MCAT Monday: Psychological, Social, and Biological Foundations of Behaviors


The Psychological, Social, and Biological Foundations of Behavior section asks you to solve problems by combining your knowledge of foundational concepts with your scientific inquiry and reasoning skills.


Practice Set #1

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 121212,067067067 people who had been closely followed over 323232 years, from 197119711971 to 200320032003. In the study, 555,124124124 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 is greater than or equal to 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: 111) an “ego-perceived friend” in which the ego identifies the alter as a friend; 222) an “alter-perceived friend” in which the alter identifies the ego as a friend; 333) 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\%57%57, percent (“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\%95%95, percent 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 to 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.


1. Which conclusion is best supported by the findings in Figure 1?

Choose 1 answer:

  • A: Friends of opposite genders only marginally increased the likelihood of obesity for the ego.

  • B: Obese persons do not seem to selectively form social ties only with other obese persons.

  • C: If a mutual friend living far away gained weight, the ego would not be more likely to gain weight.

  • D: There is almost no effect on the ego when someone in the same geographic proximity gained weight.

2. Which of the following is NOT a plausible policy implication of the peer effects findings in the above passage?

Choose 1 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.

  • 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.

  • 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.

  • D: Shared external sources contribute to obesity, so interventions that take common environmental factors into consideration can effectively target individual health behavior.

3. 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)?

Choose 1 answer:

  • A: A new independent variable would be added.

  • B: A new mediating variable would be added.

  • C: The variables would not change.

  • D: A new dependent variable would be added.

4. Which of the following statement best describes the concept of homophily? Choose 1 answer:

  • A: Obese people are more likely to change their unhealthy behaviors around non-obese people.

  • B: Individuals who are in the same geographical proximity are more likely to conform to the same health behaviors.

  • C: Individuals with similar health risks such as high blood pressure are less likely to dissolve social ties with one another.

  • D: Smokers are more likely to experience cognitive dissonance with other smokers, compared to non-smokers.

5. Which independent variable is most relevant for a study that solely investigates the impact of primary groups on health behaviors?

Choose 1 answer:

  • A: Neighbor

  • B: Religious Affiliation

  • C: Sibling

  • D: Occupational Affiliation

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