If you have a job like mine, in which a reasonably broad swath of American society presents itself to you without any clothes on, this question no doubt has popped into your head: Why are the poor so fat? That obesity is a characteristic problem of affluent countries surprises no one. But why is it also characteristic that obesity increases so dramatically as you go down the social scale?
This, of course, is not the case in developing nations. In countries such as India, where my parents emigrated from, being overweight is directly related to wealth and status and is regarded as attractive. Thus, my father's rural family of farmers is thin and wiry, while the elders in my mother's urban clan of higher caste and wealthier stock are as plump as can be. When my sister and I, two scrawny specimens of the American "upper-middle class," visit, we always evoke a good deal of concern from relatives.
The first report I found to document the effect class has on obesity was a 1965 study of midtown Manhattan residents. The researchers found obesity was six times more common among women in the bottom third of the social scale than among those in the top third. Subsequent studies, orginating from California to New Zealand, confirm these findings. In a 1996 study in Minnesota's Twin Cities, women earning under $10,000 a year weighed, on average, 20 pounds more than women earning over $40,000 a year. In advanced nations, low income is a more powerful predictor of obesity than any single factor except age, though this relationship is weaker in men than in women. In men, height is associated more strongly with status. On average, in developed and undeveloped countries alike, richer men are taller.
You'd think weight would be like height. More money means more food. So more food should mean more fat, just as it means more height, right? And, in fact, in children this is exactly the case--at young ages, certainly under age 6 in the United States, lower-income kids are less likely to be obese than higher-income kids. By the time they are adults, however, the relationship has inverted.
What's going on? As my epidemiologist friends point out to me, when two things correlate, there are always three possible explanations. One leads to the other. The other leads to the one. Or some third thing is driving both.
T he downward mobility hypothesis. One possibility is that obesity leads to lower income. Certainly the obese, particularly obese women, face severe disadvantages in both the job and marriage markets. And the evidence that this produces downward mobility is distressingly strong. Almost half the women in the 1965 Manhattan study belonged to a different social class than their parents, and those who had moved down were significantly fatter than those who had moved up. More recently, a landmark national study led by Steven Gortmaker, a Harvard researcher, tracked over 8,000 people from age 18 to 25. It found the heaviest 5 percent of women were half as likely to get married and twice as likely to become impoverished as others. Obesity affected a woman's economic prospects more than even chronic illness. In men, however, shortness led to downward mobility. One foot less of height doubled a man's likelihood of poverty. But as Gortmaker points out, downward mobility explains only a small part of the observed relationship of income to obesity. That makes sense. After all, the gap also exists in countries such as Britain, where social class is more rigid than it is here.
The genetic explanation. Could genetics be an outside factor driving both obesity and poverty? The so-called "Danish adoption study" lent some credence to the idea, finding that the obesity and social status of adoptees depended on the social status not only of their adoptive parents but also of their biological parents. The study inferred that parents can give their offspring genetic traits that tip the scales toward both heavier weight and lower status. Even so, inheritance accounted for at most a small part of the stark effect of income on obesity.
The gluttony and sloth hypothesis. So downward mobility and genes matter somewhat but, ultimately, we're left with the obvious explanation that in affluent societies the lower the income, the more people eat unhealthily, sit around, or both. Unfortunately, measuring this directly is notoriously difficult. Most people--especially the obese--fib about diet and exercise and, when observed, tend to change their behavior. The differences you're looking for are also quite small--a single calorie of extra daily intake starting in childhood can translate into 10 extra pounds by the time you're 25. Nonetheless, there's a good deal of indirect evidence--for example, TV watching is linked to eating more and exercising less, and rises substantially as income lowers. Given the weakness of other explanations, few experts doubt that lifestyle is the most important one.
The question is why. It's as if avoiding obesity were enormously expensive. But it's not--exercise is free, and eating less junk food and meat and more produce saves money. Rather, for the 30 percent to 70 percent with a genetic predisposition to obesity, it's just horrendously difficult. Being motivated is vital, and motivation is what seems to differ by class.
The 1996 Twin Cities study, for example, found marked differences in weight concern. Although women earning under $10,000 were no less successful when they dieted than women earning over $40,000, they were one-third less likely to diet in the first place. They had a greater tolerance for weight gain, saying it would take a 20 pound gain before they took action, as opposed to the 10 pound gain that would trigger action in higher-income women. They even weighed themselves less often (three times a month vs. seven times a month).