Obesity

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The Spread of Obesity in Social Networks

March 24, 2008 in Obesity | Permalink

Are you friends members of the Marathon Runner’s of America club or the Bratwurst and Philly Cheesesteak club? If the answer is the later, you are much more likely to be obese than the former.

This is the finding of a 2007 NEJM paper by Christakis and Fowler. Obese individuals are more likely to be friends, relatives, or spouses with other obese people (and vice versa). The authors contend that there are 3 explanations for why this could be the case empirically.

  1. Homophily. This means that individuals choose to associate with people who look like them. In this case, social networks would not cause obesity, it is just that obese individuals choose to hang out with other obese individuals.
  2. Counfounding factors. Siblings have the same genes. Obesity social norms within a particular geographic area may affect friends and family in a similar manner. These unobserved, confounding variables may also be the true cause of why
  3. Induction. Social influence and peer effects may effect the obesity level of each person in a group. The authors hypothesize that this explanation to be the major avenue by which social networks affect obesity.

The paper tracks a database of 12,067 individuals over 32 years. The regressions use a lagged dependent variable to eliminate problems of serial correlation.

Results

Let us define the ‘ego‘ the person as the person whose behavior is being analyze and the ‘alter’ as a person connected to the ego by a social network. When the ego’s alter is a friend and becomes obesity, there is a 57% chance that the ego will become obese. This impact is larger for same sex friendships (71% probability of become obese if the alter becomes obese) than opposite sex friendships (effect not different from zero).

How does the obesity of one’s spouse affect the ego’s obesity? According to the authors, “[a]mong married couples, when an alter became obese, the spouse was 37% more likely (95% CI, 7 to 73) to become obese. Husbands and wives appeared to affect each other similarly (44% and 37%, respectively).”

What explains this phenomenon that the alter’s obesity will affect the ego’s obesity. It is possible that the social network as a whole experiences similar life events which affect obesity. However, even when alter’s live geographically far from the ego–and thus likely have different life experience over time–this does not change the effect the alter’s obesity has on the ego. Christakis and Fowler claim that this supports their perception that social norms heavily influence obesity. Also, the spread of smoking behavior does not affect the spread of obesity. One would guess that social networks would have a similar effect on smoking and obesity. The authors claim that this finding, “…suggests that the psychosocial mechanisms of the spread of obesity may rely less on behavioral imitation than on a change in an ego’s general perception of the social norms regarding the acceptability of obesity.”

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Marital Status and Body Weight Changes

March 10, 2008 in Obesity | 3 comments

Why do people want to lose weight? While this seems like an obvious question, it does merit answering. There are two major reasons: health concerns and appearance. Being obese increases the risk of suffering from many diseases (e.g.: diabetes). On the appearance side, individuals may experience social pressure to lose (or possibly gain) weight. Further, individuals may want to maintain a healthy body appearance to attract a mate.

Jeffery Sobal is an expert in obesity studies. According to his 2003 study, activities which directly affect weight are caloric intake, physical activity and smoking.

One of the more interesting questions is how an individual’s marriage status affects obesity. It is generally found that–even controlling for age and other covariates–married individuals are more likely to be overweight than non-married individuals. Why is this the case. Sobal cites some studies which attempt to explain this.

After citing all this evidence, Sobal and co-authors state 4 hyptotheses to test:

  1. Marital trajectories that are stable are related to stable body weights,
  2. marital trajectories entering marriage are related to weight gain,
  3. marital trajectories dissolving marriage are related to weight loss,
  4. marital trajectories involving the death of a spouse are related to weight loss.

Sobal uses data from from the National Health and Nutritional Examination Survey (NHANES I). A 10 year follow up survey of the participants is collected in the National Health and Nutrition Epidemiological Followup Survey (NHEFS). The authors use an OLS specification with a lagged dependent variable (i.e., lagged BMI) in order to estimate the impact of marital status on weight. Sobal, Rauschenbach and Frongillo conclude the following:

  1. Stable marital trajectories were not associated with significant weight changes, except for weight loss among men who remained separated/divorced.
  2. Marital trajectories involving entry into marriage were associated with weight gain among women, but not among men.
  3. Marital trajectories involving dissolving marriages were associated with weight loss among men, but not women.
  4. Marital trajectories involving death of a spouse were associated with weight loss among men, but not women.
  5. Marital and other demographic characteristics were better predictors of weight loss than weight gain.

