Is obesity contagious?
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Is obesity contagious?
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
A recent study found that obesity tends to spread like a “contagion” through a social network. In other words, when a person experiences weight gain, close friends in the same networks tend to gain weight as well. The investigators conducted a detailed analysis of a mass network of 12,067 people who had been closely followed over 32 years, from 1971 to 2003. In the study, 5,124 people were used as key subjects, or “egos”, whose behavior was analyzed. Any persons linked to the egos serve as “alters”—those who may influence ego behavior. The researchers examined several aspects of obesity spread, such as clustering of obese persons within the network, association of weight gain among an individual’s social contacts, degree of dependence of association of social ties, and influence of gender or geographical distance. The researchers found that there were discernible clusters of obese persons (BMI ≥ 30) in the network at all time points.
Figure 1 shows some results from the study. The extent of interpersonal association in obesity was evaluated with regression analysis. Homophily was taken into account by including a measurement of the alter’s obesity. The researchers evaluated the possible role of unobserved contemporaneous events by separately analyzing models of subgroups of the data involving ego-alter pairings. In particular, three types of “directional” friendships are defined: 1) an “ego-perceived friend” in which the ego identifies the alter as a friend; 2) n “alter-perceived friend” in which the alter identifies the ego as a friend; 3) a “mutual friend” in which the identification is reciprocal. Familial ties (parents, siblings) and marital ties (spouses) are treated as reciprocal. “Immediate neighbor” denotes the geographical distance between an alter and an ego. For example, from the results, we can see that if an ego stated that an alter was a friend, the ego’s chances of becoming obese appears to increase by 57% (“risk of obesity”).
Figure 1: Percentage increase in obesity risk for an ego based upon his/her relationship with an alter. The dependent variable in each model is the obesity of the ego. Independent variables include a time-lagged measurement of the ego’s obesity, the obesity of the alter, a time-lagged measurement of the alter’s obesity, the ego’s sex, age, and education. Mean effect sizes (solid black dot) and 95% confidence intervals (line) are shown.
In later studies on the relationship between social networks and health behaviors, one of the researchers further found that existing social ties (especially close friendships) are more likely to dissolve between people who have health traits that are dissimilar, including health traits that are immutable such as height and personality, and traits that are mutable such as BMI, blood pressure, etc. In particular, those with similar BMIs are less likely to dissolve existing ties and more likely to form ties. Another study demonstrated that food choices also were made in accordance with social networks. In particular, spouses showed the strongest influence in food consumption behaviors, controlling for social contextual factors. Across all peers (spouses, siblings, friends), eating patterns that were most likely to be shared were “alcohol and snacks”.
Source: Adapted from Christakis, N. A., & Fowler, J.H. (2007). The Spread of Obesity in a Large Social Network over 32 Years. The New England Journal of Medicine. 357(4), 370-379. O'Malley, J., & Christakis, N. A. (2011). Longitudinal analysis of large social networks: Estimating the effect of health traits on changes in friendship ties. Statistics in Medicine. M. A. 30(9), 950-964. Pachucki, M.A., Jacques, P.F., & Christakis, N.A. (2011). Social Network Concordance in Food Choice Among Spouses, Friends, and Siblings.American Journal of Public Health, 101(11), 2170-2177.
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[question] => Which conclusion is best supported by the findings in Figure 1?
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[description] => Reason for the Correct Answer:
Mutual friends, ego-perceived friends, and same-sex friends increase the risk of obesity the most for egos.
Opposite-sex friends decrease the risk of obesity for egos.
Immediate neighbors, or social ties within the same geographic proximity, show approximately 0% increase in risk of obesity for egos.
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[0] => Array
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[each_answer] => A. Friends of opposite genders only marginally increased the likelihood of obesity for the ego.
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[1] => Array
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[each_answer] => B. Obese persons do not seem to selectively form social ties only with other obese persons.
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[2] => Array
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[each_answer] => C. If a mutual friend living far away gained weight, the ego would not be more likely to gain weight.
