Another apparent contradiction of the notion that either sedentary behavior or prosperity or a toxic food environment is the cause of obesity is that obesity has always been most prevalent among the poorest and thus, presumably, harder-working members of society. In developed nations, the poorer people are, the heavier they’re likely to be. The NHANES studies confirmed this observation, first documented more than forty years ago. In 1965, Albert Stunkard and his colleagues at New York Hospital reported that they had surveyed 1,660 New Yorkers and found that obese women were six times more common at the lowest socioeconomic level than at the highest. Thirty percent of the poorest women were obese, compared with 16 percent of those of “middle status” and only 5 percent of the richest. The poor men were twice as likely to be obese as the rich (32 percent to 16 percent). These observations have been confirmed repeatedly throughout the world, in both children and adults. Because poor and immigrant populations are considerably less likely than wealthier, more established populations to own labor-saving devices, and because they are more likely to work in physically demanding occupations, that poverty is a risk factor for obesity is another compelling reason to question the notion that sedentary behavior is a cause.

There is a tendency among public-health authorities, obesity researchers, and health writers to discuss obesity as though the problem on a societal scale were only twenty or thirty years old, but this confuses the problem of obesity with the current obesity epidemic. Because these last few decades also coincide with the spread of McDonald’s and other global purveyors of high-fat fast foods, obesity can conveniently be blamed on fast food by virtue of this association. (It has also, by this same logic, been popularly blamed on high-fructose corn syrup.) But the research literature of obesity dates back further than the epidemic, and by including all of the relevant observations over the years, we can begin to rule out competing hypotheses. Any hypothesis that purports to explain how obesity is caused, after all, should explain the emergence of obesity in any population and at any time, not just the increasing obesity in the past few decades.