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Will giving weight-loss prizes to the obese save the NHS money?

February 12, 2008 in Current Events, Obesity | 7 comments

According to the Daily Mail (…obesity epidemic…) the NHS could give “vouchers to the overweight to spend on healthy food in supermarkets” or cash prizes to those who manage to lose weight. The UK could also mandate cooking classes in school and more time for physical education classes.

The UK report on obesity states: “We need to rework the incentives for individuals and public bodies to encourage actions now, thereby avoiding much larger costs in later years.”

But will decreasing obesity save the government money? Not according to a recent paper in the PLoS Medicine titled “Lifetime Medical Costs of Obesity: Prevention No Cure for Increasing Health Expenditure.” The paper finds that healthy people have more lifetime medical costs that either obese individuals or smokers. How can this be the case? It is true, that each year an obese individual lives they will incur more medical costs than a healthy person. In particular, spending on heart disease, diabetes, and musculoskeletal diseases. Since a healthy person lives longer, however, the healthy person has more years of medical expenditures which will accumulate compared to an obese individual. Similarly, smokers have higher medical costs per year but since they have a shorter life expectancy, smokers actually incur fewer medical costs over their lifetime than healthy individuals.

It seems that giving prizes to obese individuals for losing weight will not only be costly in terms of the tax revenue needed to fund the project, but will also increase medical expenditures if people do in fact lose weight. The Healthcare Economist is proposing a revolutionary concept: let each individual choose their own weight make their own lifestyle choices without any government interference.

The financial burden of overweight elderly Americans

January 16, 2008 in Obesity | Permalink

How do we measure the financial burden of elderly obesity? At first this seems like an easy question to answer. Find the average medical spending of the obese elderly and compare that to the spending of an elderly individual of healthy weight.

Yet causation is difficult to show.

True, it is possible that the obese may be more likely to get sick and thus incur higher costs, but it is also possible that sick people can gain weight–since it is difficult to exercise when sick–which can cause more obesity. Thus, the original sickness and not the obesity may be the true cause of the additional medical expenses.

A 2007 Health Services Research paper by Yang and Hall try to examine this question. They use panel data from the Medicare Current Beneficiary Survey (MCBS). The authors use a maximum likelihood estimation strategy, modeling current BMI as a function of last year’s BMI and Acute Medical Events. Acute Medical Events are also a function of BMI in a separate estimating equation. Health care expenditure is a function of BMI, BMI-squared, functional status, chronic and acute disease and demographics.

To identify this system of questions, the authors uses the following exogenous variables: average food prices, density of fast food restaurants and air quality. Using these instruments, the authors find that “elderly men who were overweight or obese at age 65 had 6–13 percent more lifetime health care expenditures than the same age cohort within normal weight range at age 65. Elderly women who were overweight or obese at age 65 spent 11–17 percent more than those in a normal weight range.”

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PE Requirements and Overweight Youth

January 4, 2008 in Obesity | Permalink

Do state physical education (PE) requirement help to decrease the percentage of children and teens who are overweight?

This is the question Cawley, Meyerhoefer and Newhouse investigate in their 2007 Health Economics paper.

One would certainly not be surprised if PE requirements decrease the prevalence of obesity, but this may not be correct.  PE requirements may have no effect if schools do not comply with the state mandates, or increased PE exercise may lead to decreased exercise outside of school (substitution).  Further, it is possible that PE classes may do little to promote exercise.  For instance, 12 states allow students to earn PE credit online.

To find the truth, the authors use Youth Risk Behavior Surveillance System data from the 1999, 2001, and 2003.  The authors attempt to find the local average treatment effect (LATE) by using state PE requirements as an instrument for whether or not a given student has taken a PE class.

Cawley and co-authors conclude that “high school students with a binding PE requirement report an average of 31 additional minutes per week spent physically active in PE class. Our results also indicate that additional PE time raises the number of days per week that girls report having exercised vigorously or having engaged in strength-building activity. We find no evidence that PE lowers BMI or the probability that a student is overweight.”