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[each_answer] => D. There is almost no effect on the ego when someone in the same geographic proximity gains weight.
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[quiz_unique_key] => 3873426850
[question] => Which of the following is NOT a plausible policy implication of the peer effects findings in the above passage?
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[answer] => 4
[description] => Reason for the Correct Answer:
The studies all show that group peer effects identified through social networks influence risks of obesity.
The studies show that there are peer effects on various sorts of health behavior such as smoking or food consumption.
Since the studies emphasize peer effects through social groups, contacts, and networks in influencing individual health behavior, a plausible policy implication would not be the examination of external environmental factors, but rather one that focuses on utilizing social ties.
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[each_answer] => A. It may be possible to exploit variations in people’s social network position to target interventions where they may be more effective in generating benefits for the group, such as key nodes who exert stronger influence on others.
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[each_answer] => B. Group-level interventions such as Alcoholics Anonymous or other support groups that can serve as a set of artificial social network ties may be more successful than individual-level interventions.
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[each_answer] => C. If we spend $500 to get a person to quit smoking, this person’s quitting may in turn result in his or her social contacts quitting, increasing the cost-effectiveness of an intervention.
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[each_answer] => D. Shared external sources contribute to obesity, so interventions that take common environmental factors into consideration can effectively target individual health behavior.
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => If the studies were combined to investigate the effects of social networks on food selection and how this food selection is associated with the likelihood of obesity, how would this change the design of the first study (social network effects on obesity)?
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[answer] => 2
[description] => Reason for the Correct Answer:
When examining the effect of social networks on obesity, social networks would be the independent variable and obesity levels would be the dependent variable.
When taking food selection habits into account, the model becomes: social networks–>food selection–>obesity level.
A mediating variable is a variable that attempts to assess the mechanisms underlying in the relationship between an independent and dependent variable. Therefore, food selection habits as a mediating variable would help clarify the relationship between social networks and obesity.
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[0] => Array
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[each_answer] => A. A new independent variable would be added.
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[each_answer] => B. A new mediating variable would be added.
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[2] => Array
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[each_answer] => C. The variables would not change.
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[each_answer] => D. A new dependent variable would be added.
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[3] => Array
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[quiz_unique_key] => 2261298308
[question] => Which of the following statements best describes the concept of homophily?
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[answer] => 3
[description] => Reason for the Correct Answer:
Homophily is defined as the tendency for people to choose relationships with other people who have similar attributes.
Homophily in social networks also implies that individuals with similar traits are more likely to form social ties with one another, which also often impacts their actions.
Individuals that share similar health risks such as high BP are more likely to form stronger social ties with one another, or put another way are less likely to dissolve social ties with one another.
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[0] => Array
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[each_answer] => A. Obese people are more likely to change their unhealthy behaviors around non-obese people.
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[each_answer] => B. Individuals who are in the same geographical proximity are more likely to conform to the same health behaviors.
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[each_answer] => C. Individuals with similar health risks such as high blood pressure are less likely to dissolve social ties with one another.
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[each_answer] => D. Smokers are more likely to experience cognitive dissonance with other smokers, compared to non-smokers.
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[quiz_unique_key] => 574431310
[question] => Which independent variable is most relevant for a study that solely investigates the impact of primary groups on health behaviors?
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[answer] => 3
[description] => Reason for the Correct Answer:
Primary groups are often understood in contrast to “secondary groups”.
Primary groups are typically closer and longer-lasting ties, while secondary groups are more impersonal and temporary.
Siblings and other family members are considered primary groups, so the effect of siblings on health behavior is the most relevant for assessing the effect of primary groups. Religious affiliations can incorporate both primary and secondary groups so it would not be the most relevant.
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[0] => Array
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[each_answer] => A. Neighbor
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[each_answer] => B. Religious Affiliation
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[each_answer] => C. Sibling
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[each_answer] => D. Occupational Affiliation
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