Obesity: A Conjecture
My evidence for that conjecture is how much of the talk about obesity focuses on the evils of marketers cleverly manipulating people into eating junk food. While sellers of junk food do, of course, advertise their products, so do sellers of diet soda, exercise equipment, metrecal and health foods. I understand why people concerned about obesity might see the regulation of advertising as a potentially useful tool--it is at least more likely to be politically viable than an attempt to ban hamburgers and french fries from the American diet. But I do not see marketing as a plausible explanation for the increasing frequency of obesity. It seems particularly implausible given that the increase is not limited to rich countries such as the U.S.; I doubt the consumption patterns of people in India or China are much influenced by advertising.
My alternative explanation for obesity is straightforward. Humans evolved in an environment where food was costly, fat scarce, sweetness a useful signal that fruit was ripe. We are designed by evolution to put on weight when we can as a precaution against future famines and to favor fat and sugar when we can get them. In a world where food is inexpensive and plentiful we are inclined to overeat, in particular to eat more fat and sugar than is good for us.
The obvious explanation of the increase in obesity is that real incomes around the world have been trending up for decades. Now poor people in the U.S., and increasingly in poorer parts of the world, can afford to eat all the calories they want. Since all the calories they want represents more than what they require, the result is that they get fat.
There is one problem with this explanation. According to the figures I have seen, in the U.S. obesity is less common in high income groups than in low income groups. The richer you are, the less your diet is constrained by cost, so we would expect higher income groups to be at least as obese as lower income groups. To explain why they are not I must add one additional factor: Time lags in adjusting behavior and social norms to changed circumstances.
Suppose you are part of a population where food has been costly, where people engage in a lot of physical labor, and so where the problem is getting enough to eat, not avoiding too much. You, and those around you, have adapted their behavior to that environment.
Now things change; food gets cheap, wages go up, almost everyone can afford to eat as much as he wants. For a while, perhaps a generation or two, people follow the old patterns in the new circumstances; the result is that many of them end up fat. Over time, although the hardwired elements of behavior do not change--evolution is slow--the cultural elements do. Instead of demonstrating how wealthy and generous you are by urging your guests to have a second and third helping of dinner, you do it by providing them smaller amounts of particularly tasty, sophisticated, or expensive dishes. Instead of making a point of avoiding physical exertion when you can, you enter the Boston Marathon. Eventually you and those around you have adapted your behavior, although not your hardwired tastes, to the new environment.
Well off people in developed societies have been able to afford second and third helpings at every meal for a long time. Hence, if my argument is right, they have had time to adapt to a world of plenty. For poor people, being able to eat all they want of more or less what they want is a newer thing, so they are still following the old ways--the pattern of the traditional Jewish (or Italian) mother who insists that her guests have a little more of this and that before they end their meal. Hence, if my conjecture is correct, greater obesity among the poor reflects the lag in adapting to circumstances that are relatively new for them. The rich have had time to adjust.
One implication of this is that, at some point in the past, richer people should have been more often obese than poorer--back when the rich were no longer constrained by the availability of food but the poor still were. That fits my casual impression, but I have no actual data to support it.