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Breakfast of Champions: Maternal Employment and Childhood Obesity II

December 6, 2007 in Obesity | Permalink

On Wednesday, I reviewed a paper by John Cawley and Feng Liu about the mechanisms by which maternal employment can affect childhood obesity. It turns out that Cawley and Liu aren’t the only ones interested in this issue. A recent working paper by Fertig, Glomm and Tchernis investigates the same question.

The authors use time diaries from the Child Development Supplement to the PSID. They use two regressions to test their hypothesis. First, they regress BMI on the amount of time spent in various activities (e.g.: eating, watching TV, playing video games). The second regression measures how maternal employment impacts each one of these activities. In general, these regressions are estimated using OLS, but some activities are count variables (e.g.: number of meals in an average day) and the authors use a Poisson regression in these cases.

For the Poisson regression to be accurate, one must assume the mean and variance of the distribution must be the same. I would have preferred for the authors to use a negative binomial regression to increase flexibility, but I do not know whether this methodological alteration would change the results.

Some of the results of the paper are not surprising. For instance, maternal employment leads to more TV watching for the children and more TV watching leads to more childhood obesity.

Yet other results are interesting. Cawley and Liu found that working mothers cook fewer meals at home and that lead to more childhood obesity. Fertig and co-authors also found that working mothers cook fewer meals at home, but that the percentage of meals eaten at home did not affect childhood obesity in a statistically significant manner.

One of the major determinants of childhood obesity is the number of meals they eat. Missing breakfast and eating fewer meals, according to the authors, “may lead to higher concentrations of 24 hour insulin, which, in turn, can lead to increased fat deposition and higher body weight.” Children of working mothers eat fewer meals and having fewer meals significantly increases obesity.

Maybe breakfast is the most important meal of the day.

Thanks to Scott Cunningham for sending me this working paper.

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Maternal Employment and Childhood Obesity

December 5, 2007 in Obesity | Permalink

A general result in the obesity literature, is that higher female labor participation rates lead to higher obesity rates in children. For instance, the 1996 Welfare Reform act (PRWORA) increased work requirements for low-income mothers and thus increased labor participation and likely childhood obesity. One question which has not been resolved yet thorough which mechanisms does the mother’s employment increase childhood obesity.

One of the first attempts to answer this question is by John Cawley and Feng Liu (NBER 2007). They use 2003-2006 data from the American Time Use Survey to analyze how employment affects various household activities. The authors estimate a two-part model (probit for whether or not any time was spent in the activity and OLS estimating how employment affects the amount of time spent in each activity). The authors also use the state unemployment rate as an instrument in the first stage for female labor participation.

The authors find that female employment leads to fewer meals prepared at home. This is seem through a 5 percentage point lower probability of grocery shopping, 13 percentage point lower probability of doing any cooking, 10 percentage point lower probability of eating with the children. Working mothers who engage in these activities on average still spend fewer minutes preforming these tasks.

Often, when mothers spend less time with children, this results in more sedentary activity–primarily watching television. The authors found that working mothers engage in less time playing with the child, less time engaged in child care and child supervision as well.

The authors find some evidence that working women who have a husband or partner in the household reduce time spent with their children to a greater extent than single mothers since the husband spend some time with the child.

This research may lead one to believe that the Welfare-to-Work legislation may have helped reduce reliance on welfare and the number of single mothers who are unemployed, but may have also increased childhood obesity rates throughout the country.

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Marriage and Obesity

November 28, 2007 in Obesity | Permalink

Are married people more likely to be obese than single individuals? More to the point, does being married cause obesity? Married individuals are generally older than never-married individuals and since age is correlated with obesity, there could be a spurious relationship between marriage and obesity.

One may think that married individuals are not on the “single’s market” and thus may not have a strong incentive to maintain an athletic physical appearance to attract mates. As stated in Sobal (1984), “We may hypothesize that as a marital relationship becomes solidified the partners may feel less need to maintain external appearances important in attracting a mate.” On the other hand, a paper by Rand, Kuldau and Robbins (JAMA 1982) found that individuals who had jejunoileal bypass surgery to decrease obesity had improved marriage relationships. Thus, those who value their marriage may wish to avoid being overweight to make the marriage experience more pleasurable.

