Obesity¹ is socially contagious²（蔓延的、传染的）, according to research published in the past few years. How it is "caught" from others remains³ a murky⁴（不明朗的） area. But findings from Arizona State University researchers published online May 5 in the American Journal of Public Health shed light on the transmission of obesity among friends and family. Shared ideas about acceptable weight or body size play only a minor⁵ role in spreading obesity among friends, according to the findings published in the article "Shared Norms and Their Explanation for the Social Clustering of Obesity."

"Interventions⁶ targeted at changing ideas about appropriate body mass indexes 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 constraints⁷ on dietary intake," wrote lead author Daniel J. Hruschka, and co-authors Amber⁸ Wutich, Alexandra Brewis and Benjamin Morin, all with ASU's School of Human Evolution and Social Change.

Hruschka and Wutich are cultural anthropologists, while Brewis is a biological anthropologist⁹, and Morin is a graduate student in applied¹⁰ mathematics for the life and social sciences.

"When you see that something like obesity spreads among close friends and family members, this raises important questions about how it's spreading. Is it because we learn ideas about acceptable body size from our friends and family members, or that we hike together, watch TV together or go out to eat together?" said Hruschka.

"If we can figure out exactly why obesity spreads among friends and family members, that can tell us where to focus resources in curbing¹¹（控制，约束） rates of obesity. Is it more effective to change people's ideals of acceptable body size in hopes that they will change their behaviors or rather directly target socially shared behaviors that can contribute to weight gain or loss?"

To dig deeper into how clustering of body attitudes account for the observed social contagion¹² of obesity in past studies, the ASU team interviewed 101 women from the Phoenix¹³ area and 812 of their closest friends and family members.

Comparing the body mass index (BMI) of the women, their friends and family members, the researchers confirmed prior findings that the risk of a woman's obesity rose if her social network was obese¹⁴.

But the team also examined three potential pathways by which shared ideals of acceptable body size might cause obesity and body size to spread through social ties.

Hruschka explained: "You might learn what is an acceptable body size from your friends and then change your diet and exercise to try to achieve that. Or, you might not agree with what your friends or family members think, but still feel pressure from them to achieve some ideal body size. Finally, you may form an idea of appropriate body size by simply observing your friends' bodies, which in turn changes your eating and exercise habits."

The team discovered no evidence for the first and second pathways as means of transmission and found only limited support for the third, suggesting that other factors such as eating and exercising together may be more important in causing friends to gain and lose weight together.

A strength of the study was the range of approaches taken to assess body size ideals, including ideal body size, anti-obesity preference and anti-fat stigma¹⁵. For example, the participants were asked to choose whether they would rather be obese or have one of 12 socially stigmatized¹⁶ conditions, such as alcoholism or herpes. In many cases, the women would rather have more of the other conditions, with 25.4 percent preferring severe depression and 14.5 percent preferring total blindness over obesity.

While this study provides some clues, the authors noted¹⁷ that more work needs to be done to assess how other proposed factors, such as shared activities or other kinds of social norms, account for the relationship between social proximity¹⁸ and similarity in BMI.

"This study is important because it shows that while the clustering（聚集，群集） of people with larger or smaller bodies is real, it is not shared values between friends that accounts for it," said Brewis, director of the Center for Global Health in ASU's College of Liberal Arts and Science. "This gives us important clues about the best ways to tackle obesity as a public health issue; we need to focus on what people do together, rather than what people think."