individual behaviors. Network selection processes are in part driven by individuals’ health, 7 often including the very health behaviors that researchers treat as outcomes. 8–10 However, several other processes also contribute to network structures. Friendships are more likely when individuals share commonalities with respect to sociodemographic attributes (i.e., homophily 11 ), organizational affiliations, 12 spatial proximity, 13 and social connections (i.e., transitivity 14 ). Also, individuals systematically vary in their sociability and popularity. 15 Fully understanding health behavior thus requires an examination of network structures and the processes that create them. Several explanations have been invoked to understand the complex role of obesity in structuring friendships among young people. Two explanations in particular have received concerted attention. First, overweight adolescents are socially marginalized and less likely to be selected as a friend than their nonoverweight peers. 9,16–18 This is troubling given that friendships are important sources of support and companionship throughout the life span. 19 Not having or losing friends is associated with increased depression and decreased self-worth among young people, which could exacerbate the health problems associated with being overweight. 20 These negative repercussions of friendlessness may be more pronounced in middle school and high school, when intimacy and fitting into peer groups are critical. 20,21 Second, adolescents tend to develop friendships with peers who have a similar body mass index (BMI). 8,16,22,23 Friendships that are homophilous with respect to weight create the possibility for peer influence on behaviors and beliefs associated with weight. Friendships among overweight adolescents may reinforce unhealthy behaviors that further exacerbate weight problems. 24 Investigations of social marginalization and homophily have often been pursued independently, which we argue is a mistake. These patterns represent different perspectives on the more general question of how weight shapes friendship patterns. By adopting a network perspective, we recognize that the friend selection process depends on both the person initiating friendship (ego) and the friendship target (alter). Friendship likelihood can differ depending on the combination of ego and alter weight status. Assuming, for the sake of simplicity, that weight status is dichotomous, there are 4 types of friend selection dyads: overweight ego selecting overweight alter, nonoverweight ego selecting nonoverweight alter, overweight ego selecting nonoverweight alter, and nonoverweight ego selecting overweight alter. Examining marginalization requires that researchers compare friendship ties directed toward overweight versus nonoverweight alters, which disregards the ego’s weight status. By contrast, focusing solely on homophily entails comparing friendship dyads that are similar versus dissimilar, without considering whether the adolescents are overweight or nonoverweight. Examining either mechanism in isolation risks misidentifying the process underlying friend selection behaviors. A network approach demonstrates the interrelation between marginalization and homophily. Although the mechanism behind each pattern differs (e.g., avoidance of as opposed to preference for similarity), both predict that nonoverweight adolescents are more likely to befriend nonoverweight peers than overweight peers ( Table 1 ). The mechanisms differ in their predictions regarding the selection behaviors of overweight adolescents. The avoidance mechanism posits that overweight adolescents avoid overweight peers, instead choosing nonoverweight friends, whereas the homophily mechanism posits that overweight adolescents prefer overweight friends. By highlighting this distinction, we are better positioned to investigate the operation of these mechanisms. TABLE 1— Expected Friendship Likelihood Based on Avoidance and Homophily Mechanisms Avoidance Homophily Alter non-OV Alter OV Alter non-OV Alter OV Ego non-OV High Low Ego non-OV High Low Ego OV High Low Ego OV Low High Open in a separate window Note . OV = overweight. Ego refers to the person initiating the friendship; alter is the recipient. The network approach also makes clear that friend selection is multifaceted. Associations between friendship and weight status could develop indirectly through friend selection processes other than homophily or marginalization. 25 The first of these processes is social withdrawal of overweight adolescents. Overweight adolescents may be less sociable than nonoverweight adolescents, possibly because of perceived stigma 26 or lower rates of involvement in school-based activities that promote friendship. 21 Second, selection may occur on attributes correlated with weight, such as depression. Overweight adolescents may be excluded because of aversive behaviors that accompany their weight status, not because of weight itself. Third, the endogenous nature of network evolution means that the current network structure promotes some ties over others in the future. For instance, triad closure occurs when 2 individuals become friends because they have a mutual acquaintance. Consider person A, whose nonoverweight friend B has no overweight friends. Should person A form a friendship with any of person B’s friends through triad closure, those friendships will not include overweight peers. Thus, small tendencies toward homophily can become magnified over time. 27 Failure to control for alternative friend selection processes can result in biased parameter estimates. 15,28,29 Because of the equifinality of network structure, each of these processes could produce social marginalization or homophily as a spurious outcome. Our goal in this study was to offer a more detailed account of how weight status predicts friendship patterns, with an eye on homophily and the social marginalization of overweight youths. We addressed this goal by modeling friendship network data collected in several middle and high schools. Our models estimated effects related to BMI while controlling for alternative friend selection mechanisms.