If healthier individuals can more easily attract a mate, than it would be the case that married individuals will be less overweight than single individuals. Averett and Korenman (Int J Obesity 1999) found that obesity is associated with a lower probability of marriage. Gortmaker et al. (NEJM 1993) use the NLSY to conclude that individuals who where overweight in their adolescent years are 20% less likely to be married seven years later than a healthy-weighted individual. Cawley, Joyner and Sobal (Rationality and Society 2006) confirm that for adolescents “dating is less likely among heavier girls and boys and among shorter girls and boys.”

Sobal, Rauschenbach and Frongillo (Soc Sci Med 1992) categorizes the relationship between obesity as marriage through two distinct mechanisms: “marital selection” and “marital causation.” Non-overweight people are more likely to attract a mate, and thus “select” into marriage. However, if marriage “causes” weight gain–due to a more sedentary lifestyle, lower mate attraction incentive, childbirth, etc.–than a researcher may find that married individuals are more overweight on average.

The best way to control for these two conflicting effects is to use a panel data set. Cawley (JHR 2004) employs the 1979 NLSY, using lagged BMI as an instrument for current BMI and individual fixed effects to control for time-invariant individual characteristics. Other studies have used sibling weight (Avarett and Kroenman (JHR 1996), or spousal weight as an instrument for current BMI. Using data from the National Survey of Personal Health practices and consequences, the Sobal, Rauschenbach and Frongillo paper finds that “it appears that there is a relationship between fatness and marital status for men, with married men fatter and more obese.”

Nevertheless, more research is needed to refine the exact manner in which marriage affects obesity.

  • Averett S and Korenman S. 1996. “The Economic Reality of the Beauty Myth.” J Human Resources. 31(2): 304-330.
  • Averett S and Korenman S. 1999. “Black-white differences in social and economic consequences of obesity.” International Journal of Obesity. vol 23, pp. 166-173.
  • Cawley J. 2004. “The Impact of Obesity on Wages” J Human Resources. 39(2): 451-474.
  • Cawley J, Koyner K, Sobal J. 2006. ”Size Matters: The influence of adolescents’ weight and height on dating and sex.” Rationality and Society. Vol. 18, No. 1, 67-94.
  • Gortmaker SL, Must A, Perrin JM, Sobol AM, Dietz WH. 1993. “Social and Economic Consequences of Overweight in Adolescence and Young Adulthood.” NEJM. 329(14): 1008-1012.
  • Rand CS, Kuldau JM, Robbins L. 1982. “Surgery for Obesity and Marriage Quality.” 247(10): 1419-1422.
  • Sobal J. 1984. “Marriage, Obesity and Dieting.” Marriage and Family Review. 7:115-139.
  • Sobal J, Rauscehnbach BS, Frongillo EA. 1992. “Marital Status, Fatness and Obesity.” Social Science and Medicine. 35(7): 915-923.

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Too fat to work

November 26, 2007 in Current Events, International Health Care Systems, Obesity | Permalink

The incapacity benefit system in the UK is intended to provide an income support for those unable to work.  Like any government program, many of the beneficiaries are in dire need of the money and are truly unable to work, but many other individuals who are able–but not inclined–to work have taken advantage of government largesse.  Liberals will highlight the fact that these programs help the needy while conservatives will generally retort with numerous examples of how individuals are able to take advantage of ‘the system.’

Last week, The Times of London reported (‘Too fat to work‘) that “Almost two thousand people who are too fat to work have been paid a total of £4.4 million in benefit.”  Should obese individuals receive a disability benefit?  If obesity is truly a disease, than one may say yes.  On the other hand, there is a seemingly simple cure for obesity–eat less and exercise more.  For those who are obese, however, accomplishing this physiological feat is not as simple as it sounds.  It is possible that the incapacity benefit may actually make the obese worse off.  Allowing the obese to collect an incapacity benefit may reduce an overweight individual’s incentive to lose weight in order to be able to work.

Any input on this subject would be greatly appreciated.

Thanks to my colleague Mike Ewens for the referral to the Times article.

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